Author: Carol Reese, Retired Extension Specialist, University of Tennessee
Trees and shrubs provide shade, shelter, beauty, and even nourishment for the people and wildlife of Tennessee. Master Gardeners can help owners develop a landscape design and philosophy, which in turn helps determine the roles that woody plants will play. Some plants perform specific functions, such as providing screening, a windbreak, or shade, while others define and delineate landscape spaces. Designs may also lean toward a particular genre such as heirloom, edibles, or four-season excitement. Sometimes people buy a woody plant just because it is so lovely that they simply must have it, and if they have chosen an informal landscape that accommodates such impulses, they need not hesitate.
Keys to Success in Selecting Woody Plants
Regional Adaptation
Selecting regionally appropriate plants is imperative. The most important question is whether they will survive on this portion of the North American continent, where they must endure freezing winters and sweltering summers.
Hardiness zone maps describe how far north a particular plant can survive winter temperatures. Because of warming trends in recent decades, the United States Department of Agriculture (USDA) issued a revised zone map in 2023. (see Figure 1.) Most of Tennessee is in Zone 7, which means that landscape plants should be able to survive occasional winter temperatures that reach zero.
The more northern and higher elevation areas of Tennessee are in Zone 6, where winter temperatures occasionally dip to -10°F. Some southern and western areas of the state are in Zone 8, where temperatures below 10°F are not expected. Of course, nature does not like to fit into these convenient 10-degree divisions, and occasionally the winter temperatures can be more extreme. Also, be aware that a plant that should survive -10°F can be killed by a mere dip into the teens if temperatures have been fluctuating drastically between warm and cold. For example, the Easter freeze of 2007 killed many plants that are usually rock-hardy. That year an extremely warm March was followed by three nights in April that fell to the teens and low twenties. An early hard freeze in the fall can do the same, especially if a warm, wet autumn has not allowed plants to “harden off” in preparation for winter. Plants prepare for winter months by reducing and concentrating the water in their tissues, which usually takes several weeks of cooling temperatures and shorter days.
Zones may be subdivided into Zone 7a or 7b, in 5-degree increments, allowing even more room for deviation from the norm. It is also true — and of great interest to the daring gardener — that each landscape can have microclimates, spots that are a few degrees warmer or cooler than the surrounding areas, allowing the gardener to cheat a little and plant some marginal selections that may not usually survive in that particular zone.
Winter cold is only one challenge for the southern gardener. It is probable that more landscape plants are killed by the heat and humidity of Tennessee summers than by our winters. This possibility is not addressed in many books or on most plant labels, but it is of utmost importance.
In an effort to address this issue, the American Horticultural Society has issued a Heat Zone Map, establishing zones by determining how many days the temperature rises above 86°F. Unfortunately, daytime temperatures are only one-factor determining heat stress. High night temperatures are probably more important since they increase plant respiration, the reverse of daytime photosynthesis. (see Chapter 3.) This process results in the depletion of the carbohydrates produced to satisfy the plant’s needs, causing the plant to decline and become more susceptible to other problems.
These heat-related issues are most prevalent in the warmer, low-altitude western regions of Tennessee. Humidity also increases disease pressure, another factor that causes plants to fail. As a result, more information would need to be included to make the heat zone map optimally useful.
The average annual rainfall for Tennessee is around 52 inches, which would be just about perfect if it fell in ½-inch increments twice weekly throughout the year. Instead, our rains come mostly in winter and spring when we need
it least, so it is a good idea to choose plants that can tolerate weeks of summer and fall drought.
Once established, most regionally appropriate landscape plants can survive easily without supplemental watering, even though they may not look their best. Plants in our wild areas are never offered supplemental watering, yet it is rare that we see them killed outright by drought. While native-plant enthusiasts may say that is because native plants are naturally more drought tolerant, another school of thought is that the undisturbed soil is the more important factor. Human tilling and soil amending changes the texture and water-holding capacity of our native soils, and it may affect the mycorrhizae, beneficial fungi that enhance the ability of roots to take up water and nutrients, as described in Chapter 5.
In short, the most important factor for success with woody plants is selecting plants that are best suited for our challenging climate. There are lists of plants that have demonstrated their durability in university trials and many Tennessee landscapes on the website associated with this publication. These lists also differentiate the plants that may survive in the cooler mountainous regions from the ones that are better adapted to the more southern and western regions.
Since it is not reasonable for these lists to include all the plants that could survive in these regions, they are weighted toward those plants that a gardener is most likely to find in local garden centers. These lists are not printed in this manual because they are subject to change as good, new plants are introduced into commerce or old ones fall from grace, perhaps because of some newly arrived pest or disease. The lists specify some plants by function, such as screening or covering slopes, and others by site specifics, such as wet or shady. To see what these plants look like, refer to Internet search engines or look for images on UT Extension websites.
It also would benefit the thoughtful gardener to stay current with information available from the UT Gardens. The gardens comprise three sites at Knoxville, Crossville (Plateau Discovery Garden), and Jackson. Together they are officially designated as the Botanical Gardens of Tennessee and are open to the public without charge. Gardeners are encouraged to visit the gardens and see the plants, both woody and herbaceous, that are thriving in their region. Other good resources are the UT Gardens website and the UT Gardens Facebook pages, which provide photos plus links to annual reports that list the best performers. The main UT Gardens website also lists the UT Gardens “Plant of the Month,” along with an archive of the monthly winners going back for several years.
