The Benefits of Raised Bed Gardening
Raised garden beds, also called garden boxes, are great for growing small plots of veggies and flowers. They keep pathway weeds from your garden soil, prevent soil compaction, provide good drainage and serve as a barrier to pests such as slugs and snails. The sides of the beds keep your valuable garden soil from being eroded or washed away during heavy rains. In many regions, gardeners are able to plant earlier in the season because the soil is warmer and better drained when it is above ground level.
By raising the soil level, raised garden beds also reduce back strain when bending over to tend the bed. This is especially helpful to older gardeners or people with bad backs. And if the beds are built well, the gardener can sit on the edge of the bed while weeding, and for some gardeners this is the biggest benefit of all.
Raised beds are not the same as garden planters. Planters are elevated containers which have bottoms to prevent the soil from falling out. Planter bottoms usually are slatted, with some type of semi-permeable cloth barrier which permits drainage. Raised beds, however, do not have bottoms; they are open to the ground, which offers the benefit of permitting plant roots to go further into the ground for available nutrients.
Raised garden beds are available in a variety of different materials, or they can be made with relative ease.
How to Build a Raised Garden Bed
Gardeners can build their own elevated garden beds with relative ease. You will need to decide what kind of wood to use, how tall you want the bed to be, and whether you want to build the entire bed yourself or use pre-made corner braces which simplify the construction process and provide a secure corner that won’t work loose over time.
What kind of wood to use?
In most cases, cedar is the best wood to use for garden beds because cedar is naturally rot resistant. Western red cedar is commonly used, but Vermont white cedar, Port Orford (yellow) cedar and Juniper are also great choices for outdoor construction projects. Redwood is another excellent rot-resistant wood, but redwood is a more limited resource. How long the wood will last depends on the type of cedar and your local weather conditions. In our garden, we use red cedar for building the beds, and some of these beds have lasted 15 years. In general, you should expect about 10 years from a cedar bed before it begins to deteriorate.
How tall should the bed be?
You can build the bed to any desired height up to 36″. The most common height is 11″, which is the height of two stacked 2″ x 6″ boards. If you have good soil beneath the bed, the roots will go down deeper as needed to access more soil and nutrients, so you can even have beds that are only 6″ high. If you want a taller bed, remember that as you go taller, the weight of the added soil will add pressure to the sides, and will bow them outward. This is easily prevented by including cross-supports. We recommend using cross-supports in any beds which are taller than 18″, or longer than 6′.
It is also important to consider the soil depth requirements for the roots of the vegetables you want to plant. Depending on the soil conditions beneath your bed, you may want to build the sides of your bed higher for certail crops. For more information, read our article Soil Depth Requirements for Popular Vegetables.
How wide and long should the bed be?
We recommend bed width to be no wider that 4′ across. This is because it is easy to reach the center of the bed from either side, and for people with long arms, to reach across the bed. It’s important to keep the width this narrow to avoid having to step on the bed since this would compress the soil. The bed can be any length as long as cross supports are installed every 4′ – 6′ along the length of the bed to prevent bowing. We think longer beds are best, if you have the garden space.
We have built many raised beds over the years, and our construction process has evolved. The method described here is, in our opinion, the simplest method of building a raised garden bed, and it requires no special tools or expertise. Using this method, you can build your bed to any desired length, width and height.
Tools & Materials:
Use cedar “2 x” boards for the sides. These are commonly 2″ x 6″, but you can use 2″ x 4″ or 2″ x 8″ boards if this is what you have available. (2″ boards bought at a lumber yard are actually 1.5″ thick..) For the corner posts, use 4″ x 4″s, cut to 10″ longer than the desired height of the bed. If your bed is going to be longer than 8′, you’ll need extra posts to put in mid-span to prevent bowing and to provide a place to secure the cross-supports.
In the photos below, we used 2″ x 4″ boards for the posts instead of 4″ x 4″s. This is because the wood we had was full dimension, i.e., the 2″ thickness was a full 2″. If you get your wood from a mill, they can cut it full dimension for you.
Use 3.5″ #10 coated deck screws for the project. You’ll need six screws for each corner and two for each mid-span post. If you are using cross-supports, get a few 1″ stainless screws.
Buy several lengths of 1/2″ aluminum flat stock. This is available at most hardware stores, usually in 8′ lengths. It is very easy to cut with a hacksaw and to drill for the screws.
Hand saw, square, carpenter’s level, mallet (or sledge), screwdriver, hacksaw, drill.
Is treated lumber safe?
In 2003, the EPA banned the sale of lumber treated with chromated copper arsenate (CCA) for residential use. Two compounds, alkaline copper quat (ACQ) and copper azole (CA-B), have now replaced CCA wood in the residential market. Both contain copper and a fungicide but no arsenic. The copper keeps insects at bay, and the fungicide prevents soil fungus from attacking the wood. In ACQ, the fungicide is quat, which is also used in swimming-pool chemicals and as a disinfectant. The other compound, CA-B, uses copper and tebuconazole, a fungicide used on food crops. According to Miles McEvoy, who works in organic certification with the Washington State Department of Agriculture, no pressure-treated wood is allowed in soils used to grow organic food. If you want to meet this high standard, choose a different material.
Until the safety of treated wood is proven conclusively, we recommend you use a naturally rot-resistant wood like red cedar, black locust or redwood. Under most circumstances, these woods will last 10 – 20 years when used for raised beds. Recycled composite plastic lumber is another alternative, and is now available in a variety of sizes and colors.
1. Cut and assemble the basic frame, block it up to level
Clear the area where the bed will be located, because you will be building the bed “in place”.
