6c - Choosing Suitable Trees for Urban and Suburban Sites COPYRIGHTED
6c - Urban Tree Selection for Diversity COPYRIGHTED
Environmental Factors that Influence Plant Selection
Overview
Title image "Team Work" by USFWS/Southeast is licensed under CC BY 2.0
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Introduction
Lesson Objectives
Identify various environmental factors that influence landscape plant selection.
Describe the influence of photoperiod, hardiness, nativity, availability, and durability on landscape plant selection.
Key Terms
hardiness - the ability of a plant to withstand cold temperatures
nativity - describes a species that has occurred naturally in a particular region, ecosystem, or habitat without human introduction
photoperiod - the daylength (or period of daily illumination from supplemental light) required for normal plant growth; many species require specific photoperiods to initiate floral development, begin cold acclimation, or overcome winter dormancy
site adaptability - the relationship between the needs of the plant and the environment and soil conditions on the property and/or designated planting area
Introduction
Nurseries are trusted by their customers to provide high-quality plant material that will thrive in the landscape for years to come. Nurseries that supply a local or regional clientele are responsible for understanding common environmental conditions their customers may be dealing with and provide trees, shrubs, and herbaceous plants that will perform well in those situations. Clients should conduct a comprehensive site evaluation to determine the environmental conditions of the site to be planted, outline any proposed site modifications, and prepare a maintenance plan that specifies irrigation, pruning, fertilization, and cleanup. Landscapers should visit similar sites in the area, explore a local public garden, park, or nature preserve for inspiration, and reach out to a local nursery grower for advice about local availability (Gilman, 2017).
Site Evaluation
A comprehensive site evaluation that takes into account the growing environment both above- and below-ground is the first step in choosing the trees, shrubs, and herbaceous plants that will thrive in a given landscape. Researching the site’s average temperatures, light availability, topography, proximity to trees and structures, soil quality, and utilities are all important considerations when selecting plant species that will perform well in the landscape. Of course, various types of plants and gardening styles have different maintenance requirements. The relationship between the needs of the plant and the environment and soil conditions on the property and/or designated planting area is known as site adaptability. Conducting a site evaluation can take some time and research, but those who make the effort will be rewarded with a more resilient landscape and knowledge can be applied to similar projects in the future (Gilman, 2017).
Temperature
Hardiness is the ability of a plant to withstand cold temperatures. The USDA plant hardiness zones are defined according to the minimum temperatures that can be expected (Figure 6.3.1). However, these ratings only take into account average minimum temperature – not early or late freezes, precipitation, soil characteristics, or heat, which are all factors in overall plant hardiness (McMahon, 2020). For example, Knoxville, Tennessee is in the same hardiness zone (7a) as Navaho County, Arizona, Philadelphia, PA, and Walla Walla, Washington. The light intensity, day length, seasonal rainfall, and plant’s abilities to store up enough carbohydrates for winter vary for each location, but each of those factors into overall cold hardiness (Raulston, 1996).
Microclimate plays a role in overall hardiness. Even the side of a city street that a plant is grown in can play a part in plant hardiness and performance, as the north-facing side may be more shaded and cooler than the other. Planting tender plants next to a warm, south-facing wall or a water feature can offer some winter protection (Raulston, 1994)
Basic maintenance activities can also impact plant hardiness. Watering and fertilizing plants late in the season signals to plants that they should delay acclimation. The result is cold damage because plants are not prepared for winter. This is true both out in the landscape and in plant nurseries. That late season push to get a plant up to the next pot size can end up costing a nursery grower another growing season for recovery (Raulston, 1994).
Plant hardiness varies depending on which plant organ is exposed to cold temperatures. Flower buds can be less cold hardy than vegetative buds. Roots are less cold hardy than shoots because they are typically insulated by soil. Container-grown plants may be less cold hardy than their in-ground counterparts because their roots are more exposed to cold (Raulston, 1994).
With rising global temperatures, heat tolerance is another important factor to consider when selecting plants. The American Horticulture Society has prepared a map similar to the USDA hardiness map which is based on a region’s normal high temperatures (Figure 6.3.2). There are twelve zones on the map listed from Zone 1, which highlights areas with fewer than one “heat day” per year, to Zone 12, which includes locations that have more than 210 heat days each year. A heat day is a day where temperatures are at least 86 degrees Fahrenheit. Some landscape managers select species that demonstrate cold hardiness in their region and are ranked in a slightly warmer heat zone.
Many species have been tested for cold hardiness, and a growing number of plants are being tested for their heat tolerance as well. However, it’s important to keep in mind that there is often ecotypic variation within species where individuals have adapted to specific environmental conditions. (An ecotype is a genetically distinct population that has adapted to its particular environment.)
To revisit our example from Unit 6, Lesson 1: Nursery Site Selection, red maple (Acer rubrum) can be found growing naturally throughout Eastern North America, as far north as Canada and south into Florida. Tennessee growers would have a difficult time growing a Canadian ecotype of red maple that is adapted to the longer photoperiod and shorter growing season common at a northerly latitude. A Canadian ecotype would come out of winter dormancy later in the season than a Tennessee variant. Conversely, a Florida ecotype of red maple may break dormancy too early in the growing season, resulting in cold damage (Raulston, 1994).
