PROFESSIONAL LANDSCAPE DESIGN: Erosion Control Through Plants

Erosion control is an important factor in all landscape design. Nothing destroys a landscape faster or looks worse than an eroded hillside or slope. All the effort of the designer’s and installer’s work slides right down the hill if the impact of running water is not taken into account. Preventing erosion during the life cycle of the installation and planning for long-term soil conservation should be a top priority for any landscape designer.
 
Controlling erosion is a multi-approach goal. The three main areas to review in any landscape project involving soil conservation are design considerations, vegetation selection and soil treatments.

DESIGN CONSIDERATIONS. There are several design methods to help mitigate the effect of erosion on a landscape, both during and after the landscape installation:

• Examine the site carefully prior to installation – existing “wear” patterns in the surface may indicate problem areas that need special attention.
• Preserve existing vegetation as much as possible during the construction process – if there is no need to excavate an area, don’t do it.
• Minimize the length and steepness of slopes by installing benches, terraces and retaining walls or building flow diversion structures.
• Install drainage systems to capture and channel water.
 
A careful site review  in the planning and initial construction stages will help ensure the maximum efficiency of the installation and effective long-term soil conservation.

VEGETATION SELECTION. Vegetation is one of the best defenses in the long-term fight against erosion – and the most visible. Careful plant selection can both beautify the landscape and minimize the need for artificial erosion controls.
 
Plants work in two ways to mitigate erosion. First, the foliage helps to break up raindrops before they hit the ground, lessening the impact force and reducing the volume of dislodged soil.  Second, plant root systems, particularly those with dense, fibrous roots, hold the soil in place.
 
These two characteristics can work together to form several “bands” of protection, as in a hillside planted with a number of small trees, shrubs and ornamental grasses. Each band of foliage would lessen erosive force of heavy rainfall while the progressively deeper layers of roots help protect against the impact on running surface water.
 
Fortunately, a broad number of plants can be effective in erosion mitigation. Examples of plants that work very well include:

• Crimson Pygmy Japanese Barberry (Berberis thunbergil var. atropurpurea ‘Crimson Pygmy’, USDA hardiness zones 4 to 8) – This low, dense multi-purpose thorny shrub has bright reddish purple foliage.
• Cranberry Cotoneaster (Cotoneaster apiculatus, zones 4 to 7) – This low, spreading groundcover shrub has small deep green leaves that turn bronzy-red in the fall. Small, pinkish-white flowers are followed by numerous small cranberry-red fruits in the fall.
• Compact Burning Bush (Euonymus alatus ‘Compactus’, zones 4 to 9) – This densely branched deciduous shrub is useful as a hedge or screen. The medium green leaves turn fiery red in the fall and the plant has a strong horizontal branch structure that lends interest in the winter. The dense roots system helps to lock soils in place.
• Forsythia cultivars (Forsythia cvs., zones 5 to 9) – These  spring-flowering shrubs are a mainstay of soil conservation plantings due to their suckering habit and tolerance of poor soils. Low-growing selections such as ‘Bronxiensis’ and ‘Courtasol’ are particularly effective on banks.
• Hydrangea cultivars (Hydrangea cvs, zones 4-9) – The many species and selections of this genus produce spectacular flowers throughout the summer months.  Cultivars of H. macrophyyla and the native H. quercifolia are particularly effective for erosion control.
• Blue Star Juniper (Juniperus squamata ‘Blue Star,’ zones 4 to 8) – One of the most compact and interesting varieties of low-growing and groundcover juniper for soil conservation is ‘Blue Star.’  This slow-growing, rounded evergreen has rich blue needle-like foliage.
• Winter Jasmine (Jasminium nudiflorum, zones (6 to 10) – This fast-spreading plant features arching bright green stems  that root wherever they touch ground. The small evergreen leaves provide a nice backdrop for the numerous bright yellow flowers that appear in mid- to late winter.
• Variegated Lilyturf (Liriope muscari ‘Variegata,’ zones 5 to 11) – This dense, clumping groundcover has green strap-like foliage edged with white stripes and spikes of lavender flowers in summer and fall.
• Japanese Spurge (Pachysandra terminalis, zones 4 to 8) – This evergreen groundcover  related to boxwood (Buxus) is especially effective for shady and low-light applications. Pachysandra spreads rapidly, forming a dense carpet 6 to 10 inches high with leathery deep-green leaves about 2 to 4 inches long.

SOIL TREATMENTS. Adding soil amendments, such as water-retaining polymers, can also reduce erosion by augmenting the soil’s ability to retain moisture. These high molecular weight soil additives absorb excess water and release it as soils dry, reducing the need for supplemental waterings by up to half. They also can reduce soil compaction allowing for deeper, denser roots systems.
 
Aeration also helps to increase the water-holding capacity of compacted soils.  Aerated soils allow water to percolate through rather than run off – particularly important in soils with a high clay content.

THE NEXT STEP. These are just a few strategies that make up an effective erosion mitigation plan. Your local landscape supplier can help you select the appropriate nursery products that will thrive in your specific area and assist you in accomplishing both your soil protection and beatification goals.

The authors are from Shemin, The Landscape Supply Co., Danbury, Conn. Reach them at 203/207-5000 or via www.shemin.net.