A little Internet sleuthing can uncover many other sources of information about plant performance in different regions. It is best to focus on those in Tennessee or neighboring states that more closely mimic the climate at the gardener’s location. While commercial sites may offer useful information, keep in mind that University trials and botanic garden trials have no profit motive to skew their findings. In other words, keep in mind the source of information when evaluating trial garden results.
Smarter Shopping
The death of a plant in a Tennessee landscape may not be due to any transgression of the gardener other than choosing a plant that is not willing to grow here. This leads us to the question of where to shop for the best bets.
Locating High Quality Suppliers
Those who have done their homework and know a good plant when they see it have no reason to avoid picking up a healthy specimen at the big box stores. However, these stores often carry plants that are not regionally appropriate. It is not usually a good investment to pick up an unfamiliar plant based on the brief description offered on the tag. These tags are fairly useless in predicting the plant’s durability, and more often than not they provide inaccurate information about the plant’s mature size.
Based on years of experience, locally owned garden centers are more likely to carry regionally appropriate offerings. In addition, it behooves the master gardener to support these locally owned establishments. Otherwise, they may fail, leaving only the big box stores as sources for plant material. That situation could leave the gardener with few offerings chosen for regional adaptation. It could result in a more limited plant palette as the stores concentrate on what is most profitable or easiest to ship rather than what is most exciting and satisfying to the gardener.
When shopping, ascertain the plant’s preferences for the amount of sunlight, moisture, types of soils, and exposure to the elements. Never assume the information on the tag is adequate or even reliable. A plant labeled for full sun may suffer in the full sun of west Tennessee but flourish in the full sun of Oregon.
Locating High-Quality Sources of Information
While the Internet is often our first stop when doing research, it is important to learn how to distinguish good information from bad. Generally, be skeptical of commercial sites. In their efforts to promote sales, they often describe each plant as fabulous. Other sources of inaccurate information include inexperienced garden bloggers who innocently believe such commercial sites are valid places to gather information, or garden bloggers who may draw conclusions from a single experience with a plant.
Look for sites that have no ulterior motives. University sites have a clear responsibility to provide verifiable, science-based information. Along with information provided by UT, you may wish to check with other university sites, especially those in the southeastern United States with similar climates.
Good information can also be found by tapping into plant society sites that have assembled a large body of members’ experiences with plants over a period of many years. It might be thought of as naturally occurring informal replicated studies! Replicated studies are those that universities and botanic gardens set up more formally to draw science-based conclusions.
Many such societies are devoted to a particular genus of plants. There are national societies for magnolia, boxwood, camellia, clematis, holly, and rose, to name just a few. Other plant societies are devoted to broader plant groupings, such as the American Conifer Society, the Perennial Plant Society, or the Cactus and Succulent Society of America.
Local groups and chapters across the State of Tennessee provide tours, meetings, newsletters, and lectures. Examples include the Mid-South Hydrangea Society, based in Memphis, or the East Tennessee Hosta Society.
In addition, there are professional organizations, whose goal is to raise and maintain a high standard of excellence in their field. In the horticulture world, those groups cover such occupational groups as urban forestry, arboriculture, nursery and greenhouse growers, weed science, and landscape.
The characteristic all these sources have in common is their sincere desire to encourage the success and satisfaction of those seeking quality information. There is no incentive to obscure the facts. For those seeking information, a key strategy is never to rely entirely on any one source, as there is always a potential for fallibility. It is best to check several sources and determine whether there is a consensus about the information being sought.
It is beyond the scope of this book to describe the cultural needs of all the plants included on the helpful lists referenced in this chapter and listed on the website. For those who want the information in printed form, the Manual of Woody Landscape Plants, by Michael Dirr, is highly recommended. It is an encyclopedic but highly readable volume that provides the pertinent information most often needed about a multitude of woody landscape plants, including their size, cultural needs, ornamental attributes, and propagation.
It is especially useful for southern gardeners since Dirr also thoroughly addresses the heat tolerance of the plants, information that is usually scarce in woody plant books. The manual is a good investment, and most of the county agents who conduct Tennessee Extension Master Gardener classes have a copy in their office available for inspection. It is also available as an app and gets excellent reviews for its usefulness.
Site Selection
After the right plants have been purchased for the part of the state where they will be planted, the next important step is to site them according to their cultural needs. Some plants are cheerful about adapting to a variety of sites, but others are very particular and will turn up their heels in short order if sited poorly.
Drainage
Boxwoods and hydrangeas are examples of popular landscape plants decidedly intolerant of poorly drained soils. For example, if a plant that resents wet soil is placed where the condensate from the air conditioning unit seeps all summer, it is doomed.
To determine how well the site drains, dig a hole 12 to 14 inches deep and fill it with water. If all the water has drained away within 24 hours, the site drains adequately well. If it takes 48 hours, there are still many plants that can be used, but avoid those marked as being strict about well-drained soil. If it takes 72 hours for the water to drain, use only those plants specified as tolerant of poor drainage. A list is available in the online resources. The list is also useful in selecting plants for a rain garden.
