Figure 1: A root system made shallow by a high water table (top) and one reaching more normal depths (bottom) (1).
Drainage - A site’s drainage can affect its compatibility with certain tree species. Most trees develop roots within the top 36 inches of soil and, short of the 36-inch mark, roots grow no deeper than a site’s water table, with the exception of some highly water-tolerant species (2). Shallow water tables encourage shallow root systems, as roots require oxygen levels that are sufficient only above the water table. Similarly, a tree in frequently waterlogged soil would likely develop its roots closer to the surface in order to access oxygen. A shallow root system may be less stable under windy or excessively wet conditions than a deeper root system in an area with better drainage and/or a deeper water table. Since drainage can have such a great effect on a tree’s overall health and stability, a percolation test may be used to determine a soil’s drainage quality. The test consists of digging a hole, filling it with water, and measuring the rate at which the water drains (1-2 inches per hour is preferable to lower rates) (3).
Soils – It is important to consider the soil compaction and soil type of a given site as well. Most tree roots do best in soil that is non-compacted, or relatively loose and penetrable (4). This is somewhat tied to drainage, as soils that drain well tend to be less compacted, and both effect the levels of available oxygen. If the site receives a great deal of traffic or recently underwent construction, the soil may be compacted, which could inhibit strong root growth of a new tree. Soil types, with varying levels of nutrients and aeration, can also influence a tree’s health. Clay soils are rich in organic matter and nutrients, but are prone to dense compaction and poor drainage, whereas sandy soils, though lacking in nutrients, typically remain loose, well aerated, and well drained (5). In many cases, a mixture of multiple soil types is desirable.
Figure 2: The crown of this tree has been bisected in order to allow power lines to pass through (8).
Space – Available space, both below and above ground, can be an important factor when choosing a site and a tree that will work well together. Proximity of the planting location to other trees/shrubs, structures (buildings, houses), and infrastructure (sidewalks, roads, power lines) may help determine the best species for a site (6). Generally, the wider and taller the species above ground, the more belowground space it should have—this allows the tree to develop a root spread appropriate for supporting its canopy (7). If overhead power lines are present, a tree species with an expected height that does not encroach upon the power line is desirable, lest it be disfigured to avoid contact [figure 2].
Climate – The regular weather conditions of a site—rainfall, sun exposure, temperature, wind, humidity—all factor into a tree’s health. Considering a region’s hardiness zone (a measurement of minimum winter temperatures) can be a good starting point for picking a species for a given site, though it should not be the sole consideration (9). For example, a citrus tree that prefers zones 7 or 8 probably would not survive in zones 3 or 4 in a natural setting, and a balsam fir would find zone 8 too hot and dry to thrive. Climate can, of course, vary within single zones; this is where factors like drainage and soil type become more important, as well as light and water requirements and cold-hardiness. See USDA’s website for a map of zones: http://planthardiness.ars.usda.gov/PHZMWeb/#. Species Selection – Ultimately, the chosen species should be compatible with the specific conditions of the site. Expected height, canopy and root spread, rate of growth, life span, cold-hardiness, drought tolerance, disease resistance, and general aesthetic can all be important variables to consider when seeking a specific species. Some species have desirable attributes, like fragrant flowers or bright fall colors, while others have potentially undesirable ones, like excessive seed drop or foul smelling fruit. The University of Minnesota offers a comprehensive guide for selecting appropriate tree species: http://www.myminnesotawoods.umn.edu/2008/11/recommended-trees-for-minnesota-by-region/.
Figure 3: The tree to the left may develop adventitious roots and decay if left buried by mulch (13).
Soils – If soil quality negatively affects a site’s compatibility with trees, modifications can sometimes be beneficial. Before making any soil modifications, however, a site’s soil should be tested to identify its specific needs. Soil testing is available through the University of Minnesota: http://soiltest.cfans.umn.edu. Once the soil’s specific needs are identified and researched, altering the soil structure/texture can be beneficial for potential transplants, particularly when soil drains poorly or is densely compacted. Mixing in soil conditioners could improve a soil’s texture and overall compatibility with trees. There are several different types to consider, including organic, synthetic, and mineral conditioners (10). Adding conditioners could result in decreased soil compaction, improved drainage, and greater accessibility of organic compounds and nutrients, ultimately resulting in stronger, more stable root systems. Preventative measures like keeping heavy vehicles/machinery off the soil will aid in reducing future compaction.
Drainage – Poor drainage is very problematic for many trees because excess water can deny roots their essential oxygen supply for extended periods of time. Other potential negative effects of poorly drained sites include decreased stability and higher risk of root decay. As drainage is sometimes connected to soil compaction, measures to reduce soil compaction may prove beneficial for soil drainage as well. Adding organic matter, controlling the amount/frequency of irrigation (when possible), mixing in mineral or stony materials of different granular sizes (gap-graded materials), planting cover crops, or constructing a French drain trench may all help to ameliorate a site’s drainage (11). It is worth noting that, when adding organic matter, the material must be applied evenly over a large area to be effective, as opposed to just the immediate planting area (12).