What does this practically mean to nursery growers and landscape professionals? The provenance of plant material will impact success in the landscape. To continue the earlier example, a nursery grower in Tennessee that wishes to cultivate red maple should source seed or cuttings from an ecotype that is compatible with the growing environment. However, not all species are restricted to their native range. Botanic gardens often experiment by growing species adapted to environments different than the site and are important resources to nursery growers and landscape professionals who are interested in using new plants.
Above-Ground Site Analysis
It is crucial to consider the above-ground growing environment before selecting plants. If a landscape design is being provided, it is important that the designer visit the site to evaluate elements such as light exposure, slope, wind, existing trees, utility wires, lights, and other structures before putting pen to paper. This information must be communicated to the installation crew and customer to ensure the correct species and varieties are used and that proper spacing is followed.
Light Exposure
Visit the site at various points throughout the day to map light exposure throughout the landscape. Plant species are categorized as needing full sun, partial sun, or full shade. Areas that receive 6 or more hours of sun are suitable for full sun plants. Partial sun plants will need between 3 to 6 hours of direct sunlight for healthy growth. Areas with fewer than 3 hours of direct sun are best suited for shade loving plants (Gilman, 2017).
Topography
South and west facing slopes tend to be drier than other aspects and will lose more water due to evaporation and transpiration from plants. Select drought tolerant plants, avoid trees with thin bark (such as cherries and maples), and add supplemental irrigation if needed. Northern slopes are usually more sheltered with less water lost to the environment (Gilman, 2017). However, plants grown on northern exposures may leaf out later in the growing season (Morby, 1964).
Other Trees
Trees that have the potential to develop into large shade trees usually prefer 6 hours of direct sun and need at least 3 hours of sun for healthy development. If young trees are not planted in an environment with sufficient sun, they will either bend toward the light resulting in an undesirable uneven canopy (Gilman, 2017). When light is limited due to existing large trees, many species of trees will adapt to grow extremely slowly until an opening in the canopy develops after a larger tree falls.
Utility Wires, Lights, and Structures
“Look up before you plant,” is some excellent advice from Dr. Edward Gilman and Laura Sadowski in their publication “Choosing Suitable Trees for Urban and Suburban Sites”. This may sound simple enough, but a short drive through an urban or suburban neighborhood will demonstrate that too often people do not look up before planting a tree (Figure 6.3.3). Species that have the potential to grow into large shade trees should be planted at least 20 feet away from utility wires, lights, signs, buildings, and other structures. Trees that are planted too close to utility lines are usually trimmed by utility companies, resulting in uneven and unsightly canopies (Gilman, 2017).
Below-Ground Site Analysis
Soils and other below-ground characteristics are often neglected during site analysis. Plants need healthy root systems to perform well in the landscape, and a less-than-ideal soil environment can lead to serious problems in the future.
Soil Quality
A soil’s texture (the percentage of sand, silt, and clay particles) will influence soil aeration, water holding capacity, and drainage. The acidity or alkalinity of a soil (measured as pH) will influence nutrient availability. The University of Tennessee Extension’s Soil, Plant, and Pest Center offers soil testing services that can provide growers with detailed information about a sample’s texture, pH, organic matter, and nutrient content, as well as instructions on how soil can be amended to maximize crop growth.
Compacted Soil
Soil can be compressed by heavy machinery (Figure 6.3.4) and human or animal traffic, leading to a compacted layer within the soil. Compacted soils are common in urban and suburban sites but can be found rural areas as well. Compacted soils are denser than they were prior to compaction, with less pore space for air and water, decreased drainage, and a higher bulk density (McMahon, 2020).
It is much easier to prevent soil compaction than to fix compacted soils. Limit vehicle traffic across sites, especially when soils are wet. Use equipment with continuous tracks rather than wheels whenever possible. Protect the root zones of trees in high-traffic areas by installing underground cells (McMahon, 2020). Simply spreading a layer of topsoil over a site after construction will not remedy subsoil soil compaction. Plant roots are unable to colonize and anchor into compacted soils. Large trees will develop shallow root systems, making them more prone to wind damage and hazardous (Gilman, 2017).
Underground Utilities
All underground utilities, including electric, cable, telephone, water, sewer, and gas lines, must be marked before breaking ground. Tennessee residents can call 811 to reach the state’s underground utility center. The roots of large trees can take advantage of any cracks or leaks in water and sewer lines, leading to potentially expensive repairs. Avoid planting such trees within at least 10 feet of utility lines (Gilman, 2017).
Evaluate Maintenance Practices
Not all species of plant or landscape style have the same maintenance demands. Before specifying material that will require routine irrigation, annual pruning, supplemental fertilization, and pest control, ask about the maintenance plan. More clients today are shifting from heavily manicured landscapes that demand substantial water, fertilizer, and pesticide use and are looking for environmentally sustainable, biologically diverse, and native options that will have better durability.
