Eric Berg Nebraska Forest Service Bob Henrickson Nebraska Statewide Arboretum

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Eric Berg, Nebraska Forest Service Bob Henrickson, Nebraska Statewide Arboretum It’s hard to overemphasize the importance of a tree’s root system in maintaining a healthy tree and, ultimately, a healthy community forest. At the simplest level, the main functions of tree roots are to support the tree and to take up the water and minerals essential to overall plant vigor and health. Probably because they are unseen, there are many misconceptions and myths about this invisible, underappreciated and essential part of the tree. Myth: Tree roots are tough and woody. Reality: The majority of root biomass is made up of very fine feeder roots, typically averaging only 1/16” in diameter, which are responsible for water and mineral uptake. Structural roots located near the trunk help support the tree and are typically large and strong, but feeder roots represent the major portion of the root system’s surface area. The root system of a tree can be thought of as a permanent, structural woody frame with “disposable” fine roots that may last for only one season due to poor soil growing conditions, drought or freezing temperatures. In the urban environment, these roots are also very susceptible to compaction, overwatering, construction damage, water pollution and soil contamination. Trees can lose as much as 95 percent of their root system in being transplanted and therefore spend a majority of energy reserves to re-establish them.Myth: Tree roots grow deep into the soil. Reality: The majority of tree roots are found in the top 18” of soil, and typically over half of a tree’s roots are in the top 6” of soil. Tree roots require oxygen for survival. They will occupy and grow in areas favorable for survival, with good aeration and available moisture and nutrients. These types of conditions are easily created with proper mulching. While there are many instances where a given species might send roots deep into the soil, the vast majority are typically just below the soil surface—deep enough for reliable moisture, yet shallow enough for good oxygen levels. Poor soil aeration, typical of compacted soils, is one of the most limiting factors for root growth in the urban environment. When compaction is coupled with over-watering, there is very little pore space left for oxygen and, as a result, roots slowly suffocate and die. Myth: Tree roots exist only under the tree canopy. Reality: Tree roots typically have very extensive root systems growing well beyond the canopy of the crown. In forested or natural environments, roots may be found growing two to four times beyond the diameter of the tree canopy, and with drought-tolerant species such as oak, the spread can be up to six times the canopy width. Roots do not mirror what you see aboveground. To visualize this, imagine a tree represented as a wine glass connected to a wide flat plate. The glass and stem represent the canopy and trunk and the plate represents the root system extending well beyond the canopy of the tree. This is very important to consider when dealing with potential construction damage or The Truth about Roots chemical applications. Soil sterilants and materials containing Dicamba for broadleaf control should not be used around trees as these chemicals are persistent and active in the soil and will severely damage tree root systems over time. Healthy root systems result in healthy trees, whether they’re in your backyard, in your community or in natural forested areas. By better understanding some of the basic biological realities of tree roots, you can better maintain and protect them. So the next time you do construction work, apply herbicide or drive across that seemingly wide expanse of fescue, stop and consider the trees around you and the fragile but critical roots beneath your feet.Top: The major portion of a tree’s root system is made up of small feeder roots. Because they are fine-textured and close to the surface, they are easily damaged by compaction, construction damage and chemical applications. Bottom: Circling and girdled roots are a common problem in container-grown trees. (Photos courtesy of Eric Berg)Growing trees in Nebraska can be a risky proposition. If wind, hail, ice storms, tornadoes, droughts, hot summers and cold winters don’t kill a tree, insects, diseases and people often seem ready to finish the job. Thank goodness for hackberry (Celtis occidentalis). This tough-as-nails cousin to the elm has proven its worth in yards, parks, farmsteads and shelterbelts across the state for generations. Hackberry possesses many positive attributes that make a good tree for the Great Plains. Its broad, arching growth habit makes it an ideal species for street side plantings; its tough