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How to build a raised garden bed on sloping, uneven ground

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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.

By Greg Seaman, Eartheasy.com Posted Apr 9, 2010

raised-garden-bed0Our 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.

raised-garden-bed1Set 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.

raised-garden-bed2Cut 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.

raised-garden-bed3This 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.

raised-garden-bed4Slip 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.

raised-garden-bed5Spanners 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.
raised-garden-bed6
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.

raised-beds-a

raised-beds-2

To learn more about gardening in raised beds, click here.
Eartheasy has the widest selection of raised beds anywhere. To buy one of our raised beds, or to see what we have to offer, click here.

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  • svere

    Awesome. Great information to build a raised garden bed on sloping. Thanks for sharing.

  • wall art decor

    Thanks a lot for providing such a nice information with us.Very informative and useful article.

  • Nick

    just what i needed!

  • Paul Williams

    What a beautiful garden! I would love to see it when it is all planted and in full production.

  • McLoughlin's from MN

    Thought you would like to know that my husband and I are modelling our garden based on the beautiful garden in your picture. Our plot has a gradual slope and we want the same effect of having all the beds level. We are very excited about the project! Thanks!

  • michael

    wow! thanks alot this will really help my garden this year!

  • some guy

    i didn't make mine with a taper piece, it was easier to just level the ground below using a tiller. meaning, chop out the hill.

    you only need to level the spot the same size as your bed. the edge keeps the same taper look because it becomes buried. once its level you use the dirt and fill the outside hill back in.

    it looks exactly the same as yours when it is complete.. plus you may need to til the ground below anyways.

    yours look awesome but my way might be easier for someone not handy with a saw.

    • Greg Seaman

      I used to do the same thing – bury the high side so it's down to level. However, the buried wood rots pretty quickly, and it's tedious trying to establish level as you dig down and keep checking with a board and level.
      Ths method described in this article does require more skill in cutting the taper. But the benefit is that the tapered offcut, rather than being buried, can be used to fill in the space for the other side, or the next bed. In making 10 beds last summer, I was able to use all the taper offcuts and save a lot of wood.

      • David

        If you were going to bury the boards (that's what I'm thinking of doing, mostly because I only have to build one 13×7 bed, and would have to buy additional boards to cut the taper pieces), then would it make sense to get 2×10's or 2×12's? I feel like as much as seven inches or so might be buried on the high side of my plot.

        Your way definitely looks awesome. But I'm scared I'll screw up the sawing. I'm a real novice with this kind of stuff.

        • Greg Seaman

          The 2 x 12s are going to give you more depth, which is nice.
          When you pick your boards out at the lumberyard, look for ones without the pith running through them so they're less likely to cup and check. Ask the yard staff for advice if you're not sure. Wider boards are more likely to cup and this pulls apart the corners. A nice edge-grained board will be more stable and give you a longer lasting construction.

          • David

            Thank you for this advice!

  • Elizabeth

    How deep into the soil do I need to put the end posts. I'm dealing with some serious roots and at at least one corner won't be able to get the post into the ground at all.

    • Greg Seaman

      It's OK if some of the end posts do not go into the ground. Once the bed is filled with soil, the weight of the soil keeps the bed in place. Some raised bed designs do not have the end posts going into the ground at all.

      • Elizabeth

        Is there any reason not to make the long sides out of smaller sections? I'm thinking three 6' sections so the wood fits in my truck. Making the box will be more challenging, of course.

  • Elizabeth

    awesome! thanks

  • Lauren

    I love raised bed gardening. I wish I would have seen this article earlier in the year. I had a buddy of mine who is a San Jose handyman help me build about 12 boxes for my garden in the spring. I was able to plant all kinds of stuff. I had tomatos, chili peppers, lettuce, asparagus, pumpkin, watermelon, carrots, blueberries, and raspberries. It was great until some local rodents got in my yard and ate all the fruits of my labor. Oh well, lesson learned, next year I’ll put up a little rabbit fence!

    • grommet

      A little rabbit fence likely won't do the trick. Did you line the bottom of your planters with galvanized gopher wire, 1/2" octogonal? You will also want something to keep the rabbits out. The same galvanized gopher wire at 2" above ground and 12" below grade (yes, you'll need to trench all the way around) is required.

  • Leigh Sabey

    Thank you for sharing! We were just finishing up plans for 4 raised garden beds on a slope, and thankfully I googled the subject and found your article! This will be much easier than burying the boards as we had planned.

