A raised garden bed is a self-contained structure placed on top of the existing landscape, typically constructed from wood, metal, or stone. The primary benefit of using a raised bed is gaining control over soil quality and drainage, but this advantage is compromised if the bed is not installed on a level surface. A level base is important for maintaining the structural integrity of the frame, which must withstand the considerable outward pressure of wet soil. Furthermore, an uneven bed causes water to pool on the downhill side while the uphill side dries out quickly, leading to uneven moisture distribution and poor plant health. Preventing soil erosion and ensuring all plants receive a uniform share of nutrients are the main reasons to ensure the base is flat before construction begins.
Initial Site Assessment and Preparation
Before moving any earth, the intended site must be thoroughly cleared of obstructions. This preparation includes removing all existing vegetation, such as sod and weeds, along with any rocks, roots, or large debris that could interfere with leveling. A clear area allows for an accurate assessment of the ground’s topography.
To determine the lay of the land, you should mark the perimeter of the future raised bed using stakes hammered into the four corners. String or twine is then tied between the stakes to establish the outer boundary of the bed. A line level, which hangs directly on the string, is used to adjust the string until it is perfectly horizontal around the entire perimeter.
This level string line acts as a reference point to measure the slope and identify the highest and lowest points within the planned footprint. By measuring the distance from the taut, level string down to the ground at various points, you can calculate the total difference in elevation across the area. This difference is the measurement that dictates the method needed for leveling, and whether you will need to dig down or build up the base.
Techniques for Achieving a Level Base
For areas with only minor unevenness, meaning a difference in height of less than six inches across the length of the bed, the ground can be leveled using simple cut-and-fill techniques. The process begins by removing soil from the highest points, often using a shovel or a mattock for precision digging. This excavated soil is then systematically moved to fill in the lower depressions and dips within the defined perimeter.
A long spirit level or a straight edge, such as a long board or screed, is placed across the bed’s footprint to check for flatness as you work. This tool should be used both diagonally and across the width and length of the area to identify subtle slopes. Once the soil is redistributed, it is important to compact the base lightly using a hand tamper or the back of a shovel.
This slight compaction of the native soil helps prevent future settling of the frame once the bed is filled with growing medium. If the native soil is heavy clay, waiting until the ground is workable and not overly wet will make the leveling and compacting process much easier. This simple movement of soil ensures a stable and uniform foundation beneath the bed frame.
Addressing Significant Slopes or Uneven Terrain
When the terrain features a significant slope, generally greater than six to ten inches of elevation difference end-to-end, simple redistribution of soil is often insufficient. In these situations, the gardener must choose between two main approaches: extensive excavation or building a stepped foundation. Extensive excavation involves digging down the entire uphill side of the bed until the base is level with the lowest point.
Alternatively, especially on very steep grades, a terracing approach is often employed, where multiple beds are stepped up the slope. This method involves building a retaining barrier or a partial frame that is buried into the hill on the uphill side and extended significantly on the downhill side. The frame itself is constructed to be level, allowing the soil to be retained against the natural slope of the land.
If extensive filling is necessary to raise the low side, it is advisable to use crushed stone or gravel beneath the bed to create a stable, well-draining sub-base before adding soil. This practice prevents the frame from sinking over time due to the weight of the water-saturated soil. Regardless of the method, the goal is to ensure the top edges of the bed frame, not the surrounding ground, establish a perfectly horizontal plane.
Final Placement and Stabilization
With the ground prepared and level, the next step is placing the raised bed frame onto the stabilized base. Once the frame is positioned, a final check with a spirit level across all four sides and diagonally is necessary to confirm the level surface has not shifted during placement. This verification ensures the frame is not twisted or resting unevenly, which could lead to structural strain when filled.
To secure the frame against the outward pressure of the soil and prevent shifting, especially on larger beds, stakes or rebar should be driven into the ground at the interior corners. Utilizing twelve-inch pieces of rebar hammered through the frame and into the ground provides exceptional anchoring, which is particularly useful for beds on slopes that function as partial retaining walls.
Before the bed is filled with the growing medium, a layer of weed barrier fabric or hardware cloth is often placed on the ground inside the frame. While a weed barrier prevents unwanted vegetation from growing up into the bed, a layer of hardware cloth, typically galvanized mesh, offers a physical deterrent to burrowing pests like gophers or voles. These final steps prepare the bed for its purpose, ensuring longevity and protection against common gardening challenges.