Gardening on Slopes and Hillsides

Creating a garden on a slope can present unique challenges and fun opportunities. Slopes are highly prone to erosion. The steeper the hillside, the more likely it will be to have erosion issues, especially when no plant root systems are in place to help hold the soil.  This means these areas should be planted.  

However, steep slopes have special planting considerations.  Without proper planting, the trunk flare of a tree or the crown of a perennial could easily be partially buried.  They can be difficult to navigate, making everyday garden tasks, like weeding, challenging.  They are also difficult to irrigate.  Water will run down the hill before it can soak in.  For this reason, many slopes have drier soil conditions.  

With the right strategies, you can transform a sloped area into a beautiful, thriving garden that helps with erosion control and water management. 

Understand Your Slope  |  Control Erosion  |  Choosing the Right Plants  |  Planting  |  Watering  |  Maintenance  |  Recommended Species  |  More Information

Understand Your Slope

Before you begin planting, take the time to evaluate the slope.

Gradient

The potential for erosion depends on the degree of slope and factors like soil type, vegetation cover, and the amount of surface water coming from the top, among other things. Slopes are measured by calculating their rise over run - the number of feet the hill goes up over a horizontal distance.  Slope can be expressed as a ratio (such as 1:12, a rise of one foot for every 12 feet) or as a percentage (1:12 = 8.33%).  Gentle slopes of less than 5% (1:20) generally have few special planting considerations related to slope.  Once slopes are moderately steep at around 8-16% (1:12-1:6), it is important to consider special slope planting requirements. These slopes can be difficult to walk and garden on and difficult to water as the water will be more likely to run off before it soaks in.  Slopes of 20% (1:5) or more are considered steep and require special planting considerations.  Those of ~30% (1:3) or more are too steep to mow, and plantings other than turfgrass need to be used.  A slope of 50% (1:2) is often considered the steepest slope that can support plant growth.   

Soil & Drainage

As a general rule, steep slopes are drier.  This means that utilizing drought-resistant plants may be necessary.  Observe the soil conditions on the hillside.  Do some areas tend to be wetter or drier?  For example, the soil at the base of the slope is often wetter than areas further up the hill.  Are there areas that currently have gullies or other signs of erosion?  These spaces can be modified to slow or redirect surface water to prevent erosion.  Are the soils rocky, organic, heavy, or sandy?  These are important considerations when selecting plant materials.

Growing Conditions

Be sure you know the site's growing conditions. Is the slope in sun or shade?  Is it exposed to prevailing winds or more protected?  You can reduce maintenance by establishing plants that are well-suited for the growing conditions.  This is an important consideration since accessing and navigating a hillside can be difficult. 

Current Vegetation

Often, when looking to plant a hillside, it already has some plants growing on it.  It is frequently turfgrass, and mowing on an incline is inconvenient and possibly dangerous.  Annual plants often don't help reduce erosion in winter or early spring months, and invasive plants may cause problems in nearby gardens or natural areas.  These are frequently the reasons for establishing a garden or intentional planting on a hillside.

Some or all of the existing vegetation must be cleared out before planting.  When turf is present, it can be killed but left in place to stabilize the slope. Use a non-selective herbicide with a short residual (such as glyphosate), allowing you to replant in just a few weeks.  The new plants can be planted right through the dead vegetation while the dead vegetation can help hold mulch in place while decomposing to add organic matter to the soil.  

Control Erosion

Erosion can be a significant issue on slopes. Several strategies should be considered to mitigate this issue.  

Terracing

Create flat areas on your slope using retaining walls, boulders, field stones, bricks, or wooden timbers to slow down water runoff and create planting zones.

Cover the Soil

There are multiple ways to cover the soil and prevent soil splashing and erosion.  Apply a thick layer of mulch around plants to retain moisture, suppress weeds, and protect the soil. Coarse-textured, natural mulches with angular edges work well.  Mulches like shredded bark, leaf mold, and grass clippings are less likely to wash down the hill than mulches like bark chips and river rock.  

The soil should also be covered with plants.  Plant foliage of different sizes at various heights helps scatter heavy raindrops, reducing erosion. Groundcovers can be particularly effective at covering the soil surface, but layers of woody trees and shrubs with perennials and groundcovers underneath can be even more effective.  

Reduce Surface Water Flow

Slowing water down and allowing it to soak in before it reaches the slope or while it is on the hillside is beneficial. Stagger plantings to eliminate water gathering and running down the rows, taking soil with it.  If plants need to be planted in rows, run them perpendicular to the slope.  Planting along the contour lines of the slope can help slow down water flow and reduce erosion.  

Reduce surface water flow from the top of the hill by redirecting water from driveways, downspouts, and other areas at the top of the hill. Less water running down the hill means less erosion. Excess water can be redirected by digging small trenches along the top of the slope, installing drainage tile, or even surface drains to keep excess water off the hillside.     

Choose the Right Plants

All plants for a hillside garden should meet these requirements.  A list of species well-suited for slopes can be found at the end of this article.

Well-Suited for the Site

Select plants that are well-suited for the growing conditions of your slope. Consider light (sun vs. shade) and soil conditions (wet vs. dry) and plant only those species that grow best in those conditions. Many slopes are well-drained, so selecting drought-tolerant plants is beneficial.

Low Maintenance

Utilize low-maintenance plants.  Navigating up and down a steep hill to frequently deadhead, divide, water, or prune is undesirable.  Plant only those species requiring minimal care to reduce the time you have to spend on maintenance chores.

