How Much Space Does My Shade Tree Need? Planting Space Recommendations for Medium and Large Trees in Florida Cities1
Trees provide urban landscapes with shade, beauty, and habitat. They can also help lessen the effects of flooding and urban heat buildup while storing carbon dioxide, a major greenhouse gas. When planted in the wrong place, however, trees can damage urban infrastructure like sidewalks. Most of a tree's roots, including the large structural roots, occur within the top two feet of soil. In urban settings, this means as the roots grow, they can displace or crack nearby sidewalks, roads, and curbs (Randrup et al. 2001). On the other hand, root-cutting activity that commonly occurs with the installation and repair of infrastructure can result in tree instability, decline in health, or even death (Benson et al. 2020). Shade trees such as live oaks (Quercus virginiana) are commonly planted in small planting areas, only to be removed a decade or so later when they begin damaging infrastructure or declining in health given limiting planting space.
Premature tree removal represents a significant economic and environmental loss. When determining the economic value of urban trees, one must consider the resources spent growing and maintaining the trees (e.g., nursery production costs, water, transportation, mulch, etc.), as well as the benefits they provide. When a tree is planted in the landscape, it can take years before its ecosystem services (e.g., carbon storage, shade, and stormwater mitigation) outweigh the earlier costs associated with production and planting. Once this point is reached, tree benefits continue to increase as the tree grows. By supporting the growth of healthy, long-lived trees, communities can save substantial amounts of money. For example, recent research on Florida's urban tree canopy has shown that the urban forest in the Tallahassee metropolitan area provides an estimated value of $300 million each year by reducing air pollution, stormwater runoff, and carbon dioxide (Mclean et al. 2020). If practitioners could adjust planting practices and provide adequate space to support tree health and survival, they could increase tree benefits, including the positive effects trees have on human health and well-being (Wolf et al. 2020).
Credit: Deborah R. Hilbert, UF/IFAS
To maximize the benefits provided by urban trees, we need better-informed tree selection and larger planting spaces with the capacity to support big-canopy trees (particularly street trees). Information is available for recommended soil volume for urban trees in planting pits, which is the amount of soil in an area needed to support a tree's root system. However, minimum soil volume recommendations do not look directly at the conflict that occurs between infrastructure and the base of the tree when root space is not limited (e.g., in the space between a road curb and a sidewalk). This article provides a new piece of the puzzle by presenting recommendations on how far a tree should be planted from infrastructure using data from field measurements. This article focuses specifically at ground-level infrastructure like sidewalks and streets, not belowground utilities.
This fact sheet is intended to help arborists, urban foresters, landscape designers, landscapers, and anyone else responsible for the planting of trees in developed areas make informed decisions regarding the planting width requirements of the trees they select.
Credit: Deborah R. Hilbert, UF/IFAS
Credit: Deborah R. Hilbert, UF/IFAS
Applying Urban Tree Research to Planting Space Recommendations
Prior research conducted by UF/IFAS scientists and colleagues looked at the size of common street trees at two different areas of the trunk: 4.5 feet off the ground (a common forestry measurement called diameter at breast height, or DBH) and ground level (a measurement called trunk flare diameter, or TFD) (Figure 4). It is tricky to measure the diameter of the trunk flare, so researchers created equations allowing users to predict TFD by plugging in the more common measurement, DBH (Hilbert et al. 2020; North et al. 2015). This research focused on medium- and large-stature trees (Table 1), but future work will look at small-stature trees such as crapemyrtle (Lagerstroemia indica) and Japanese privet (Ligustrum japonicum).
Credit: Deborah R. Hilbert, UF/IFAS
For this article, we looked at the maximum diameter at breast height of common urban trees by searching through champion tree registers. This gave us an idea of how large a species' trunk can grow in natural conditions (i.e., the maximum genetic potential of the trees). Then we plugged those DBH values into a series of equations from our abovementioned research (Hilbert et al. 2020) to calculate these species' TFDs. To determine recommended planting space, we added 4 feet of buffer to either side of the trunk flare to account for large supportive roots that (while hidden below the soil surface) could still lift or crack sidewalks (Hauer et al. 2020; Johnson & North 2016; Perry 1992). The figures below help explain this.
