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Common Questions - Raised Beds

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 high-quality choices for outdoor construction projects.

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. Recycled composite plastic lumber is another alternative, and is now available in a variety of sizes and colors.

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.

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 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

Lumber 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. Fastenings 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. Cross Supports 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. Tools Hand saw, square, carpenter’s level, mallet (or sledge), screwdriver, hacksaw, drill.

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. 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. 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. 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. Burrowing pests? 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 planters available for above-ground gardening. These planters are designed to be easy on the back. 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, read our articles: Leave a generous width between beds for the pathways It helps when pathways between beds are wide enough for a small wheelbarrow. For grass pathways, make sure they are at least wide enough for a weed-eater or a small mower. (In our raised bed gardens the pathways are 22″ wide.) Mulch the pathways between beds Weeding pathways is a nuisance that you can avoid by putting a double layer of perforated landscape cloth over the pathway and covering 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.

Can a raised bed be installed on a concrete surface, such as a patio? Yes! Many beautiful raised bed installations are set on concrete surfaces, but there are a few things that need to be taken into consideration.

Typically, patio and driveway surfaces are not level. This is intentional so water can drain away from the main house structure. Raised beds, however, should be built to level, the same as if being constructed on soil. This means the bottoms of the beds will need to be blocked up on the low end just enough to facilitate and direct drainage.