Yards aren’t created equal. Some are beautifully flat and easy to work with, but others may be set on a sloped surface going either up or down. Without doing 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 you do, but the rewards will be worth it.
So let’s cover all the essentials you’ll need to know to build your 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 speaking, a raised bed does not absolutely need to be level. Plants grow on slopes naturally, after all! But a level raised bed can help with many different things.
In a level garden bed, watering will be easier. Applying water on a slope means the top is more likely to dry out before the bottom of the slope does. You want your bed to have an even distribution of soil moisture, so a level bed is best for that.
Similarly, nutrients dissolved in the water will flow to the lowest point. Having level beds ensures the even distribution of fertilizers and other amendments you add to your soil.
As rain hits a non-level raised bed, another thing can be unevenly distributed: your growing medium itself. You can end up with some rather severe erosion issues if you don’t level off your beds.
Finally, it’s a lot easier to work on a level surface. Gardening on a slope can be a bit of a challenge in the best of times, so when you have the option of leveling things out, you should take it! Your raised garden bed will perform better overall.
Reinforcing A Steep Hillside
If your hill near where you’d like your beds to be is a steep slope, it can seem daunting to get everything set into motion. Since water flows downhill and carries dirt and debris with it, erosion could be a major factor with a heavily-sloped surface. Reinforcing your steeper hillsides will help you to protect your beds from damage as well as protecting the rest of your yard from flooding.
The easiest method of reinforcement is by building retaining walls or reinforcing fencing to hold back the hillside. This should be done during a time when rain is not likely to interrupt your work, as that way you don’t run the risk of erosion while halfway through your project!
You’ll need to carve out the side of the hill to make a level, stepped location for your raised beds. Leave enough room for both the raised bed and your intended reinforcement wall, plus additional room if you want to access the side of the bed closest to the reinforcement. You’ll likely need to remove quite a bit of soil to make a level surface.
Once the soil is out of the way, you may need to remove a little more soil to add leveling sand and tamp it down for a good base for your wall. This is especially true if you’ll be building a concrete wall, as you don’t want it to sink into the soil underneath it! Take time to make sure it’ll be on even, secure footing now, or you’ll be rebuilding it again later.
After the soil is stable, it’s time to build your wall. Both concrete block and wood are options depending on the steepness of your grade. A very steep grade should use concrete block, while a shallower one may be able to be held back with wood.
A good concrete retaining wall can easily be 24” to 28” thick to ensure it can handle the weight of the hillside behind it. Typically, these walls will be constructed of two layers: one which leans inward towards the slope, and a secondary wall that acts as the front surface you’ll see. Between the inner wall and the hillside, you’ll need to add some gravel to allow excess water to drain and to help stabilize the soil.
Wooden retaining walls are often much thinner, but usually have a moisture barrier directly against the soil, then a heavily reinforced fence panel against that. Often the fence panel will have runners that extend out into the upper slope as well to ensure that the fence stays stationary. Rebar or heavy lumber posts will add even more stability.
Take time planning out your retaining wall in advance and make sure you allow for plenty of room between that and your future gardening space. Building a retaining wall can open up quite a lot of room for your future garden!
A terraced garden style can be done with or without raised beds. But for the best results, the addition of some raised structures can really enhance your environment.
In technicality, these multi-level plots are another variation on raised gardens. After all, you’re just leveling and stair-stepping your garden for maximum working area. Something like this can be incredibly effective and essential when dealing with heavily sloped surfaces!
With a terraced garden, you’ll definitely want to build sturdy retaining walls to prevent erosion. Something on the weaker side may allow a lot of soil to spill out around the edges. This gradually will reduce the amount of good soil you have in your upper levels and can overflow the lower ones, so it’s important to start out right.
Building individual raised beds to hold the majority of your soil in will prevent the erosion issues that come with terraced gardens. You can even use the raised beds in conjunction with your terracing for a great, sleek-looking space.
Gardening On A Shallow Grade
A shallow sloped surface is much easier to work with and requires a lot less soil to be moved. For a vegetable garden in a location like this, you’ll need to level the sloped surface just a little bit before you begin construction.
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 of the sort that is sold through the Epic Gardening store is a great choice to build. Construction of this style of bed is incredibly easy, as it’s ready to go right out of the box. Build the configuration you want if you’re going with one of the 8-in-1 beds or 6-in-1 beds by using the included screws to secure it together in the shape you desire. The fixed-size bed pieces are also easily assembled with screws.
Once your bed is assembled, you’ll need to make sure that the bottom of the frame is flush with the ground and that the top is level. If you’ve done your preparation in advance, this is easy. Otherwise, remove small amounts of soil to level out the sides of your bed.
If you’ve got problems with moles or gophers, place some hardware cloth underneath your bed to prevent them from digging up your plots. Check the level once more so you make sure your bed isn’t tilting downhill, then fill with your potting mix of choice and you’re ready to grow!
One last aspect with this style of bed: while the planting medium should prevent your bed from moving, if you’re dealing with a heavily sloped surface you may want to confirm there’s no chance of the bed sliding forward due to the pressure of the hill. Sturdy stakes made of rebar or wood can help. Pound the stakes into place along the inside of the bed to secure it in place. Lumber will deteriorate over time when covered with damp potting mix, so using rebar or other metal stakes is generally a better option for use over many years.
Wooden Raised Beds
Wooden beds on a slope work out well, too. This style of garden is more of a custom build than an out-of-the-box project, but it’s easy to source wood and screws to assemble them.
As with galvanized garden beds, you’ll want to make sure your bed isn’t sloping or sitting on an angle. Level the ground in advance where your bed will be.
Most wooden beds are simply a box made from boards that have been cut to size. Cedar is one of the favored lumber types for this style of build due to its longevity, but you can opt for Douglas fir or other woods as well. Avoid pine, as this will break down quickly when exposed to moisture. Gardens are always moist!
I prefer 2 x 6 lumber for the sides of my garden bed, but depending on your design you might opt for other sizes. The perk of a 2 x 6 board size is that two of them stacked atop each other is one foot in height, so it’s easy to plan for your desired height and buy lumber. Most people prefer raised beds with sides at least 12” deep, and these pieces will work well for that!
4 x 4 posts make good reinforcing corners. If you’d like to anchor your bed securely in place, you can make the corner posts about 12” longer than necessary for the height of your side panels, and bury the extra 12” at each of the corners under the ground. This ensures that the frame won’t move or shift from its position.
Once the construction of your box is done, set it in place and check the level. Adjust the placement of the bed’s sides to eliminate sloping. As with metal beds, you can anchor your bed with stakes if necessary. Fill with your garden mix of choice, plant it out, and you’ll have a beautiful garden in no time!