Building a Raised Bed On A Slope: How To Do It Right

Yards aren’t created equal. Some are beautifully flat and easy to work with, while others may be set on a sloped surface, either going up or down. Without major terraforming, you’re stuck working with the landscape you’ve got. Thankfully, building a raised bed on a slope is an option, and there are multiple ways to do it!

Depending on how steep the grade is where you’d like to place your raised beds, different variations may be necessary. You’ll have to do some digging and leveling no matter what, but the rewards will be worth it.

So, let’s cover all the essentials you need to know to build raised garden beds on a sloping surface. Gardening is possible no matter what your landscape looks like to start, and you’ll love being able to plant your garden every spring!

Do Raised Beds Need To Be Level?

Technically, a raised bed doesn’t absolutely need to be level. After all, plants naturally grow on slopes! However, a level raised bed can offer several important advantages.

In a level garden bed, watering becomes much easier. On a slope, the top is likely to dry out faster than the bottom, making it harder to maintain even soil moisture. A level bed helps ensure consistent hydration across the entire area.

Similarly, nutrients dissolved in water tend to flow to the lowest point on a slope. Level beds ensure that fertilizers and soil amendments are evenly distributed, benefiting all your plants equally.

Rain can also cause uneven distribution in a non-level bed—this time, affecting the growing medium itself. Without leveling, you may face significant erosion issues that can deplete your soil.

Finally, working on a level surface is much more convenient. Gardening on a slope can be challenging even under the best circumstances, so leveling your raised beds when possible will make your gardening experience far more enjoyable. Plus, your raised garden bed will perform better overall.

Reinforcing A Steep Hillside

If the hill near where you’d like to place your beds is steep, the task may seem daunting at first. With water flowing downhill and carrying dirt and debris, erosion can become a significant issue on a heavily sloped surface. Reinforcing steep hillsides will not only protect your beds from damage but also safeguard the rest of your yard from potential flooding.

The simplest method of reinforcement is to build retaining walls or install reinforcing fencing to hold back the hillside. Be sure to tackle this project during a dry period to avoid the risk of erosion while you’re still in the middle of your work!

Step 1: Create Space

You’ll need to carve into the side of the hill to create a level, stepped area for your raised beds. Be sure to leave enough space for the raised bed, your reinforcement wall, and additional room if you plan to access the side of the bed closest to the reinforcement. Expect to remove a significant amount of soil to achieve a level surface.

Step 2: Build a Base

Once the soil is cleared, you may need to remove a bit more to add leveling sand, tamping it down to create a solid base for your wall. This step is especially important if you’re building a concrete wall, as it needs a stable foundation to prevent sinking. Take the time to ensure the footing is even and secure now to avoid having to rebuild it later.

Step 3: Choose Your Materials

Once the soil is stable, you can start building your wall. The choice of material depends on the steepness of your slope. For a very steep grade, concrete blocks are the best option for stability and durability. On a shallower slope, wood may be sufficient to hold back the soil effectively.

Step 4: Build the Wall

A good concrete retaining wall is typically 24 to 28 inches thick to ensure it can handle the weight of the hillside behind it. These walls are usually constructed with two layers: one that leans inward toward the slope and a secondary outer wall that forms the visible front surface. Between the inner wall and the hillside, you’ll need to add gravel to allow excess water to drain and help stabilize the soil.

Wooden retaining walls are often much thinner but usually include a moisture barrier directly against the soil, followed by a heavily reinforced fence panel. These panels often have runners extending into the upper slope to keep the fence stationary. Adding rebar or heavy lumber posts provides additional stability.

Take the time to plan your retaining wall carefully and leave plenty of room between it and your future gardening space. Building a retaining wall can open up quite a lot of room for your future garden!

Terraced Gardens

A terraced garden can be created with or without raised beds, but adding raised structures often enhances both functionality and aesthetics.

Technically, these multi-level plots are a variation of raised gardens, as they involve leveling and stair-stepping the terrain to maximize usable space. This approach is especially effective and often essential for heavily sloped surfaces.

With a terraced garden, sturdy retaining walls are crucial to prevent erosion. Weak walls may allow soil to spill out around the edges, gradually reducing the quality and quantity of soil in the upper levels while overwhelming the lower ones. Starting with strong, well-built walls will save you trouble down the line.

Incorporating individual raised beds to contain the majority of your soil can effectively mitigate erosion in terraced gardens. You can even combine raised beds with terracing for a sleek and functional garden design.

Gardening On A Shallow Grade

A shallow sloped surface is much easier to work with and requires significantly less soil to be moved. For a vegetable garden in this type of location, you’ll only need to level the surface slightly before starting construction.

How to Build Raised Beds on a Slope

Make sure that the surface for your future beds is level, tamped down if you want it to be a bit more compacted, and that everything is out of the way. Then it’s time to construct your beds!

Steel Raised Beds

A galvanized steel garden bed, like those sold through the Epic Gardening store, is an excellent choice. These beds are incredibly easy to assemble, as they’re ready to go straight out of the box. Put together the shape and secure it with the included screws. Fixed-size bed pieces are just as easy to assemble with screws.

Once your bed is put together, ensure the bottom of the frame is flush with the ground and the top is level. If you’ve prepared the site properly, this should be straightforward. Otherwise, remove small amounts of soil to level the sides of the bed.

If you have issues with moles or gophers, place hardware cloth underneath your bed to prevent them from burrowing into your plots. Double-check that the bed is level to ensure it isn’t tilting downhill, then fill it with your chosen potting mix, and you’re ready to grow!

One final consideration with this style of bed: while the planting medium should keep the bed stable, a heavily sloped surface might cause the bed to slide forward due to hillside pressure. To prevent this, use sturdy stakes made of rebar or wood. Drive the stakes into the ground along the inside of the bed to secure it in place. Since lumber can deteriorate over time when exposed to damp potting mix, rebar or metal stakes are a more durable option for long-term use.

Wooden Raised Beds

Wooden beds on a slope are a great option, offering flexibility for a custom build rather than an out-of-the-box project. It’s easy to source wood and screws to assemble them to fit your space.

As with galvanized garden beds, it’s important to ensure your wooden bed is level and not sitting at an angle. Prepare and level the ground in advance where the bed will be placed.

Most wooden beds are simply a box made from boards cut to size. Cedar is a popular choice for its durability, though Douglas fir and other woods can also work. Avoid pine, as it deteriorates quickly when exposed to moisture—a constant in gardens!

I prefer using 2 x 6 lumber for the sides of garden beds. This size is convenient because stacking two boards gives you a bed one foot in height, making it easy to plan your desired depth and buy the appropriate amount of lumber. Most gardeners prefer beds with sides at least 12 inches deep, and 2 x 6 boards are ideal for this.

For reinforcing corners, 4 x 4 posts work well. If you want to anchor your bed securely, cut the corner posts about 12 inches longer than the height of the side panels, and bury the extra length at each corner. This keeps the frame stable and prevents shifting over time.

Once the box is constructed, set it in place and check for levelness. Adjust as needed to eliminate sloping. Like with metal beds, you can use stakes to anchor the bed if necessary. Fill it with your preferred garden mix, plant your crops, and enjoy a thriving garden in no time!