Raised Beds in our gardens are popular, and for good reason. Here in the Pacific NorthWest, getting a slightly earlier start on the gardening season is definitely desirable, especially for edible crops, and a raised bed both warms up a little earlier in the spring, and drains the rains earlier to allow the soil to be worked. The only real downside is that raised beds not only drain away spring rains faster, they drain away the water you add to the soil. You’ll need to water a little more often.
Preparing for a Raised Bed
Look over your gardening space. A good location for a raised bed will be a level area over natural soil. Placing a raised bed over concrete or pavement turns it into a container, which presents a whole different set of challenges. Before building the raised bed, loosen the soil several inches down over the proposed planting area. This will make it easier for plant roots to access the subsoil. Also be sure the area has good drainage, and is not too close to other structures. Most people will be kneeling outside the bed, and bumping into other objects will irritate you as long as this new bed exists! 24” spacing is probably a minimum. (The author is intimately acquainted with a tall old guy who built beds only 18” apart and is paying the price!)
To many gardeners, a raised bed implies some kind of border. Although borders have some advantages, they are not necessary. Simply piling extra dirt in the area where plants will be raised, often by digging out the pathways, makes a perfectly usable raised bed. Such a bed will probably have sides at approximately a 45 degree angle, and thus a little less planting space on the top of the bed, but they can work perfectly well. They are also the least expensive, and can be almost any shape.
If you are short on space, or like the look of vertical borders, there are a lot of options. Concrete blocks can be used, as can bricks, although using bricks will limit the height of your borders. At the Multnomah County Demonstration Garden, there are beds with brick borders, borders made from upside-down wine bottles, and a woven border called wattle. The wattle border relies on vertical stakes every 2-3 feet, and long, straight, roughly finger thick shoots from plants woven between the stakes. Young Alder shoots are a good choice.
Wooden borders are probably the most common. They are fairly quick to build and can be quite sturdy if built right. Since the wood will be in contact with the soil, a rot-resistant species is highly desirable. Here in the Pacific Northwest, the best choices are Western Red Cedar or Juniper.
How high should your border be? That’s up to you - considering what you will plant and any physical limitations you may have. Keep in mind that the higher the border, the more expensive the wood. If you will be importing soil, that cost will go up as well. Most borders are as tall as a single board is high. Common lumber heights are 6”, 8”, 10”, and 12”. If at all possible, chose lumber that is 2” thick. The weight of the contained soil, especially when wet, is considerable, and 1” thick boards will bend away from that weight. It is quite possible to build higher using multiple boards per side. It is advisable to tie the boards together with a vertical board inside the bed near the center of any long sides.
Constructing a 4’ x 8’ wooden bed
At your local lumber yard, purchase 3 boards, 8’ long, as wide as you wish your bed to be high. Let’s assume 8” for now. You will be asking for “eight foot cedar two by eights”. You should also acquire a short (3’) piece of 2”x2” or 4”x4” for corner supports. Assuming you have access to a circular saw, cut one of the 8’ boards exactly in half, making two 4’ boards. Most lumber yards will do this for you on request. These will be the ends of the bed, and the longer remaining two boards will be the sides.
You’ll be more pleased with your construction in the end if you now take a close look at the boards to decide how you want them arranged. It is quite likely that one side will appear better than the other, and should be on the outside of the bed. Likewise, one edge will appear better, and should be on the top. Take a moment to lay set the boards on their edges in the arrangement you prefer. Be sure you arrange both ends the same way, perhaps having the 4’ board overlapping the ends of both 8’ boards. Now, using a permanent marker, identify each corner on the inside bottom of the bed. A common method is to call one corner ‘A’, and write that near the end of each board that will form that corner. Proceed around the bed marking the remaining corners. (If this author’s experience is any predictor, you will be glad you’ve done this! Ending with an ugly side pointed out is irritating!)
Joining two boards at the corner
The simplest joint that can be used is the butt joint. Simply overlap the two boards and screw through the outside board into the inside board. It is simple, but unfortunately it is not strong. If you take a look at the very end of any board, you can see that the grain, called “end grain” is coming straight at you. Imagine trying to screw into the end of a broom - the “grain” will just move out of the way with little force to retain the screw. A joint is far stronger if all screws go into “cross grain”. This is where the extra short piece of wood comes in. Cut it into four pieces, each the same length that your main boards are wide. It is fine if it is a little shorter, but a nuisance if it is longer. By placing one of those pieces vertically in each corner, you can screw through the outside “main” board into the corner piece, meeting cross-grain in both pieces.
Drill pilot holes through the main boards where the screws would roughly hit the center of the corner piece. Two screws on each side should be plenty. If you are using 2” thick main boards, use 3” screws at a minimum. Be sure the screws are rated for outdoor use. Before you assemble the corners, double check that both ends are overlapping the side boards in the same way. If the 4’ board overlaps both sides, your bed will be 4’ wide by 8’ 4” long. If the side boards overlap the end board at both sides, your bed will be 4’ 4” wide by 8’ long. A mistake here will result in a very nice trapezoid, but possibly not what you wanted.
Before you actually screw the sides to the corner pieces, make sure the bed is “square”, meaning all corners are 90 degrees. The foolproof way to do this is to measure the two diagonals, which should be equal. If they aren’t, nudge the boards until they are. Then screw them tight.
Building longer beds
A 4’ x 8’ bed may suit you well, but many will wish to construct longer beds. (Give careful thought to this - you will be walking around them often!) Two inch thick cedar boards are usually available up to 12’ long, and occasionally longer. It is also possible to attach two pieces end to end with an overlapping thin connecting piece on the inside. Either way, however, the side pressure of the contained dirt, especially if well-watered, may cause the sides to bulge in longer beds. By the time this happens in the second or third year, it would be a major undertaking to dig out the dirt and fix the problem. Better some prevention now.
The simplest solution is to pound in stakes at multiple places along the outside to prevent bulging. If you choose this method, realize that you may be bumping into those stakes with your lawn mower and weed whip many times each summer. Also, it can be difficult to drive the stakes straight down if the soil is at all rocky. One alternative is to install a cross-connector about every 8’ of bed length. This is a piece of cedar perhaps only 1” x 4” that will attach to each side, but be hidden under the dirt in the bed. As with the main boards, using a butt joint will probably be too weak. It is better to use a small vertical piece of wood at each end so all screws can go into cross-grain.
If you use materials that will resist rot, and pause occasionally to make sure the pieces are aesthetically arranged, you can quickly build an addition to your garden that will serve you well for years.