What Should I Not Plant Close to a Retaining Wall?

Retaining walls often combine concrete block with a decorative facade.

Retaining walls often combine concrete block with a decorative facade.

Retaining walls should augment a garden's beauty while fulfilling their intended purpose. Proper construction holds soil in place and increases usable space while creating a specialized planting zone behind a wall. Selecting plants ill-suited for those conditions is a sure recipe for disappointment. Understanding the planting environment can help you avoid inappropriate plants. Your retaining-wall garden will be a source of enjoyment instead of regret.

Bypass Water Lovers

Retaining-wall success depends on excellent drainage. Water must move quickly and be directed away from behind the structure or pressure can cause walls to fall. Trapped water seeps into mortar and blocks, weakens the wall and compromises its integrity. To prevent this, proper design demands a drainage system that includes backfilling the area behind a wall with gravel. This creates a shallow, fast-draining planting area often subject to drought-like conditions. Avoid plants such as leopard plant (Ligularia dentata), hardy in U.S. Department of Agriculture plant hardiness zones 3 through 8, that spiral downward when deprived of consistently moist, wet soils. Splendid foliage becomes a pile of droopy leaves.

Shun Wandering Roots

Trees with aggressive, spreading root systems spell trouble behind retaining walls. The same trees that damage home foundations, septic systems and pipes cause retaining walls to tumble. Fast-growing trees such as Italian poplar (Populus nigra "Italica"), hardy in USDA zones 3 through 9, display equally vigorous activity underground. Aggressive, invasive roots seek out water, wiggle beneath retaining wall foundations and penetrate mortar crevices with finer roots. Misbehaving tree roots have no place in your retaining wall planting. They'll destroy a wall just as easily as they heave sidewalks and driveways.

Avoid Alkalinity Haters

Most modern retaining wall structures incorporate poured concrete or blocks along with drainage gravel. High in alkalinity, concrete products and limestone gravel can increase soil pH and create an inhospitable environment for acid-loving plants. In alkaline soil, these plants suffer from reduced absorption of soil nutrients and become nutrient starved. Shy away from plants such as azaleas (Rhodendron spp.), hardy in USDA zones 4 through 9, that require low pH, acidic soil. You'll avoid having chlorotic, unattractive plants and the never-ending task of trying to keep soil pH low.

Circumvent Dissatisfaction

Part of your wall's purpose lies in creating satisfying outdoor living space. Refrain from high-maintenance plants unless the wall's height makes work easier. Topiaries, for example, require consistent pruning. Close to a wall, this means constantly shifting from one level to another. Working sides from different angles can yield unpleasant results. Steer clear of plants best appreciated from above, especially if your seating area would provide a view of stems instead of flowers. Avoid thorny, prickly plants at eye-level for playing children or pets, and keep aesthetics in mind. If foliage or flower colors don't complement your wall's facade, select a different plant.

About the Author

Jolene Hansen


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