What Should I Not Plant Close to a Retaining Wall?

Surface Roots

• Trees with large, shallow, root systems can damage a retaining wall, especially if not planted far enough away from the wall's base. While almost any mature, large tree can develop surface roots, certain species naturally have shallow root systems, according to the Purdue Extension. These include the silver maple (Acer saccharinum), a fast-growing, deciduous tree that reaches heights of 70 feet. These shade trees are very easy to grow, but their rapidly spreading roots often damage walls, sidewalks, underground pipes and septic systems. Spruce (Picea spp.) trees also tend to have surface roots. Common ornamental species include the white spruce (P. glauca), a 60-foot-tall evergreen, and the Colorado blue spruce (P. pungens), which grows to 90 feet tall. Boxelders (A. negundo) also have surface roots.

Water-Seeking Roots

• Certain tree species have water-seeking roots. These trees have vigorous, strong, root growth that can damage a retaining wall or other structures. Water-seeking species include poplar (Populus spp.), willow (Salix spp.) and elm (Ulmus spp.). The Eastern poplar (P. deltoids) quickly grows to 100 feet tall with a 75-foot spread. This deciduous tree's shallow root system makes it easy to transplant, but a risk around retaining walls. The weeping willow (S. babylonica) has invasive roots that cause frequent damage to water and sewer pipes. This deciduous tree spreads to 40 feet wide and grows to 50 feet tall. Plant water-seeking trees at least 50 feet from retaining walls.

Acid-Loving Plants

• Do not grow plants that only thrive in acidic soil near concrete retaining walls. Many concrete structures contain limestone or other alkaline materials, which leach into the surrounding soil raising the pH level. Alkaline-intolerant plants include the black spruce (P. mariana), a 40-foot-tall evergreen tree; azaleas (Rhododendron spp.), a genus of shade-loving evergreen shrubs; European white birch (Betula pendula), a 50-foot-tall shade tree; and most coniferous species, including pines (Pinus spp.), hemlocks (Tsuga app.) and firs (Abies spp.).

Rain Garden Plants

• Do not plant rain gardens behind retaining walls, as they create weight and excess moisture that causes walls to fail, according to Kansas State University. Many rain garden plants thrive in moist, wet or flooded sites, exactly the type soil that harms retaining walls. Such plants include perennials and grasses including wild blue flag iris (Iris virginia-shrevei), bottlebrush sedge (Carex hystericina), hop sedge (C. vulpinoidea), tussock sedge (C. stricta), soft rush (Junus effuses) and blazing star (Liatris pycnostachya).