Credit: Deborah R. Hilbert, UF/IFAS
Credit: Deborah R. Hilbert, UF/IFAS
How to Use the Planting Space Recommendation Tables
The tree species in Table 1 and Table 2 are medium- and large-stature trees featured in the planting lists for 8 communities representing north, central, and south Florida (i.e., Fort Lauderdale, Gainesville, Miami, Minneola, Ocala, Pensacola, Tallahassee, and Tampa). Table 1 outlines species information including hardiness zones (north, central, and south Florida), status as a Florida native tree, mature tree height and spread, drought tolerance (high, moderate, and low), and the DBH from state champion tree registers.
Table 2 outlines the planting space recommendations for these species. "Planting Distance" tells you how far to plant a tree from a single road, driveway, sidewalk, etc. "Planting Width" gives the necessary width of a planting strip, such as the space between a road and sidewalk or the width of a median. See Figures 5 and 6 for help visualizing this. "Max Genetic Potential" refers to the space needed to support a tree that will grow to the diameter of the referenced champion tree. Because few trees grow to their full genetic potential, we included columns to show the planting width required to support a tree that grows to be 75%, 50%, or 25% of the maximum size listed in Table 1.
Table 3 can be used if the species you are interested in is not featured in Tables 1 and 2, or if you simply want to accommodate a tree to a given DBH (i.e., 10 in, 20 in, 30 in, or 40 in). Using this table requires some research and only applies to medium and large shade trees, not small trees. To determine the species' typical habitat, visit the USDA PLANTS database and look at its "wetland indicator" status for the Atlantic and Gulf Coastal Plain region (or whatever region your state falls within). Additionally, it may be helpful to use a champion tree register to make sure you are not selecting an expected diameter that is beyond the maximum genetic potential of the species.
Future research will need to look at how these planting space recommendations should be modified when technology like suspended sidewalks is used. We suspect the 4-foot planting buffer can be reduced if suspended sidewalks are used because large, supportive roots are situated below infrastructure in these designs. By following planting space recommendations, damage to urban infrastructure can be minimized while supporting the growth of healthy, long-lived shade trees. When planted for long-term success, mature shade trees provide ecological, financial, and environmental benefits to communities for many years.
Acknowledgments
The authors wish to thank the following people for their contributions to the research that laid the foundation for this article: Robert J. Northrop, county Extension forester, UF/IFAS Extension Hillsborough County; Eric A. North, assistant professor, University of Nebraska–Lincoln; Richard J. Hauer, professor of forestry, University of Wisconsin-Stevens Point; Michael Andreu, associate professor and Extension coordinator, UF/IFAS School of Forest Resources and Conservation; and Stephani Parbs, student researcher, University of Wisconsin–Stevens Point.
References
Background and Equations
Benson, A. R., A. K. Koeser, and J. Morgenroth. 2020. "Response of mature roadside trees to root severance treatments." Urban Forestry & Urban Greening 46 (2019): 126448.
Hauer, R. J., A. K. Koeser, S. Parbs, J. Kringer, R. Krouse, K. Ottman, R. W. Miller, D. Sivyer, N. Timilsina, and L. P. Werner. 2020. "Long-term effects and development of a tree preservation program on tree condition, survival, and growth." Landscape and Urban Planning 193 (2020): 103670.
Hilbert, D. R., E. A. North, R. J. Hauer, A. K. Koeser, D. C. McLean, R. J. Northrop, M. Andreu, and S. Parbs. 2020. "Predicting trunk flare diameter to prevent tree damage to infrastructure." Urban Forestry & Urban Greening 49 (2020): 126645.
