Planting a tree on sloped terrain is entirely possible, but it requires different techniques than planting on flat ground. Slopes introduce immediate challenges related to water flow, as rapid runoff prevents moisture from soaking into the soil, leading to insufficient hydration for the root ball. Uncontrolled runoff also significantly increases the risk of topsoil erosion, potentially exposing and damaging new roots. Special preparation of the site is therefore necessary to ensure the tree receives adequate hydration and remains stable as it establishes its root system.
Preparing the Site to Manage Water Flow
The first step in slope planting is managing the trajectory of water flow, as water naturally runs downhill, often carrying away valuable topsoil and leaving the planting site dry. To counteract this, a water-catching structure must be built before the main hole is dug. This involves creating a small, level planting shelf by excavating soil from the uphill side of the intended location. The removed soil is then used to construct a crescent-shaped ridge, known as a berm or shoulder, on the downhill side of the shelf.
This structure acts as a dam, capturing runoff and pooling water near the root ball, allowing gravity more time to pull the moisture into the soil. The uphill cut and downhill berm create a miniature basin that is level with the tree’s base, ensuring that rain or irrigation water is directed toward the root zone. The berm should be compacted to prevent it from washing away during heavy rain events, stabilizing the water retention area and maximizing infiltration.
The Specifics of Planting on a Grade
Once the water-catching shelf is established, the physical act of planting requires precision. The planting hole should be dug wide—ideally two to three times the width of the root ball—but only as deep as the root ball itself. It is important to ensure the base of the hole is level, which may mean digging slightly deeper into the uphill side of the prepared shelf. Before placing the tree, the root flare must be located, which is the slight outward curve where the trunk transitions into the roots.
The tree must be positioned so the root flare sits level with or slightly above the new soil grade of the shelf. Positioning the tree too deep is a frequent error that restricts the flow of water and nutrients, jeopardizing the tree’s long-term health. During placement, the trunk should be held vertically, perpendicular to the level horizon, and not angled to match the pitch of the slope.
Before backfilling, gently loosen or score the sides of the planting hole to encourage new root growth outward into the native soil. If the root ball is tightly bound, gently loosening the outer roots is necessary to prevent them from continuing to circle. Use the original excavated soil for backfilling around the root ball, carefully tamping it down to eliminate air pockets that could dry out the roots.
Ensuring Long-Term Stability and Soil Health
After the tree is planted and the immediate area is prepared, long-term establishment depends on maintaining soil integrity. Applying organic mulch is a necessary step, providing a 2- to 3-inch layer across the prepared shelf and inside the water-catching berm. This mulch conserves moisture by reducing evaporation and helps regulate the soil temperature, which is beneficial for root development. The mulch layer also serves to break the impact of rain, preventing surface erosion within the basin area.
Keep the mulch material several inches away from the tree’s trunk to prevent moisture retention against the bark, which can lead to disease or decay. If the tree is top-heavy or planted on a steep grade, temporary staking may be needed for the first year to prevent movement and allow the roots to gain a secure hold. Stakes should be flexible and removed after the first growing season so the trunk develops the necessary strength to stand independently.
Beyond the tree’s immediate zone, planting ground cover or companion plants around the area helps anchor the surrounding slope against erosion. The diverse and deep root systems of these plants reduce pore-water pressure by drawing moisture from the soil, contributing to the overall stability of the hillside.