Trees for a Sloping Backyard

Trees help control erosion and runoff on slopes.

If you have a sloping backyard, choose ornamental trees carefully, as hilly sites present many gardening challenges. Runoff from heavy rainfall can cause a great deal of damage or even uproot trees. Slopes also have microclimates; the direction that trees face, amount of sun they receive and elevation all contribute to the creation of warm and cold areas that determine which species can grow. Choose trees with cultural requirements and U.S. Department of Agriculture plant hardiness zone ratings that best match your growing conditions.

Slope Gardening

Before planting, determine the site’s light exposure, soil type, pH level and annual average temperature range. Many slopes have poor soil from runoff and erosion, so amend the site with organic compost. Note the slope’s exposure; southern- and western-facing slopes usually receive more direct sunlight, creating hot, dry microclimates, while northern exposures may experience cooling winds and less sunlight. When planting, don’t insert trees directly into the hill, as roots on the upper side grow too deeply and stunt the plant’s growth. Instead, build out the planting site with soil and adjust the grade, creating a level planting surface.

To Zone 3

For cool sites, choose trees hardy to USDA zone 3. The sweet birch (Betula lenta) is hardy in USDA zones 3 through 8. This 55-foot-tall deciduous tree thrives in sun to partial shade and a range of soils, from sand to clay. It has a spreading canopy and turns yellow in fall. It grows best on cool slopes with northern exposures. The Eastern red cedar (Juniperus virginiana) is hardy in zones 2 through 9. This fast-growing evergreen reaches heights of up to 40 feet with a 20-foot spread. It has gray-green foliage and produces blue, wildlife-attracting berries. Eastern red cedars tolerate drought, heat and a variety of soils, and thrive in sunny sites. Its natural range includes slopes of varying grades and slopes with northern exposures.

To Zone 5

For USDA zone 5 slopes, which reach average lows of minus 20 F, plant willow oaks (Quercus phellos). Their natural range includes slopes adjacent to wetlands and floodplains. Hardy in zones 5 through 9, these deciduous trees grow 80 feet tall and tolerate a variety of site conditions, including compacted soil, heat, drought and standing water. Willow oaks’ deciduous foliage turns yellow in fall. The Fraser fir (Abies fraseri) reaches heights of up to 50 feet and is hardy in zones 4 through 7. This evergreen has dark needles and a dense, conical form. It prefers sun or light shade and well-draining, moist soil. In the wild, Fraser firs often grow on very steep slopes, where the windy conditions help disburse their winged seeds and the trees' roots help reduce erosion in shallow soils.

To Zone 7

USDA zone 7 temperatures reach 0 F. The Japanese alder (Alnus japonica) grows well on slopes and, in the Phillipines, is used as a reforestation species to prevent erosion on slopes. This deciduous tree grows to 25 feet tall, is hardy in zones 6 to 9, and thrives in sun to partial shade. It tolerates poor, wet and flooded soil, and is pest- and disease-resistant. The China fir (Cunninghamia lanceolata) grows slowly to 75 feet tall. An evergreen, this pyramidal tree has pendulous branches covered with blue-green needles. It tolerates sun to shade and well-drained to heavy soil. China firs are hardy in zones 7 through 9. In its native China, this tree grows on gentle slopes but doesn't tolerate steep slopes as well.

References

• Washington State University Extension; Gardening on Steep Slopes; April 2005
• Colorado State University Extension; The Science of Planting Trees; David Whiting, et al.; June 2011
• Garden Aesthetics; Design Dilemmas the Garden Slope What To Do?; Linda Engstrom
• "Garden Magazine"; The Great Terrain Robbery - Erosion Prevention in Gardening; Keith Muraoka; July 2000
• NC State University: Index of all Plant Fact Sheets - Scientific Names
• Ohio State University: Ohio Trees
• University of Wisconsin Extension: Eastern Redcedar
• NC State University: Quercos Phellos
• University of Wisconsin Extension: Fraser Fir
• Purdue University: Alnus Maritima Nutt.

Resources

• United States National Arboretum; USDA Plant Hardiness Zone Map; Henry Cathey; January 1990
• University of Minnesota Extension; Steep Slopes; y Connie C. Collison
• Western Massachusetts Master Gardener Association; Planting A Slope: Tips and Plant Suggestions; Judy Lochbrunner; Spril 2002
• Clayton-Bush Lab: Lists of Species