As a general rule, plants that can grow in poorly drained soils can also prosper on higher and drier sites, exhibiting reasonable drought tolerance once they are well established. A few good examples include common bald cypress, winterberry holly, and Virginia sweetspire. However, the reverse is not true. Any plant that is specific to drier sites will quickly succumb in soils that stay wet because their roots require good air exchange.
Sun and Shade
The terms full sun, part sun or part shade, and full shade describe the sun exposure of a site. Generally, 6 hours of sun or more per day is considered full sun, 4 to 6 hours is considered partial sun, and less than 4 hours constitutes full shade. This is yet another area with a spectrum of variation, so if 4 hours of sun come during the middle of the day, the light and heat would affect the plant differently than if the 4 hours consisted of 2 hours in the early morning and 2 hours in the late evening. Sometimes it just takes experimentation to find the best location for a plant. If a plant is not happy, do not hesitate to move it. Remember that sun and shade can change over time as trees grow, are limbed up, or die.
Some plants may survive in less-than-ideal light but may not reach their full potential. For example, abelias and viburnums are tolerant of some shade, but they will be more dense and floriferous in full sun. Azaleas (see photo on page 8-9) will survive in full sun and flower heavily, but they are more prone to attack from lace bugs and spider mites when grown in a hot, sunny site. They would prefer shade during the hottest parts of the day, and they thrive in filtered light.
Bigleaf hydrangeas and Japanese maples, among others, perform better when they get several hours of morning sun, but they suffer from hot afternoon sun. It is the Goldilocks syndrome: not too hot, not too cold, but “just right!” Some plants are just worth it.
Soil Quality
Some plants are very particular about pH, such as azaleas and blueberries (see photo on page 8-9) that demand acidic soil along with good drainage. Others can prosper in most any soil. Willow oak (see photo below), black gum, and yaupon holly come to mind. While amendments can affect soil pH, the soil always tries to revert to its original pH, which can make for a frustrating battle. It is best to choose plants suited to the native soil.
For soils with low organic matter, plants can be found that will not only tolerate that condition but can help it improve over time. Many cover crops, especially legumes, can help accomplish this goal. Remember that some plants actually prefer what are sometimes called “lean soils” and would not prosper in those with high organic matter, so pay attention to their origins in the wild for clues. Note that plants found in bottomlands that often flood are frequently the species successful in plantings where the roots may be deprived of good air exchange, such as settings with lots of concrete. These plants have already adapted to poor conditions for roots.
The moral of the story is to be thoroughly acquainted with a plant’s cultural needs, or, looked at another way, purchase a plant that experience has shown will succeed in the intended setting.
Avoiding Troublesome Plants
While survivability is usually considered a good thing, most gardeners have had experience with a successful plant they wish would die! What factors might make a plant an undesirable choice for the landscape?
Aggressiveness
The term invasive is most often used when a plant is introduced from another country, but some of our native plants can also be overbearing. A few trees that aggressively reseed include boxelder and red maple, but even oaks can be guilty of this habit. Other trees and shrubs spread rhizomatously and form large thickets if not controlled, such as white poplar, purple leaf plum, pawpaw, sassafras, and sumac.
Root Issues
Some plants have roots that stay near the surface and rob any other nearby plants of nutrients and moisture. Maples and poplars can be exceptionally guilty, but any tree exhibits this characteristic to a degree. Other trees’ roots seem to have a knack for finding and invading sewer lines. Maples are on that list as well, along with willows. Magnolias and eastern red cedar are known for having competitive root systems that make it difficult for plants grown beneath them to survive.
Litter Problems
Most people dislike the prickly balls of sweetgum or the large green husks of hickory trees that can permanently stain clothing. Sweetgum balls may also be a safety hazard, rolling underfoot. The large leaves of southern magnolia trees take a long time to break down, which is why it is advised never to remove the lower limbs of this tree. Instead, leave them to hide the trunk and the ground below and be sure to site the tree accordingly. Sycamore leaves are also large and unsightly.
Mess Production
Certain trees are susceptible to aphid and scale infestations, unseen in the canopy overhead; however, the insect’s excrement forms a sweet film on everything below, providing the perfect substrate for a dark little fungus, giving rise to a problem known as sooty mold. Pecans, tulip poplars, and some crape myrtles (especially the older forms) are especially prone to this problem, and even the leaves on the lower portions of the canopy are covered with this black film.
Another mess factor might be a small fruit that is not visible at a cursory glance but is favored by birds. A good example is hackberry, whose small fruits are not visually annoying. However, that cannot be said for them once they have passed through a bird and become purple blotches on everything below. Birds love them.
Pest-prone and Short-lived Species
Certain trees are especially prone to borers. Thin-skinned trees such as young maples, ornamental cherries, and peaches are likely to be attacked. The fungal disease entomosporium leaf spot is common on photinia and raphiolepis. Scale is common on euonymus. While these problems can be prevented by a regular preventive spray program, the gardener must decide whether they are worth the extra effort when there are other plants available that do not require special care. Sometimes the answer is yes when a plant is one that has been inherited and is an important statement in the garden or is simply one that the gardener loves.
