Planting on a hillside presents unique challenges, primarily due to the constant threat of soil erosion, rapid water runoff, and difficult access. Gravity pulls soil, mulch, and water downhill before they can benefit the plants. Addressing a sloped landscape requires specialized planning to ensure water infiltrates the soil rather than accelerating down the surface. Successfully planting on a hill transforms an unstable area into a self-sustaining landscape that actively prevents further land degradation.
Site Preparation and Stabilization
Successful hillside planting requires strategic preparation to stabilize the ground. The first step involves assessing the slope’s steepness, as vegetation alone is most effective on grades less than 30 degrees (a 50% slope). Steeper slopes often require structural intervention, such as terracing or retaining walls, to create level planting areas and slow water velocity.
Modifying the soil is necessary to maximize its ability to absorb water and nutrients. Incorporating organic matter, such as compost or peat moss, into the top 8 to 12 inches of soil significantly increases its water-holding capacity, especially in sandy or coarse soils. Adding just 1% of organic matter can allow the soil to store thousands of gallons of additional water. This preparation transforms the soil into one that retains water, which is paramount for plant survival on a slope.
To provide immediate, temporary stability, secure erosion control materials to the prepared soil surface. Biodegradable products like erosion control blankets, netting, or coir logs help hold the soil and newly planted seeds or small plants in place. These mats protect the ground from the impact of raindrops, which dislodge soil particles, and slow surface water flow until the plants establish root systems.
Strategic Plant Selection
Hillside plants must possess specific physical characteristics that enable them to anchor the soil effectively. The primary criterion is a deep, dense, and fibrous root system capable of binding soil particles and strengthening the ground structure. Plants with extensive underground networks, like native grasses such as switchgrass and little bluestem, are superior to those with shallow roots for long-term erosion control.
A layered approach using a diverse mix of plant types offers the best stabilization and ecological benefits. Low-growing groundcovers, like creeping phlox or vinca, create a dense mat that shields the soil surface from the force of rain and runoff. Shrubs and trees, such as dogwood or sumac, add structural stability with deeper, woody root systems that penetrate the slope. Selecting native species is recommended because they are adapted to local soil conditions and moisture availability, requiring less maintenance once established.
Slope conditions favor plants tolerant of both drought and intense sun exposure. Sloped areas tend to dry out quickly because gravity pulls water away and constant exposure to sun and wind accelerates evaporation. Choosing hardy, drought-tolerant varieties ensures the plants survive the initial establishment phase and thrive through periods of low rainfall.
Installation Techniques for Slopes
Planting on a hillside requires specific techniques to counteract gravity and manage water flow. When excavating the planting hole, dig a small, level pocket or terrace to allow water to collect and soak in, rather than run off immediately. The excavated soil can then be used to form a small, crescent-shaped berm or lip on the downhill side of the plant.
Angle the plant slightly upward, or set the root crown level with the uphill side of the hole, to ensure the plant does not get buried by soil wash-down. This slight uphill tilt helps the crown, where the stem meets the roots, remain exposed, preventing rot. Proper spacing is important, as dense planting encourages quick ground coverage, which is the most effective long-term erosion solution.
After placing the plant, gently tamp the soil to ensure good contact between the roots and the surrounding soil without excessive compaction. Creating a small water basin around the plant helps direct irrigation water directly to the root zone before backfilling with the downhill berm. For very steep slopes, use organic materials like rocks or small logs on the downhill side of the newly planted area as a temporary mini-retaining wall until the roots take hold.
Water Management and Ongoing Care
Efficient water delivery is a major challenge on a slope, as traditional sprinklers lead to significant runoff before water can penetrate the soil. Drip irrigation or soaker hoses are the preferred solution, applying water slowly and directly to the plant base, minimizing waste and preventing surface erosion. The system should use the “cycle and soak” method, which involves short, repeated watering cycles separated by rest periods to allow water to fully absorb.
Mulching is necessary to retain soil moisture and protect the surface from erosion, but specific materials and application methods are required for slopes. Coarse, chunky mulches, such as shredded bark or wood chips, are less likely to slide down the hill than fine-textured materials. Applying a layer of 2 to 4 inches is recommended, but on steeper slopes, biodegradable netting or erosion control fabric may be used to secure the mulch in place immediately after planting.
Initial monitoring of new plantings is necessary to ensure successful establishment. The first two seasons are when plants are most vulnerable, and deep, infrequent watering encourages the development of strong, drought-tolerant root systems. Long-term care focuses on preventing water channels from forming and ensuring the dense plant cover remains intact to provide continuous stabilization for the hillside.