character makes it very useful in parks and other public spaces; its distinctive warty bark and clean branching gives it a unique natural character; it is native to the region and thus uniquely acclimated to the Great Plains environment; it suffers from few disease or insect problems; it tolerates both wet and dry soils; and its fruits are prized by cedar waxwings and other colorful birds. Hackberry is not perfect. It has been over-planted in some communities and it tends to break up in wind and ice storms when not properly pruned and cared for. It can also be very opportunistic (nice word for “weedy”). Birds are able to spread its seeds far and wide and seedlings are often found growing in alleys, An Ode to Hackberry woodlots and other out-of-the-way places within a community. This opportunistic nature, however, has one good advantage –free trees are available for those willing to transplant them. Indeed, hackberry is hard to find in nurseries, so digging a seedling is sometimes the best option. The important message here is not to promote the widespread planting of many more hackberries. Rather, the point is to encourage us all to appreciate the many hackberries we already have, work hard to maintain them properly, and when possible, assist in their renewal by planting a few here and there so that they will be in our communities for years to come. As I sit and watch the cedar waxwings enjoy a late winter meal of hackberry seeds, on the tree that will almost fully shade my neighbor’s backyard in the coming months, I can’t help but feel very thankful for a species that gives so much while asking for so little in return Pine wilt disease and the potential threat of emerald ash borer have many Nebraskans concerned about the future of some of the state’s most popular trees. Here are some fast facts about these threats. For more information, go to: Pine Wilt Pine wilt has already killed thousands of otherwise healthy Scotch pine in southeast Nebraska and also Pine Wilt Disease and Emerald Ash Borer has begun to kill Austrian pines. The disease is advancing north and west across state and shows no signs of slowing down. Scotch pine and Austrian pine have been the two most common pines planted in the state, especially in shelterbelts, over the last 50 years. The potential loss from this pest is significant and could eventually total millions of trees. As such, Scotch pine is no longer recommended for planting throughout most of the state. Susceptible trees: Scotch pine is highly susceptible and Austrian pine somewhat susceptible. Symptoms: Faded, gray-green needles eventually turn brown. Trees die rapidly, sometimes branchby-branch, usually between late summer and early spring. Cause: Pinewood nematode attacks tissues in the wood, decreasing water flow and eventually killing the tree. The nematodes are carried to other trees by pine sawyer beetles, which tunnel in the wood of dying pines (including those dying from pine wilt). Emerging beetles can carry thousands of nematodes on their bodies. As they feed on new shoots of healthy pines, they create wounds through which nematodes infect the tree. Control: Diseased trees cannot be saved and should be destroyed (burned, buried or chipped before beetles emerge from the wood) to prevent spread to healthy trees. Highly valuable trees can be protected with a trunk injection of abamectin, available from certified arborists. Emerald Ash Borer Over the last few years, the emerald ash borer (EAB) has killed millions of native ash trees in Michigan, Ohio and Indiana and it has the potential to kill most of the ash trees in the eastern U.S. Many experts believe it is likely that the insect will reach Nebraska (probably on firewood) within the next several years. The potential impact is enormous since green ash is one of the most common trees found throughout Nebraska in communities, on farms and in native woodlands. Because of the threat of EAB, native ash species including green ash (Fraxinus pennsylvanica), white ash (F. pennsylvanica), blue ash (F. quadrangulata) and black ash (F. nigra) are no longer recommended for planting in the eastern half of Nebraska, and should be used only with caution and in limited numbers in the western part of the state. This recommendation includes all of the common native ash cultivars such as ‘Patmore’ and ‘Marshall’s Seedless’ green ash as well as ‘Autumn Purple’ white ash. Susceptible trees: All North American ash (Fraxinus) species and cultivars are susceptible (not mountain-ash or Sorbus, which is not a true ash). Symptoms: Exit holes in ash bark about 1/8” in diameter and “D-shaped”; zigzag tunnels under the bark; tunnels packed with sawdust; dieback Even a quick look at trees growing in an urban environment makes it easy to see the stresses they have to endure. We plant them in pits, parking