In short, a woody plant is a living thing and likely to have a few faults. Certainly, some are more tolerable than others, especially if they have more positive traits than irksome ones. Sometimes careful siting can be a fix. For example, a crabapple that drops messy fruit onto a parking area may be a problem, but those same fruits further out in the yard may attract desirable birds for pleasurable viewing.
Other times, plant gender may be an issue. If the plant is dioecious (having both male and female plants), the problem can be avoided by selecting the male. Ginkgo is a good example, as the female has golden round fruits that exude a powerful stench. Other times, the female and the resulting fruits are worth the trouble, such as holly with its pretty red winter berries.
Becoming a More Knowledgeable Buyer
After plant species or varieties and their sites have been selected through careful research, it is time to shop for the plants themselves. In that process, it is important to know what to look for in a high-quality plant. As stated earlier in this chapter, there are reasons to patronize a locally-owned garden center. They often have well-established relationships with the growers of the plants they carry, and those plants can be expected to be of consistent, reliable quality.
Quality is an issue often overlooked in procuring plant material. Most people would agree that it is important to pay more for a quality tool or a pair of shoes, but when it comes to plants, they cannot see why they should pay more when they can get the same plant cheaper elsewhere. What might identify a plant as one of poor quality?
To start, it is the retailer’s responsibility to carry plants that are accurately labeled. Most gardeners have had the experience of planting a tree or shrub that turned out to be something other than what they expected.
Also, take note if the plants carried are those that are recognized as “improved,” or proven cultivars. In fact, take heed if there is no cultivar indicated for certain types of plants. For example, if a crape myrtle is labeled simply as “pink,” buyer beware. There are many pink crape myrtles. Some are dwarf, some are meant to be grown as small trees, some have fantastic bark, and others may be less cold hardy or get powdery mildew, not to mention that there are several shades of pink.
A reputable garden center should carry durable, disease-resistant cultivars that grow to a predictable size and exhibit a specific color. If the seller has not done the homework to provide this service to customers, it is not a good place to shop, no matter how inexpensive. Other times, named cultivars are of little importance, such as with oaks. While there are particular cultivars of oak that are selected for unusual characteristics, most are seed-grown and will yield a predictable habit that makes a good shade tree.
Another example of quality differences pertains to field-grown trees that have poor root systems. An experienced grower of landscape trees is usually familiar with the tree sizes often in demand by the local landscape trade. An example might be a 2½-inch caliper willow oak. (Caliper refers to the diameter of the trunk just above the root flare.) Field-grown trees are dug once fully dormant (that is, they have dropped their leaves with the onset of cool weather), and the root balls are wrapped in burlap (or a similar material) for shipping to the retailer or directly to the landscape site. This is why you may hear them called balled-and-burlapped plants, or simply “B&Bs.”
A good grower concerned with producing quality trees knows that there is an official Nursery Standard for Nursery Stock, stating that for every inch of caliper, there should be 1 foot diameter of root ball. That means a 2½-inch caliper willow oak should have a root ball that is 2½ feet wide. Still, that is not a very large root ball for a tree when you consider that the feeder roots can extend as far as or farther than the spread of the canopy, which could be several feet wide for a tree of this caliper. Digging only 2½ feet of root ball will mean that the majority of the roots are left in the ground. This would result in a tree that is less likely to survive or will put its energy into growing a new root system before it begins to send energy toward its canopy. This all means there will be a long wait before it begins to prosper.
A grower of quality trees can prevent this problem. Since the intention is to harvest trees at a caliper of 2½ inches, the grower will prune the root ball at 2½ feet on a regular basis as the tree grows. This pruning can be done with a sharp shovel, but most growers of any size use a tree spade, a piece of equipment that mechanizes this laborious process. Just like a cut stem will send out new shoots, a cut root will send out new roots. Thus, when the tree is dug with a 2½-foot root ball, it contains a massive amount of feeder roots, and once transplanted, it will be able to send more energy toward the growth above ground rather than below.
Since it is not reasonable for the shopper at a garden center to unwrap the root balls and check for good root systems, it comes down to shopping with a local garden center that can be trusted to purchase from growers who produce quality plants. However, it would be possible to measure the caliper and the root ball to see if they are close to nursery standards.
When it comes to container plants, buyers can check the roots. Carefully tip the plant to one side and try to get a little space between the roots and the plastic container. The roots should visibly fill out the container but should not be crowded and circling. Circling roots can cause problems later if not teased out to spread, or even pruned heavily before planting. Sometimes shoppers purchase a 3-gallon plant and find that when pulled from the container it has a 1-gallon root ball that has recently been planted into a 3-gallon container with fresh potting soil. The buyer has paid for a 3-gallon root system, and it would be wrong of the garden center to sell the plant until it had filled that container. A conscientious grower would have put such plants to one side until it was ready for sale, or at least explained that the plant was recently bumped up to a larger pot so the customer could decide whether or not to buy it.
In recent years, another problem has surfaced. In order to get plants to market more rapidly, many growers stick several small plants into one pot, giving the illusion of a larger plant. In reality, it is several plants competing for space, water, and nutrients. A quality grower would have stuck one plant in the pot, perhaps pruning it several times to produce the desired bushy growth habit, and may have had to wait a couple of years or more before it was ready for the market.