Tree – Unforeseen problems can arise well after a tree has been transplanted. Tree size is a common cause of many conflicts with nearby infrastructure, buildings, and other plants. It is important to thoroughly research prospective tree species both before and after planting in order to recognize potential issues or maintenance needs later on. Early, regular pruning can be beneficial for maintaining size and shape, removing weak branch unions, and avoiding future conflicts. In some situations, mulching may also be appropriate and helpful. Particularly on sites with little precipitation, fast drainage, or extreme summer and winter conditions, mulching would help to retain moisture and insulate roots from extreme temperatures. It may also help protect the tree from mechanical damage, such as accidental encounters with lawn mowers and other garden instruments. It is imperative to note, however, that too much mulch (more than 2-4 inches) or mulch burying the stem of a tree could lead to issues like excess moisture, root and stem decay, development of stem girdling roots, and even toxicity (14). To prevent these problems, it is advisable to lay mulch with a slight downward slope near the stem to avoid stem-to-mulch contact [figure 3].
Figure 4: All of these boulevard trees are ash trees, making this neighborhood vulnerable to species-specific disease and pests (15).
Selecting an appropriate species – Diversity among plant species in a given area is integral to the health and stability of the local ecosystem. Homogenously planted areas are at greater risk for extensive damage from pests and disease. It is worthwhile to consider the existing plant demographics of surrounding areas in the selection process. Ideally, the species would be compatible with the site, non-invasive, and a different species than the majority of nearby trees (assuming only a few species dominate the area). For example, if an area consists of mostly ash trees, a compatible tree that is not an ash species—perhaps a species of oak—would be a suitable, responsible choice that diversifies the area and potentially reduces its susceptibility to collapse from emerald ash borer.
Homeowner and municipal considerations to species selection – As described above, it is in everyone’s best interest to promote diversity in a given ecosystem. There are several proposed “rules” to consider when selecting species to plant. The 10% Rule, for example, suggests a single species should make up no more than ten percent of a given population/community (16). Another more extreme rule proposes no more than 5% of a community consist of trees of a single genus (17). Other considerations may include the size, longevity, susceptibility to disease, undesirable features, and general aesthetic of a species, keeping in mind nearby structures, houses, infrastructure, and neighbors. For instance, planting a horse-chestnut tree next to a high-traffic bike route may prove problematic as the fruit are large, barbed, and could potentially damage bicycles or cause injury.
Figure 5: This tree’s trunk flare and lateral branch roots have lifted this section of sidewalk. The least harmful solution would be to move the sidewalk, rather than prune the tree’s roots (22).
Schedule of maintenance – Trees should be watered immediately after transplanting. Regular watering is important during the first few growing seasons as the tree expends energy to establish its roots in the new landscape. Colorado State University offers a helpful watering guide for newly planted trees based on size and need in Table 1 on its website: http://www.ext.colostate.edu/mg/Gardennotes/635.html (18). Young trees may also require some formative pruning in order to meet desired safety and aesthetic standards. Pruning can aid in maintaining clear sightlines, lessening height conflicts (pedestrians, vehicles, houses, infrastructure), promoting good tree architecture and preventing failure of weak branch unions and other potential problems. The University of Minnesota provides helpful information on why, when, and how to prune landscape trees: http://www.extension.umn.edu/garden/yard-garden/trees-shrubs/pruning-trees-shrubs/#planting-time (19).
Longevity of the site – All sites will likely undergo some changes during a tree’s lifetime. Ideally, the tree would be tolerant of these changes and not develop any serious issues as a result. Moving forward, factors like soil erosion, water & light availability, development/construction, other plants, disease/pests, people & animals, and climate change could all potentially affect the regular conditions of a site. Some of these are somewhat controllable, particularly pertaining to soil and water. Depending on the cause(s) of soil erosion, managing water runoff, promoting good drainage, implementing windbreaks, and planting other trees/shrubs may also help to lessen erosion (20). If water availability changes, implementing or amending a watering regimen could also help reduce adverse reactions. Above all, however, thorough research of a given species’ needs is the best tool in prevention and maintenance.
Trees and infrastructure – Infrastructure considerations tie in with tree species and maintenance as mature tree height, width, and root spread all factor into a tree’s success in a given site. Common conflicts involve the canopy encroaching on power lines, the trunk flare/large roots damaging curbs/sidewalks/driveways, and too little belowground space to properly support the canopy. Consumers Energy has some very useful guidelines and suggestions for avoiding conflicts with power lines: https://www.consumersenergy.com/content.aspx?id=1592 (21). The issues of appropriate planting space and damage to curbs/sidewalks are often closely related, resulting from a tree’s trunk and roots outgrowing its allocated space. The best method to combat this is prevention, specifically, planting large trees only in large enough spaces (larger than an 8-ft wide boulevard) and instead planting smaller trees and shrubs in the smaller spaces. The Morton Arboretum provides some information about root issues, treatments, and alternatives: http://www.mortonarb.org/trees-plants/tree-and-plant-advice/horticulture-care/tree-root-problems (23). The common practice of root pruning, however, is not advisable as it can debilitate a tree’s stability, particularly in extreme weather events. Instead, leveling the sidewalk or simply diverting it away from the tree would be preferable alternatives. Allowing the root system enough space to properly establish itself is beneficial not only to infrastructure, but also to the tree’s stability and, subsequently, public safety. Trees are long-lived and can add great value and beauty to a landscape when allowed enough space to properly establish themselves.
Lake Harriet, MN: Head to the Lakes! Rethlefsen, Melissa. http://npc.mlanet.org. [Online] 2011. http://npc.mlanet.org/mla11/?tag=lake-harriet New York, NY: Trees may play a big part in the urban healthcare of the future. John Benjamin. Urbanful. [Online] https://urbanful.org/2014/08/19/trees-may-be-the-urban-healthcare-of-the-future/