    • Greg Seaman

      I think your boards will also last longer when no part of the board is buried. This has been our experience here in the wet northwest.

  • Wwerth

    What do you need to do to prepare the ground underneath the raised bed?  I live in Arizona where the dirt is clay-like.

    • http://eartheasy.com/ Greg Seaman

      To prepare the ground beneath the bed, dig approx. 18″ to clear the soil of rocks or debris. This gives you a chance to see the underlying soil conditions which can be corrected with amendments. If soil is clay-like, it can be lightened with peat. Lime may be needed as well, but first ask a local gardener if your local sol is acidic or alkaline.
      Digging the soil also breaks it up which improves drainage and favors root growth.Once you have prepared the soil, then the bed is not stepped on again.

  • monette

    I’m starting a raised bed garden on a slope bordered by white oaks. I don’t know how many acorns are buried under my site and don’t have access to a tiller. Do you think a couple of layers of that black landscaping cloth would keep ‘em from sprouting into my new garden?

    • http://eartheasy.com/ Greg Seaman

      Unless your bed is over 12″ – 18″ tall, I think your plants will benefit more by having access to the soil beneath your bed. We recommend that you dig the soil beneath the ground in perparation for the new bed, so you can remove any rocks or debris. You may find acorns to remove during this process. A tiller is not required for digging, just use a shovel. I would forego the landscape cloth.

  • Jen

    My south slope is over my septic system. Will I need to put some sort of fabric at the bottom to prevent the roots from growing into the ground above the system?

    • http://eartheasy.com/ Greg Seaman

      I think you’ll be fine without any ground cover for a cegetable garden. However, avoid planting woody perennials.

  • Keith Hanigan

    When you dig in preparation for building the beds on sloped ground do you move the soil around to accommodate the slope?

    • http://eartheasy.com/ Greg Seaman

      We originally moved the dirt into terraces which fit the natural slope. The boards which held up the front of the terraces would collapse every few years and need replacing. Eventually we built the beds to replace the terraces.

  • Fantasticdad

    Awesome post and wonderful shots of your garden! I love using raised garden beds, especially for growing vegetables. Thanks for your ideas, really helpful!

  • ny

    Hello,
    I hope you get my message soon I’d love to know what did you use to seal raised bed and in what shade. I just love the color of your raised bed and thought they would be fabulous in mine.

    Thanks so much.

    • http://eartheasy.com/ Greg Seaman

      The lumber is unsealed. It is recently milled Western Red Cedar in its natural color. After a few seasons the wood will fade to sliver-grey. If you want to retain the original color you can finish the wood with a sealer such as tung oil finish.

  • http://eartheasy.com/ Greg Seaman

    Just go to any hardware store and ask for lengths of aluminum flat bar stock, 1/2″ wide. It comes in 8′ lengths, is easy to cut with a hacksaw, and should only cost about $1 per foot. Use pan head screws to screw it to the center uprights.

    • http://www.facebook.com/jickfoo Justin Hickey

      how on earth do you get 16′ wood home ?

      • http://eartheasy.com/ Greg Seaman

        Had a friend with a flatbed truck put it in a nearby cove, then floated it to my place and carried it up the hill and into the garden!

  • http://eartheasy.com/ Greg Seaman

    Get some 1/4″ galvanized mesh and cut to 6″ wider and longer than your bed. Fold the edges up 3″ around, then set it on the ground inside your bed and staple the edges to the inside of the bed. Then fill with soil.

    • http://www.facebook.com/jickfoo Justin Hickey

      Galvanized mesh is expensive, especially the 1/4″ variety. Any alternatives ?

      • http://eartheasy.com/ Greg Seaman

        You could use 1/2″ mesh to save a few dollars. Or poultry mesh (1″) which costs even less but won’t last as long.

  • sam

    this article is exactly what I need for our raised beds~ thanks so much for sharing!

  • http://eartheasy.com/ Greg Seaman

    You can prepare the soil as usual for a garden bed, but go easy on the lime for the Azaleas. Here is an article with more information: http://eartheasy.com/blog/2012/01/6-tips-for-building-soil-for-your-raised-garden-beds-and-planters/

  • http://eartheasy.com/ Greg Seaman

    Can you cut some tapered pieces of cedar to fill in the gaps between the fence and ground?

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