The shortcut grower prepares the plants for sale much more quickly by using several cuttings and babying them with plenty of water, fertilizer, and light. However, once the cluster of plants gets into the landscape and the plants continue to grow, some will be crowded out. Suddenly a “part” of the shrub is dead through no fault of the gardener but rather because of competition among individual plants in the cluster.
This practice is especially obvious in crape myrtle production, where one often sees three cuttings stuck into a pot and sold as a “multistemmed” crape myrtle. A true multistemmed crape myrtle consists of one plant that was cut back to the ground in its youth. From the regrowth, several stems, typically five to seven (but always an odd number), were selected for training up to display their lovely muscular wood and picturesque bark. The cheaply done three-to-a-pot method is not likely to perform well in the landscape. What typically happens is that two of the stems do relatively well and the third one suffers. A discerning buyer or landscaper will specify a ‘true” multistemmed crape myrtle despite the added time and effort incurred by the grower.
Any plant should be examined for diseases or pests, broken limbs or twigs, and any damage to the bark. It is especially important to recognize borer symptoms and inspect for them.
Many ornamentals and the majority of fruit trees are grafted plants. Remember that a grafted plant is the desirable cultivar joined to the rootstock. (see the propagation section in the online supplemental materials.) Usually, the graft union is just a few inches above the root ball. Look at this union to see if it is healthy and sturdy. If there is any new growth coming from below the graft, it is being issued from the rootstock and should be removed as it will be more vigorous than the growth from the scion and can overpower the desired cultivar. The rootstock should be monitored regularly over the years as the plant grows, and any rootstock shoots should be removed.
Planting
The traditional planting process has been to dig a generous hole and fill it with a mixture of the original soil amended with organic material, such as finely milled bark, known as soil conditioner or compost, and sometimes peat moss. (see Figure 2.) Modern research has shown that this method can create problems over the long haul since it basically creates a “bucket” of differently textured soil. In Tennessee that often translates to a more porous soil than the surrounding clay-based soil. This causes the “bucket” to fill with trapped water during wet times and to dry out faster during spells of drought. Often the roots are reluctant to leave the amended area and instead grow in a circle, causing the plant to be poorly anchored and possibly leading to girdling of the roots. (see Figure 3.)
Another traditional preparation for planting is tilling the soil. However, tilling can be destructive to the many helpful organisms in the soil, including microbes, insects, earthworms, and mycorrhizae, that contribute to soil fertility and aeration. (see the breakout box in Chapter 6.) The 21st-century thinking on soil preparation is that we should mimic more closely what nature does to create a healthy soil environment for plants. That means improving soils from the top down using compost and mulch. In a wild setting, this process is what would occur as decomposing leaves are recycled into plant nutrients over time, and pore space is created by activities such as burrowing insects, decomposing roots, freezing and thawing, and natural aggregation of soil particles.
Of course, there are times when the soil available is terrible and will not provide a viable substrate for plant material, perhaps where grading has been done and all topsoil is absent. In that case, it is certainly advisable to construct a better root environment. In some cases, a load of topsoil may be required — perhaps just as a layer or as a raised berm. If those options are not desirable, tilling in other materials to the existing soil may be the only alternative. However, it is important to till the entire area so that the plants roots are not restricted to a virtual bucket or upper layer of improved soil. Also, be certain that water can move through and away from the improved soil. Compost, ground bark, municipal biosolids, and thoroughly aged cotton gin trash or decomposed sawdust are a few examples of materials that may be available and can be mixed into the soil.
If there is an adequate amount of relatively undisturbed soil, a fresh school of thought encourages gardeners to consider minimal soil disruption. The soil is simply broken up enough to place the plant, making the hole wider than the root ball of the plant but no deeper. This method allows the plant’s roots to become well integrated into the natural soil that will support it for its lifetime. If the hole is deeper than the root ball, the plant can sink lower than the surrounding soil, setting it up for trouble.
While it seems counterintuitive, it is best to vigorously disturb the root ball just before putting the plant in the ground. The more the roots are loosened, the better, as it would be detrimental for them to continue to grow in any circling motion. If they are already tight and girdling, take a sharp serrated saw or some good pruners and snip through them on three or four sides. (see photo on page 8-17.) Just as a cut stem will generate new shoots, those snipped roots will send out new growth into the surrounding soil. Also knock or rinse away the potting material as it is often porous, bark-based material that is very different from the soil the growing plant must adapt to, which can cause problems, as described earlier.
For balled and burlapped (field grown) plants, newer research has shown that it is also a good practice to shake or rinse away the soil in the root ball, encouraging the roots to integrate into the new soil. Always remove the burlap, twine, and wire basket if present. Anything that may restrict or girdle the growing roots should be removed. If the root ball is so heavy that once wrestled into its hole it may be difficult to remove the burlap from the very bottom, at least snip away as much as you can reach, as roots grow mostly horizontally anyway.