Use a square to mark the ends and saw the boards to desired length. Put two screws in each corner to hold it together for now. Set a level on the frame and place blocks beneath it to keep it level. Do this for the ends and the sides.
2. Drive in corner posts and screw the boards into them
Cut the post pieces longer than you will need. You can saw a point on the bottom of the posts, although is it not essential.
Set the first post into the corner of the frame and drive the post into the ground a few inches. Screw the frame into the post, using two screws per side. Set the other posts in place and screw them in the same way.
3. Fill in boards to ground. Saw post tops flush to sides
Now add the bottom row of boards down to ground level, using the same method of simply screwing into the posts. You may have to dig into the ground in places to get the boards to fit.
Using a hand saw, cut the posts where they stick up, so that they are flush to the sides of the bed. Smooth the ground in the pathway and start filling the bed with soil.
4. Add cross-bracing
If your bed is longer than 8′, or taller than 18″, it’s a good idea to use cross-bracing. This will prevent the bed from bowing outwards in the center of the span.
Use a hacksaw to cut the aluminum flat stock to the exact width of the bed. Drill a hole in each end, and use a 1″ stainless screw to attach the cross-brace to the posts at either side of the span.
5. Top up the bed with soil and get gardening!
Use your best garden soil to top off the bed. If there are rocks, the soil can be screened through a piece of 1/2″ mesh. Or you can just pull out any rocks you come across.
Add soil amendments such as peat, lime, rock phosphate and organic fertilizer, as needed. Spray the soil with a fine spray, and top it off again because the water will lower the soil level a bit.
Now your bed is ready to plant!
Raised Bed Installation / Tips / Layout Suggestions
Lay out the beds so they are horizontally facing south
It’s best if the long side of the bed faces south. This assures equal light exposure to all the plants growing in the bed. If your bed is aligned the other way (the ends facing south), you may have planting limitations because taller plants in front can block the sunlight to small plants in back.
Double-dig the bed area
If the ground has never been used for gardening, it should be ‘turned over’ (dug) to a depth of 16”. This gives you a chance to pull rocks, and to see the composition of your soil. Leave soil piled up in the center, away from the sides, so you can set the bed in place without obstructions.
Check for roots
As you dig the soil, keep an eye out for any roots which may be growing beneath your beds. If left to grow, these roots will steal the organic amendments you add to the soil. Pull any roots back towards their source, and pull the main root clump. If the source is a living tree, you may need to install a root barrier by digging a narrow trench outside the perimeter of the bed, and deeper than the roots, and then insert a barrier such as heavy plastic sheeting.
Level the bed
Use a level for this task. This may seem overly meticulous, but after several waterings the soil will settle to level, and you’ll want the bed to be the same. Set a stiff board (2×4) on top of the bed sides, across the span, and set your level on this board. Tap down the sides as needed till you get a level reading. Be sure to check for level both along the length and across the width of your bed.
If your garden has burrowing pests such as moles, a layer of 1/2″ or 1/4″ hardware cloth (galvanized mesh) can be laid across the bottom, before soil is added. The mesh should continue at least 3″ up along the insides of the bed and be stapled in place. If you plan to grow root crops, such as potatoes or carrots, you may want to set the mesh lower in the ground by digging deeper when you are setting up the bed. There are also raised garden planters available for above-ground gardening. These planters are designed to be easy on the back, but they also keep burrowing pests, rabbits, cats and other critters out of your garden.
Spread soil out evenly
Add any planned soil amendments, such as peat, compost or lime, and spread the soil evenly across the bed. Water the bed with an even, fine spray. This will settle the soil; add more soil to “top off”. (Over time the soil will settle an inch or two more.) Rake the bed once more to even out the soil and you’re ready to plant. To learn more about developing the ideal soil for your raised beds, read our article 6 Tips for Building Soil in your Raised Beds and Planters.
Avoid stepping on the bed
Once the soil is added and the bed is planted, make it a policy to never step on the bed. Stepping on the bed will compact the soil, reduce aeration and impact root growth. Pets should also be trained to stay off the raised beds.
It’s very helpful to have a ‘spanner board’, a short sturdy board, like a 2 x 6, that’s just longer than the width of your beds. This board can be laid across the bed, setting on top of the bed sides, and can be used to set buckets on when weeding or adding amendments, and it can be used to step on if you must step on the bed. It also makes a handy seat when weeding or tending the bed.
Leave a generous width between beds for the pathways
It helps when pathways between raised beds are wide enough for a small wheelbarrow. For grass pathways, make sure they are at least wide enough for a weedeater or a small mower. (In our raised bed gardens the pathways are 22″ wide.)
Mulch the pathways between beds
Weeding pathways is a nuisance which you can avoid by putting a double layer of perforated landscape cloth over the pathway, and cover this with a 2- 3″ layer of bark mulch or coarse sawdust. When laying down the landscape cloth, allow it to come up 1″ against the bottom board of the bed, and staple this to the bed. This will not be visible because the mulch will cover it.
When buying mulch, ask the seller if they have had any complaints about weed seeds in the mulch. It is very common for bark mulch to have weed seeds which sprout in your pathways. Some weeds will still appear on your pathways regardless of the mulch. Wait until it rains before pulling them out, or you may rip the landscape cloth. The weeds will come out easily if the ground is wet.
Add a trellis to your raised bed
Raised beds are sturdy enough to support a trellis, even one large enough for pole beans which grow to 8′ tall. Here is our favorite trellis design which won’t mark your raised bed with visible holes, yet is easy to disassemble for storage or to relocate to another raised bed.