islands, compacted construction sites and basement sub-soils and expect them to perform. We forget that Nebraska is a plains state where prairie once ruled and where trees were often found only along streams and rivers. The plains climate has always been tough on trees, with damage from wind, ice, heat, drought, insects and diseases, to name just a few. We are familiar with these problems, but we forget that these stresses are even more magnified in the urban environment. Arborists and other tree professionals often consider only the top of the tree—leaves, branches and trunk—in diagnosing problems. The rootstock or root system, often the most important part of the tree, is more difficult to examine. After planting and caring for trees over the years, I’m constantly reminded that, for the health and performance of a tree, “it’s all in the roots.” If roots are stressed from poorly drained, oxygen-deprived soils or from compacted or contaminated soil, the tree will be more vulnerable to insects, diseases and other problems. While poor soil or planting depth is an obvious cause of problems, there is another stress that is rarely considered. According to John Ball of South Dakota State University, “many of our urban trees suffer from a split personality… they’re really two trees, the trunk and canopy is one species, the roots another.” Named cultivars or clones are a group of cultivated plants distinguished from other plants of the same species by a Trees with a Split Personality characteristic retained in propagation. Many tree cultivars are propagated by a grafting technique called budding, in which a single bud taken from the desired cultivar is nestled within a fold made in the lower stem of a seedling tree. The initial sideward growth from this single bud is what causes the slight crook at the base of the stem. The top of the tree, referred to as the scion, originates from the single bud taken from a specific tree or cultivar. The bottom, called the rootstock or understock, is beneath the scion and consists of the lower few inches of the stem (where the bud is grafted) and the entire root system. In the past, many shade tree cultivars were selected for desirable growth habit or fall color, with little regard for drought- or heat-tolerance or adaptability to various soil types. Fortunately, today’s cultivars are selected not only for shape, size and fall color, but also for how well they tolerate urban growing conditions. Two common cultivars in the urban landscape include Autumn Purple white ash (Fraxinus Americana ‘Autumn Purple’), selected from a single specimen in Ohio known for its outstanding deep purple fall color and ‘October Glory’ red maple (Acer rubrum ‘October Glory’) selected from a tree with crimson red fall color. When the scion wood is collected from these trees we know where the trees come from, but the origin of the lower portion of the tree or rootstock is usually a mystery. Very rarely are we aware of the origin of the rootstock (and yes, that does matter when you consider that red maples grow naturally from northern Minnesota to Florida). Most white ash cultivars, including ‘Autumn Purple’, are budded onto green ash rootstock. Green ash is commonly used as an understock for many ash species and for this reason has been dubbed “the universal donor.” Though grafting has enabled us to select cultivars with superior ornamental qualities, these trees lack the full genetic representation of a species, including its ability to overcome a wide variety of stress factors. One of the questions we need to ask is “Will these trees be prone to failure before they reach maturity?” There are well-documented cases of graft incompatibility, even when the scion and the rootstock are of the same species. In the 1970s, many red maples grown from budded trees began to decline for no apparent reason. Researchers determined that the problem was delayed graft incompatibility, i.e. the failure of the graft does not occur until five to 10 years after budding, when the tree is already established in the landscape. Maples were showing premature fall color, dieback and shedding bark; many were dying. Because of this, nursery growers began developing maple cultivars on their own roots via cuttings or tissue culture rather than grafting. Ornamental trees endure a lot of unique, stressful situations in the urban landscape. Their “split personality” may not be the primary stress they have to overcome, but it is a problem not shared by their forest cousins. To truly diversify the urban community forest, we need to consider planting more trees that are produced from seed. By simply planting one seed-grown red oak or Kentucky coffeetree for every ‘Autumn Blaze’ maple or ‘Skyline’ honeylocust, we can make a good start toward diversity and sustainability in our community landscapes tordas