Many beginning gardeners plant too deep. It is important to remember that roots like air exchange and that most woody plant roots are found in the top few inches of soil. Burying those roots can kill the plant over time. Though some plants will put out new roots on the buried trunk, they struggle in their growth. The surface of the root ball should be even with or possibly a bit higher than the surrounding soil. This can cause water to run away from the root ball when watering, so it may be necessary to make a moat a few inches from the root ball to retain and direct water to the roots.
Be sure to give the newly planted tree or shrub a deep soaking to help settle the soil in around the roots, even if rain is expected. An average rain shower or even turning on the irrigation system for a while is not likely to accomplish this important goal. Whether irrigation is even desirable over the long term is discussed in the next section.
Don’ts
Do not put fertilizer into the planting hole. In fact, do not fertilize shrubs and trees at all for the first year. Fertilizers are salty, and salt draws water from the roots. Do not bother with root stimulators, either. There is no scientific research that shows any benefit. The money saved can be used to purchase more plants.
Do not put plastic film down around the plant’s base because it will block air exchange to the roots. Use organic mulches to cool and improve the soil and to provide a buffer zone between the plant and the mower or string trimmer. Shredded bark or pine straw is a good choice for mulch.
Do not go overboard with watering newly established plants. Any plant can wilt during a warm, breezy day, even if the soil is wet because the plant cannot replace the water in the leaves as fast as it is leaving them through evaporation in that kind of weather.
While there are many different ways to irrigate the landscape, as described in the landscaping chapter, they are not always necessary and some may be undesirable. Consider an above-ground drip irrigation system that can be amended as needed, as described in the landscaping chapter. (Also see the section on irrigation in the online supplemental materials.) Many plants are killed by irrigation systems that deliver the same amount of water to plants that need it as to those that do not like much water. Watering properly is crucial to success and a task that is often misunderstood, which can lead to plant death. This topic is discussed in more depth in the next section.
Next Steps
After proper planting and siting of regionally appropriate plants, consider the further steps described below to help ensure plant success.
Protect the Bark
Some plants do not survive because of “weed-eater disease,” a term often used when plants die because the bark was beaten off of shrubs and trees. Sometimes it is a string trimmer that did the damage; other times, the lawnmower skinned the bark. If too much bark is removed, the phloem is so disrupted that the plant cannot distribute photosynthate from the leaves, and the roots slowly die from lack of nourishment.
Sometimes just a ding is all it takes to kill a tree, as the wound can attract borers. Borers can be the larvae of beetles or moths. Usually, the adult insect lays eggs on the bark, and the young larvae bore in after hatching. As they tunnel about while feeding, they destroy vascular tissue, limiting the tree’s uptake of water and nutrients from the roots or the flow of photosynthate from the leaves to the rest of the tree. This damage can be fatal, though it may take more than one year of infestation for the tree to die. If the problem is caught in time, the tree may be saved, but of course, prevention is preferable.
How does wounding a tree attract borers? Adult insects are attracted to damaged trees by scent and they sometimes lay their eggs in the wound or in cracks or damaged areas of bark to make it easier for the young to get a start.
Apply Mulch
The best way to avoid mechanical wounds to trees and shrubs is to apply enough mulch around the base to suppress weeds so there is no reason to use a mower or string trimmer near the trunk. Make a V-trench with a sharp shovel to keep a crisp line between the turfgrass and mulch so the mulch will not be knocked away from the tree little by little each time the grass is mowed. Mulch also cools the root zone, helps slow the loss of soil moisture, and improves soil properties if organic mulches are chosen that decompose over time.
Wood mulches are easily obtained. Using those that originate as byproducts of a responsible timber industry certainly qualifies as a sustainable practice. While some may flinch at the idea of cutting trees, any trees, forestry has come a long way in embracing what are generally called “best management practices” that consider the health of the ecosystem as a whole. While you can still find those who disdain good stewardship, much of the wood-harvesting industry recognizes the value of taking care of plants, wildlife, soil, water, and air quality. Many have adopted chain-of-custody certification programs that ensure the products they sell are sustainably grown wood products. Unfortunately, these practices haven’t yet trickled down to many mulch products, but in time that may change.
Cypress mulch has been the subject of extensive debate, with reports that whole cypress trees harvested from crucial wetlands are being chipped up for sale as a high-value mulch product and that replanting and regrowth are not keeping up with the losses. While some say that cypress mulch is still simply a byproduct of a responsible industry, some of the larger retailers have become concerned enough to remove any cypress mulch harvested in Louisiana from their inventories.
In studies done in Florida, cypress mulch was not shown to be superior to mulches made from pine or hardwood anyway, despite some claims to the contrary, so there is no compelling reason to buy it.
Sometimes local sawmills sell mulch directly, although the quality may vary. This locally produced wood mulch should figure into the green equation since local sourcing would reduce the fossil fuels consumed in transporting the product to your garden. Hardwood and pine mulches seem to be equally good.
Does mulch need to be “aged” before use? Much has been made of raw wood products “stealing” nitrogen from plants as they break down. Research using raw wood chips as mulch has demonstrated that well-established plants are not harmed because the amount of soil affected by the breakdown process is very shallow. However, tilling in raw wood products with new plants is certainly detrimental, so only well-composted material should be used in that scenario.