Diverse Mix

Look to include a wide variety of species.  Not only does this increase the interest and beauty of the hillside, but it can help reduce erosion.  Plants with diverse sizes and growth habits can help scatter raindrops better, reducing the potential for surface erosion during heavy rains.  Additionally, varying root systems (large, small, fibrous, shallow, deep, etc.) help to hold soil in place better.

Planting Design

From a design perspective, slopes provide a unique opportunity.  When viewed from the bottom, it is easier to see different plants of a similar height. Despite this, like with more level garden beds, taller plants should still be placed towards the back (further up the hill) so as not to block the view of smaller plants in the front. Be careful not to select plants that are significantly taller than those in front of them, or they will appear to tower over the garden. 

When the sloped garden space is viewed from the top, the planting heights may be reversed.  Often, it is desirable to maintain the view, so planting shorter plants at the top and taller ones towards the bottom makes it less likely that views will be obstructed.

Weeping forms of trees and shrubs can be particularly attractive on slopes.  Vining and rambling plants also add a lot of movement and interest to the planting design.

Planting the Slope

Plants can be established on a slope as transplants or from seed.  Which you choose will depend on your the size of the planting area, the slope of the hillside, and your budget. 

Transplanting

Plant material can be sourced from your garden's divisions or purchased as bare root or potted plants from a garden center.  This option can have a higher cost, especially for large areas but the plants will establish quicker and cover more ground sooner than from seed. Additionally, less soil must be disturbed, as new plants can be planted through dead vegetation.  Erosion can be reduced by leaving most of the soil surface undisturbed and previous plant material killed by herbicide (and their roots) in place.

When planting potted or bare root plants, be sure to get them at the appropriate depth.  Do not bury the trunk flare of trees or the crown of perennials.  It is easy for soil to wash down and partially (or entirely) cover these things after planting.  Set the root ball of the plant level with the upper side of the hillside and then mound up the soil on the lower side to cover the root ball.  Stones, bricks, cardboard strips, or half of a pot can help hold soil in place on the lower side.  These items could remain in place long-term or be removed after a few years when the roots are fully established. With cardboard, it typically breaks down by the time the plant is established.

Alternatively, small planting terraces or tables can be created by carving out soil from the upper side of the hill and placing it on the lower side.  These planting terraces need to be 2 to 3 times wider than the root ball.  Small stones, bricks, or other natural items can prevent the soil from the upper side from washing down and covering trunk flares or plant crowns. 

On more moderate slopes, bare root plants, plugs, and other small plant material (not trees) can be planted at a slight angle so the root ball can be fully covered.  While the plant may appear to lean slightly, it will grow upward once established.  Do not plant large woody plants and trees at an angle as it takes many years for them to grow upright again.

Seed

Establishing new hillside plantings by seed can be cost-effective, making it particularly well-suited for large areas. A good seed bed needs to be created, which often requires the removal of any plant material previously established on the slope.  After seeding, cover the soil with straw or chopped cornstalks to help conserve moisture and reduce the likelihood of seeds washing down the hill when watering or during rain events.  A temporary ground cover or living mulch can be established alongside the desired species to provide a quick cover for the ground, reducing the potential for erosion. Annual cover crops like annual rye are good choices as they will quickly fill in but die as the permanent planting establishes.  

Watering

Proper watering is crucial for a successful slope garden. Irrigating newly planted seed and plants is often one of the most challenging parts of establishing hillside gardens.  Newly installed plants need regular water to establish, but water can quickly run down the hill before soaking in.  This makes adequately wetting the root zone difficult.  

Drip irrigation is an efficient way to provide consistent moisture without overwhelming the plants.  Drip irrigation, soaker hoses and other low-volume irrigation systems are the best options for watering plants on a slope.  

When drip irrigation is unavailable, water plants until excess water starts running down the slope.  Move on to the other plants and circle back to rewet the plant again.  Repeat this cycle until the root zone is thoroughly wetted.  Creating a small water basin by mounding up the soil on the lower side of the root ball can assist in holding water in the root zone long enough for it to soak in.

When watering seeded areas, consider applying water using a mist or other low-flow sprinkler.  This will prevent seeds from washing down the hill.  Turn off the irrigation once the water starts to run down the soil surface.  You may have to water in several stages, pausing between each to allow the water to soak in.

Maintenance

As with other garden areas, sloped gardens require regular maintenance.  When low-maintenance plants well-suited for the growing conditions are planted, maintenance tasks are limited, especially once the garden is fully established.  Stay on top of weeds, which can quickly take over and compete with your plants for nutrients and water. Monitor how water flows on the hillside and make adjustments to prevent erosion if gullies or runnels start to form.

Recommended Plants

Groundcovers, grasses, and native plants are good choices for many hillsides as they are typically low maintenance and well-suited for the site. Most species of trees can be grown on a hillside.  Below are several lists of plants that are well-suited for hillsides. A mix of groundcovers, grasses, herbaceous, and woody plants is the best choice for a hillside planting.

Grasses and Sedges

Herbaceous Groundcovers

Herbaceous Perennials

Woody Shrubs

More Information

• Groundcovers for Sun
• Groundcovers for Shade
• Groundcovers (publication)
• Landscape Plants for Difficult Sites
• Landscape Plants for Dry Soil Conditions
• Establishing a Lawn from Seed (publication)
• Lawn Alternatives to Turfgrass