How to build a raised garden bed on sloping, uneven ground
Building a raised garden bed over sloping ground is simple if you build it ‘in place’. Here’s the easiest way to build a level bed over uneven ground.Posted Apr 9, 2010
Our garden is situated in a large, open south-facing area which provides ideal exposure and growing conditions. The ground is uneven, however, and slopes downhill in a southward direction. The sloping ground provides good drainage for the orchard, but presented some problems for the vegetable beds. We needed to terrace the ground into level ‘steppes’ or build raised beds for the garden vegetable plots in order to keep the valuable soil and amendments in place.
Terracing the ground proved difficult. Each ‘steppe’ was supported by a large mill slab or stacked timbers, and held with stakes. But over the years, we grew tired of replacing rotted stakes and fallen boards each spring. Building raised beds was a better solution because they held together for years and required no maintenance.
Building the raised beds over uneven, sloping ground was another matter. We wanted the beds to be level, to ensure even water distribution (in beds that are not level, plant roots at the high end get less water than those at the low end.) and we like the way it looks when the beds are level. So some of the bed sides need to be tapered to fit the ground contour, which makes the carpentry a bit more complex.
Below is my current favorite system for building a raised garden bed with tapered sides. The project is simplified by building the bed ‘in place’, requiring no measuring of the slope grade, and can be built working alone – you don’t need someone on the other end to hold boards or move the finished bed in place. The technique described here will work on varied grades and even if the slope runs off in two directions.
1. Make a simple box.
Using 2 x 6 lumber, or wider if you prefer, cut two lengths and two widths to make the size bed you want. Screw the butted ends to make a simple box. Don’t worry if it’s a little flimsy, it will get stronger later in the construction. In the pictures here, the bed is 16’ long and 4’ wide. The lumber is full dimension 2” x 6” which is thicker than standard construction lumber, but a standard 2” x 6” (actually 1.5” x 5.5”) will work fine. For fastenings, I used 4” decking screws throughout the project, and pre-drilled the screw holes with a 1/8” bit. If you use standard lumber, use 3” decking screws. We recommend coating lumber with a non-toxic sealant or wood treatment to extend its natural lifespan and improve rot resistance.
2. Set the box in place on the ground. Lift up the low ends until level and block it up.
Set the box right where you want it, allowing for pathway widths. Set a level in the middle of the long span, on top of the board. Go to the low (downhill) end and lift it until you see the bubble centered in the level. Push a block in underneath the end to hold it in place. The bed is now level end to end. Next get the level and set it on top of the end board (as pictured), and lift the low side until the bubble appears. Push in another block where needed. Now you can stand back and see the box is level both across the width and along the length. This whole process takes about two minutes!
3. Add corner stakes and screw the bed sides to the stakes.
Cut corner stakes from 2” x 4” stock and tap them into the soil, and ensuring they fit flush against the corner pieces. Leave extra height on these pieces if you intend to build higher sides to the bed. In the bed pictured below, I wasn’t sure if I wanted higher sides so the corner pieces were left long until the bed was finished, then it was easier to make a final decision on height. Screw the bed sides into the stakes. Now you can kick out the blocks and the box will stay in place, and be level.
4. Cut the tapered pieces to fill in the sides to ground level.
This is easier than it looks. Select a piece of lumber wide enough to fill the gap between the box sides and the ground at its widest point. In the picture below, the bed was 4” below level, so I was able to use a 2” x 4” piece for the tapers. Simply lay the board on the ground against the space to be filled and mark the line from the inside. You can set the board to be cut right on the box while you cut it. I used a chainsaw, which is fast and easy. Another option would be to use a skill saw. The cut, as you can see below, doesn’t have to be perfect since it’s going to be set along the ground. In the bed pictured below, I was able to use the off-cut (extra piece) for the taper on the opposite side of the bed.
5. Insert the tapered pieces and screw them into the stakes.
Slip the tapered pieces into place, you may need to clear a bit of soil to get it right, then screw them into the stakes. In the bed pictured here, two more sets of stakes were placed along the long board, at 5’ intervals, for both screwing the sides into and to keep the middle of the bed from bowing outwards once it is heavy with soil. You can see the tapered board in place, below, and the second taper on the other side is ready to slide into place. The bottom end piece has also been added.
6. Add spanners to prevent bowing if bed length is more than 6’ long.
Spanners are lengths of aluminum, steel or wood which are fastened from mid-stake, across the bed, to the opposite mid-stake, for the purpose of preventing outward bowing. Set them several inches beneath the soil surface. The taller your bed, the more you’ll need spanners, since more weight will be against the sides. The spanners pictured here are made of scrap aluminum ¾” angle stock; flat ¾” bar stock would be better. I just used what I had. You may think you won’t need spanners, but the soil gets heavier when wet and over time the sides begin to bow outward.
If you want to build a taller bed, add the extra top pieces now by screwing them into the stakes. Once you are satisfied with the height, saw off the stakes level to the top of the boards. Use a hand saw for this.
7. Fill in with soil, add soil amendments and you’re ready to plant!
Add soil and any amendments you need such as peat moss, rock phosphate, compost and lime, and rake the soil smooth and level with the top of the beds. Over time the soil will settle, allowing room for more soil or for adding mulch. Because you sawed the stakes off level with the top of the bed, you can see where they are. This way you know where the cross-spanners are located so you don’t dig into them during future plantings and harvestings.
Here is the finished bed, level at top and tapered to fit the ground slope. A rosemary plant has already been transplanted and now getting used to its new home.