Shredded wood mulch is a good product and performs well. Pine bark “nuggets” are not as good. Nuggets tend to shift around rather than pack down tightly, and in heavy rain, they may even float away. This tendency to locomote makes them an especially poor choice for use on a hillside.
It may be best to stick with the natural color of shredded hardwood mulch. The dyed mulches stand out like a bad carpet, and they age to something that looks like a faded bad carpet
Be sure to apply mulch properly. About 3 inches deep is good, and the depth should be closer to 1 or 2 inches as it nears the trunk of the tree. Mulch should never be piled up around the trunk. Make the diameter generous — remember that the further the mower is kept away from the plant, the better.
Pine needles make excellent mulch, and it is often said that this product is best suited for acid-loving plants. It is true that the soil under pine forests is notoriously acidic, which is a result of decades and decades of pine needle decomposition. However, it would take many years of using pine straw mulch to change the soil pH in a landscape appreciably. Also, most ornamental plants prefer soil on the acidic side anyway.
Staking … or Not
Should a plant be staked? Not if it can be avoided. Much research has been done on the subject, and the results have shown that when trees are not allowed to move freely in the wind, they become spindly specimens. A staked tree will be weak, floppy, and have a thinner trunk than one grown without stakes. In fact, if there is a weak-trunked young tree in the landscape right now, it should be given a good shaking often. The trunk will begin to thicken up faster than those of unshaken trees.
If the reason for staking is that the tree is blowing completely over, consider a way of pinning the root ball to the ground instead of staking the tree. Drive a piece of steel rebar or a long wooden stake through the root ball of the tree and deep into the ground. Wooden stakes will eventually rot, but by then the tree roots should have gained a purchase in the soil. Drive at least three of these “nails” at opposing angles so they will more effectively fasten the root ball to the hard ground below. (see Figure 4.) It is possible to hammer the rebar or stake completely into the ground so it can be mowed over, but if a generous amount is left sticking up, it can help ward off mowers. It would be better to have a little ratty grass around the tree’s trunk than to have it skinned by a mower. If the ratty look is not tolerable, mulch the whole area or apply a little grass-killing herbicide of a type that will not damage the tree.
If this strategy is not sufficient to hold a large tree upright, a different arrangement can be used. Lay two strong boards over the root ball and nail them to wooden stakes driven deeply into the ground beyond the root ball. (see Figure 5.) This undisturbed soil plus the wider distribution of pressure on top of the root ball can work well.
If traditional staking is used, with lines attached to the tree’s trunk running to anchors in the surrounding soil, be certain to use soft material to pad the line where it comes into contact with the trunk. Canvas strapping can be used, but stockings or bicycle inner tubes are also options.
These should be tied loosely, allowing the tree some freedom of movement in the wind. They should be placed at a height that still allows the tree to sway naturally. Move your hand up and down the tree and push it back and forth to find the right balance between anchor and movement. Remove stakes as soon as the tree has anchored.
Another solution that allows some movement is to use a loose figure-eight loop that includes the tree trunk with just two stakes driven into the ground alongside.
Watering
New plantings need supplemental water until well established. Watering during the first year is crucial, and the water needs during the second year bear watching. Usually by the third year a plant that has been properly sited for its normal water needs should be fine without supplemental watering except during an extraordinary drought.
There are no standard guidelines to use in developing an exact schedule for watering, since water needs are determined by many factors. Is the soil sandy or mostly clay? Is the weather damp and cloudy, or bright and breezy? How much foliage has the plant produced? More foliage means more water loss to the air since leaves are the “pump” that pulls moisture through the plant. Pruning a plant may reduce water needs.
The answer is to water as needed, which can easily be determined by using the sensitive instrument found at the tip of one’s arm. Stick a finger into the soil and when it begins to feel dry, add water. Do not let the soil get bone dry, and be sure to stick the finger into the soil, not just into the mulch. Water slowly and deeply. Water can be applied slowly with a dripping hose, or if far from a faucet, with a leaky bucket or jug. Commercially available devices can be used as well, often consisting of bags or plastic containers shaped as rounded rings that can be filled with a hose or water tank.
Fertilizing
It is possible, even likely, that a tree or shrub will be a healthy specimen even if it is never fertilized, as most Tennessee soils have adequate fertility. Some people choose to use fertilizer to optimize growth and to address situations where degraded soils lack certain elements in sufficient quantities. A soil test can help determine if the soil on a planting site needs additional fertility.
Certainly, a bit of a nutritional boost can speed growth and performance, but it is always better to err on the side of restraint. It is human nature to think that more is better, but too much fertilizer can cause plant damage or even death. Too much lush growth can increase the likelihood that the plant will be attacked by pests and diseases, and it can reduce the potential for flowers on blooming trees and shrubs.
Avoid any fertilization for the first year or until plants are well established and past any transplanting shock. Then fertilizer may be applied a few times each year, perhaps beginning with late winter and continuing with a light application once a month through July. Do not fertilize (or prune) after the first of August, as doing so may stimulate the production of new growth that does not have time to “harden off” sufficiently before the first frosts of autumn. This tender new growth will be damaged. Not only is it an ugly sight to see through the winter, but the energy the plant expended to produce the new growth will be wasted. Those squandered carbohydrates could have gone into storage in the stems, limbs, and roots for next spring’s growth.