Unfortunately, in some areas, client choice is limited due to lack of availability of more durable selections. Online and mail order specialty nurseries have been filling the gap by producing specialty seed, plant plugs, and liners for delivery. Local nursery growers can keep up with demand by producing drought tolerant and low maintenance plants as well as species that are native to their region.
Environmentally Sustainable Choices
The most durable, resilient landscapes are the ones where plant material is tailored to the unique environment of the site. Putting the right plant in the right place will reduce maintenance demands and will lead to healthier plantings. Healthy plants are less susceptible to pest and disease pressures than stressed plants (Martinez, 2020).
Use the information gathered in the site evaluation to select species that will thrive with little or no need for supplemental irrigation or fertilizer once established. Choose drought tolerant species (such as juniper, some oaks, and elms) to grow in dry soils. Select plants that will tolerate “wet feet” (like bald cypress, river birch, or red maple) for sites that have poor drainage. Choose compact or low growing varieties for areas where visibility, wires, structures, or aesthetics would otherwise require regular pruning.
Biologically Diverse Selections
Plantings that include representatives from different plant families are usually more resilient in the face of pest and disease pressure. In the past, many designers have opted to create mass plantings of just one or a few species. While these monoculture plantings do have uniform growth and aesthetic impact, exclusively planting just one species or using primarily members of the same plant family allows pest and disease to spread easily within the landscape.
Lack of biodiversity in landscapes have led to catastrophic loss of urban tree canopy across the country. Dutch elm disease (Ophiostoma ulmi) was introduced to the United States in the 1930s. This devastating disease was spread by bark beetles and quickly decimated populations of American elm (Ulmus americana). This had widespread impacts on both street tree plantings and natural areas (Windham, 1999). The emerald ash borer (Agrilus planipennis) was accidentally introduced to the United States from Asia in 2002. By 2010, this destructive pest had spread to Tennessee through infected nursery plants, firewood, and other wood products and has quickly killed ash trees (Fraxinus spp.) growing in cultivated and natural areas through much of the eastern half of the state (Emerald, n.d.).
While individual species can be more susceptible to some pests and disease pressures, members of the same plant family are often predisposed to the same health problems. Not only are the same species commonly repeated across plantings, genetically identical clones of popular varieties are usually preferred. Studies have shown that forests that are high in genetic diversity are less vulnerable to health problems caused by pests, diseases, and climate change (Hilbert, 2020). Avoid over-use of clonal varieties and cultivars in mass plantings, aim to combine different species of plants, and research plant families to create a resilient, genetically diverse landscape (Figure 6.3.5).
Native Plants
The term nativity in the green industry can mean different things to many people. Some growers mark a species as a native plant if it is native to North America. Other nurseries reserve the term native for species that naturally occur locally to their area. According to the National Wildlife Foundation, “A plant is considered native if it has occurred naturally in a particular region, ecosystem, or habitat without human introduction.” Native species have precise relationships with endemic wildlife, and many species of animals are dependent on specific families, genera, or species of plants to complete a part of their lifecycle. Keep in mind that a plant will not necessarily thrive in any landscape within its native range. As with any other species of plant, natives should be selected based on their suitability to the site.
Dig Deeper
"Choosing Suitable Trees for Urban and Suburban Sites: Site Evaluation and Species Selection" by E.F. Gilman & L. Sadowski, University of Florida IFAS Extension. Copyright © University of Florida IFAS Extension. Used with permission.
"Urban Tree Selection for Diversity" by D.R. Hilbert, A.K. Koeser, R.J. Northrop, University of Florida IFAS Extension. Copyright © University of Florida IFAS Extension. Used with permission.
Attribution and References
Attribution
Title image "Team Work" by USFWS/Southeast is licensed under CC BY 2.0
References
Emerald ash borer. (n.d.). Protect TN Forests.org. Retrieved December 2021 from https://www.protecttnforests.org/emerald_ash_borer.html
Gilman, E.F. & Sadowski, L. (2017). Choosing suitable trees for urban and suburban sites: Site evaluation and species selection. University of Florida IFAS Extension. Retrieved June 2021 from https://edis.ifas.ufl.edu/pdf%5CEP%5CEP31000.pdf
Hilbert, D.R., Koeser, A.K. & Northrop, R.J. (2020). Urban tree selection for diversity. University of Florida IFAS Extension. Retrieved June 2021 from https://edis.ifas.ufl.edu/pdf/EP/EP58900.pdf
Martinez, A. & Williams-Woodward, J. (2020). Common landscape diseases in Georgia. University of Georgia Extension. Retrieved December 2021 from https://secure.caes.uga.edu/extension/publications/files/pdf/B%201238_8.PDF
McMahon, M. (2020). Plant science: Growth, development, and utilization of cultivated plants (Sixth edition). Pearson Education, Inc.
Raulston, J.C. & Tripp, K.E. (1994). Exploring the complexities of plant hardiness. Arnolida, 54(3): 22-31.
Windham, A.S. (1999). Diseases of shade and ornamental trees. University of Tennessee Extension. Retrieved December 2021 from https://extension.tennessee.edu/publications/Documents/SP546.pdf