Johnson, G., and E. North. 2016. "Total Infrastructure Planning: Mind the four R's for successful street landscapes." Arborist News 25 (3): 69.
McLean, D. C., A. K. Koeser, D. R. Hilbert, S. Landry, A. Abd-Elrahman, K. Britt, M. Lusk, M. Andreu, and R. Northrop. 2020. Florida's Urban Forest: A Valuation of Benefits. ENH1331. Gainesville: University of Florida Institute of Food and Agricultural Sciences. https://edis.ifas.ufl.edu/ep595
North, E. A., G. R. Johnson, and T. E. Burk. 2015. "Trunk flare diameter predictions as an infrastructure planning tool to reduce tree and sidewalk conflicts." Urban Forestry & Urban Greening 14 (1): 65–71.
Perry, T. O. 1992. "The ecology of tree roots and the practical significance thereof." Journal of Arboriculture 8 (8): 197–211.
Randrup, T. B., E. G. McPherson, and L. R. Costello. 2001. "A review of tree root conflicts with sidewalks, curbs, and roads." Urban Ecosystems 5 (2001): 209–225.
Wolf, K. L., S. T. Lam, J. K. McKeen, G. R. A. Richardson, M. van den Bosch, and A. C. Bardekjian. 2020. "Urban trees and human health: A scoping review." Int. J. Environ. Res. Public Health 17 (12): 4371. doi:10.3390/ijerph17124371.
Habitat Designation and Species Information
First Choice
FNPS. "Native plants for your area." Visited May 11, 2020. https://www.fnps.org/plants
UF/IFAS Environmental Horticulture Department. "Tree selection for urban and suburban landscapes." Visited May 11, 2020. https://hort.ifas.ufl.edu/woody/selection.shtml
USDA, NRCS. 2019. "The PLANTS Database." National Plant Data Team, Greensboro, NC 27401-4901 USA. Visited May 11, 2020. http://plants.usda.gov
Second Choice (used if the above three did not have the information, such as for some non-natives)
CABI. 2020. Invasive Species Compendium. Wallingford, UK: CAB International. Visited April 24, 2020. www.cabi.org/isc
Lady Bird Johnson Wildflower Center. 2020. "Native Plants of North America." Visited May 11, 2020. https://www.wildflower.org/plants-main
ISSG. 2020. "Global Invasive Species Database." Visited May 11, 2020. http://www.iucngisd.org/gisd/
IUCN. 2020. "The IUCN Red List of Threatened Species." Visited May 11, 2020. https://www.iucnredlist.org/
Nomenclature
Wunderlin, R. P., B. F. Hansen, A. R. Franck, and F. B. Essig. 2020. Atlas of Florida Plants (http://florida.plantatlas.usf.edu/). [S. M. Landry and K. N. Campbell (application development), USF Water Institute.] Tampa: Institute for Systematic Botany, University of South Florida.
Max DBH for Champion Trees from
American Forests. 2020. "The 2020 National Register of Champion Trees." Visited May 11, 2020. https://www.americanforests.org/get-involved/americas-biggest-trees/champion-trees-national-register/
Florida Department of Agriculture and Consumer Services. 2020. Champion Trees. Visited May 11, 2020. https://www.fdacs.gov/Forest-Wildfire/Our-Forests/Florida-Champion-Trees
Texas A&M Forest Service. 2020. "Big Tree Registry." Visited May 12, 2020. http://texasforestinfo.tamu.edu/BigTreeRegistry/Lists
Ward, D.B. and R.T. Ing. 1997. Big Trees: The Florida Register. The Florida Native Plant Society. 223 pp.
Tables
Species information for common medium and large native Florida trees, used to create planting space recommendations.
Planting space recommendations based on big-tree records and equations from UF/IFAS researchers. The final three sections provide the planting space recommendations for trees if they were to reach 75%, 50%, or 25% of the champion tree diameter.