This storage phenomenon allows a small window of time when woody plants can benefit from fall fertilization. The timing target is to apply fertilizer to the plant before it goes into dormancy but too late for it to initiate new growth. The result is a plant with a high carbohydrate load that will help it to sustain winter cold and be poised for a great burst of growth come spring. If in doubt about the timing — and this can be difficult to predict because of annual weather fluctuation and even differences among species of plant — avoid this last fertilization target.
What fertilizer is best depends on your soil and the specific requirements of the plant species. Time-release types should be used early in the year so as not to push plants at the wrong time.
Do not assume that all plants love organic sources of fertilizer such as manure. Acid-loving plants such as blueberries and azaleas do not appreciate the alkaline qualities of manure.
Deep-root fertilization and “tree spikes” are a waste of money. The feeder roots of trees are in the top few inches of soil, and fertilizer scattered on the soil surface will deliver the nutrients to the roots quite adequately. Understory trees and shrubs may appreciate extra fertilizer and also water because they must compete with the large tree roots for moisture and nutrition.
Pruning
Pruning of woody plants is a topic that deserves an entire book, and indeed many books have been written on that subject. However, a few general rules can be useful. Before pruning any woody plant, it is best to do some research on the particular species as to how the pruning should be accomplished. Keep in mind that parts cut off cannot be put back!
Though it seems many gardeners get a yen to prune come fall, that is not a good time to prune any plant. Remember that during the growing season, woody plants have been storing carbohydrates in the limbs and stems to enhance chances of winter survival. Removing them is self-defeating, plus it opens the plant up to cold, drying winter winds. Besides, who wants to look all winter at a plant that has been cut back?
The end of winter is a good time to prune some shrubs and trees, just before they begin to put out new spring growth, especially evergreens such as boxwood or holly.
The plants that should not be pruned in fall or winter are those that produce spring flowers. Spring-flowering shrubs formed their bloom buds the previous summer, so any pruning done will remove those buds. They are said to bloom on “old” (that is, last year’s) wood. It is a good general rule to prune these shrubs just after they have finished flowering.
On the other hand, plants that produce blooms on new growth (produced in the current growing season) can be pruned just before the growing season and not be affected. Some even benefit by producing more flowers, such as landscape roses or rose of Sharon.
Again, it pays to learn about the plant before cutting. Hydrangeas are a good example of a group that must be researched well since some of them bloom on old wood and others on new. Winter pruning or cold winters that kill the flower buds are often the cause of flower loss on the classic bigleaf hydrangea. Fortunately, recent breeding work has produced many cultivars that initiate flowers on the current season’s growth.
Crape myrtles bloom on new wood, but cutting them back is not advised as it ruins the lovely sculptural form of an elegant crape myrtle in winter with its richly colored bark and sinewy trunks. Look for reliable sources of information on how to properly prune crape myrtles.
Tree topping is arguably the most appalling crime of the many committed in the name of pruning. Robbing the tree of its entire source of photosynthate (the leafy canopy) causes it to send forth thousands of weakly attached sprouts that will snap and fall in the coming years. The stubbed-off limbs left by topping cannot properly heal, so they begin to rot and allow entry of rot organisms down into the trunk.
If a limb needs to go, it should be cut off at the place where it emerged, whether that be the next large limb down, or all the way back to the trunk. It should not be cut flush with the trunk but just at the point where the bulge near the trunk has narrowed to become a limb. A tree cut at this place can compartmentalize the tissues around the wound, which develops a nice rounded callus at the cut that fills in completely over time, resealing the protective layer of bark. Do not apply any “tree wound paint” as it has been shown that it actually slows healing.
The technique of going back to the next limb or to the trunk to remove a limb is sometimes called “drop crotch pruning” (because one goes back to the lower crotch before making the cut). It works beautifully on small trees and shrubs that may have an errant limb touching the house or crowding the sidewalk. The end result is that there are no visible stubs that send out unsightly, disproportionate bristles of new growth. This method can also be used to reduce the size of overgrown shrubs while still preserving a graceful form.
If tree pruning is a large job and more than you can handle, always do your best to find a certified arborist in your area who has been trained in proper pruning techniques. Any tree pruning company that offers topping as a service is not a good candidate. Operators of businesses worth their salt would refuse to top and explain the options.
Summary
Success with woody plants begins with selecting plants adapted to the region and planting them in the landscape where their cultural needs are met. If plants that are resistant to diseases and pests are chosen, they should not require any spray programs, though borers can be problematic on some species and may warrant special measures. Implement an attentive watering regime, and take care not to inflict wounds or breakage. Mulch to cool the root zone and improve the soil. Apply fertilizer sparingly, and only after plants have become well established. Learn the optimum timing and methods for pruning those plants that need it. Be careful with broadleaf weed killers in the lawn and landscape, as they can affect or kill woody plants.
Be sure to visit the UT Gardens in Knoxville, Crossville, and Jackson, Tennessee, to see the extensive collections of plants at those sites and study how they might be used in the home landscape. Also look through the archives of the UT Gardens Plant of the Month to read about some of the best performers in these gardens.