Note from Kirsten: Today’s DIY post is brought to you by Ken Charm. Ken works on the sales counter at our Everett location and recently got involved with Dunn DIY through the Northwest Flower & Garden Festival.
Ken studied environmental science at Western Washington University and worked as an environmental consultant for 14 years, so he knows a thing or two about working with various wood species in a garden setting. Today, Ken is sharing how to build a self-watering raised bed (also referred to as a planter bed, planter box, garden bed, etc.). Take it away, Ken!
I'm excited to show you how Kirsten and I created a durable raised bed, which is a great addition to any garden. I chose to make this raised bed a self-watering planter because, well, I knew I wouldn’t spend the time needed to keep my veggies watered. I also wanted the "insurance" of being able to leave for a long weekend and not worry about my veggies dying (or asking a neighbor to water the plants).
A self-watering bed consists of two parts: the reservoir and the soil. The reservoir holds water and the soil wicks it up and stays moist. The physics involved here aren’t terribly complicated, but the important thing to remember is that water can only wick so far—12” to 18” max. I’m kind of pushing the limits with 28” planter beds, but I want to make sure my veggie-loving pug doesn't get into it. Let’s get started.
Step 1: Choose a Spot
Make sure the spot you choose for your bed gets the light you need for what you’re planting. In my case, I’m planning on veggies, so full sun is critical.
You can level the area at this time, or (like me) level the area after the framework is completed.
Step 2: Build the End Frames
I decided to build a 4' x 8' frame. The scale of those dimensions fit my yard nicely, and it happened to coincide with the heavily discounted materials I’d found (this is where it helps to know people at your local lumberyard). I’ll use those dimensions for this how-to, but you can alter as needed to fit your space.
To start, you'll need to cut some of your lumber into shorter lengths. (Remember, every cut you make to pressure-treated lumber should be treated with End Cut Solution.) Cut two 26" long pieces from your 10' four-by-four, and cut one two-by-six in half to give you two, 48" long pieces. Also cut one of your two-by-eights in half; you'll use one of these 48" pieces on each end of the planter. Build one short end of the planter by attaching two 48" two-by-six boards to the top and bottom of two 26" long four-by-fours (it should make a rectangle when you're done, and once standing upright, will make the short end of the bed). The two-by-six boards make up the side of the this end frame, and the four-by-fours are two of the corner posts. Attach a 48" piece of two-by-eight between the two-by-six boards for additional support. Once you’ve finished one end, repeat on the other end.
Quick note: My beds are 28” tall because I found discounted copper colored metal roofing at my local lumberyard that was 26" tall. (Note: Dunn Lumber stocks a plain galvanized metal roofing and colored roofing generally needs to be special ordered.) I needed a taller bed to keep one of my pugs out of the veggie garden—she likes to graze on anything that may be edible, especially fresh lettuce—and at that height, she can’t quite jump in. Make your bed as tall or short as you need to, as long as it’s within the 12” to 18” parameters of what a wicking bed is capable of.
Step 3: Attach Long Boards to the Ends
Once you have the two short ends completed, attach the long (8') two-by-sixes and two-by-eights to the four-by-four posts, making a rectangular frame with the four-by-fours on the inside. By having the four-by-fours on the inside of the framework, we can avoid putting screws into the end grain of the wood, which is far less capable of holding a screw than the side of a four-by-four. I used two to three screws in the two-by-sixes, and three to four screws in the two-by-eights.
Step 4: Level the Site
Once the frame is built, level the site. My yard has all sorts of humps and dips in it, so leveling only under the frame and not under all 32 square feet is far less work. Some people may call it lazy—I call it efficient!
Step 5: Clad One Side of the Frame (Optional)
My raised bed happens to be up against a fence. If yours will be freestanding, skip ahead to Step 6.
Now that we have a bed frame and a leveled site, it's time to add the cladding. This is where it all starts to come together. Using either a grinder with a cutting wheel or a circular saw with an old carbide blade turned backwards, cut the metal roofing to length.
For the side against the fence, I’ve been using parts of the cover sheets the manufacturer uses to protect the roofing during shipping. It’s the same profile with all the same protective barriers minus the pretty paint, but who cares about the paint on a panel facing a fence? Once cut, pull the frame away from the fence and attach the paneling. At this stage, I'm only adding the cladding on the side that rests against the fence. The remainder of the cladding will be added in Step 8. For the hardware, I chose to use a plain galvanized screw instead of one color-matched to the siding. The galvanized screws are readily available, and I like the look of the contrasting color, but the choice is yours. Once done, move the bed back against the fence.
Step 6: Secure Liner to the Frame
Lay pond liner into the empty box, smoothing the liner on all sides so that you minimize wrinkles and folds. There will be folds in each corner, but we don’t want too many. Don't skip this step—the liner creates a sealed environment inside the planter box which is required if you want the bed to be self-watering.
Staple the liner to the top of the frame. I used my pneumatic staple gun. You could use an electric one as well, but there are so many staples here that while you could use a manual staple gun, it would be worth renting or buying something that would give you some mechanical assistance. I bet I used at least 500 staples, and I’m certain I wouldn’t want to do that with my hand-powered staple gun!
Using a utility knife with a fresh blade in it (remember: a sharp knife is a safe knife), trim the excess liner away.
Step 7: Mark Screw Holes
Now it's time to add the rest of the cladding. Use a tape measure and find the center of an 8’ piece of metal roofing. Mark the center with a dot, and add a dot every foot in a nice straight line, working outward from the middle along a “valley” of the roofing (where the screws will go into the framing of the box and hold the roofing in contact with box). When you’re done, there will be three rows of identical dots along the two 8’ sides and similar rows of dots on the remaining 4’ side.
Step 8: Attach Metal Roofing
For this step, it helps to have an extra set of hands—at least while you get the first screws in. Place the top of the roofing level with the top of the frame. The screws are self-tapping, so there’s no need to pre-drill. Put a screw through one of the dots near one end while a friend holds up the other. And don't forget to wear gloves! The cuts one can get from metal roofing are unpleasant (don’t ask me how I know). Repeat this process on each side.
Step 9: Attach Fence Board
The edges of the cut metal roofing can be sharp—to cover these, we'll use fence board. Using the planter box as a measuring tool, mark the height of the planter on a piece of fence board, then cut the board to length. Attach this board with screws through the ridge part of the metal siding. You could use one-by-six fence boards, but I prefer the narrower line of a one-by-four board. These aren’t structural, so go with your eye. You can also order metal flashing that matches or complements the metal roofing to do the same job: hide the sharp edges.
Step 10: Add Trim
To finish the frame, we'll add a lumber trim along the top. To do this, set a two-by-six ten-footer flat on top of the planter flush with the end of the box. I chose to use pressure-treated wood here, because a customer told me that the copper prevents snails and slugs from entering the bed. I don't yet know if this works, but I thought I'd give it a try. Measure for a miter cut so that the end of the two-by-six hangs a fence board–width over the edge. If it helps, use a scrap piece of fence board for spacing. Then, make the cut. Remember to apply end grain sealer (that wood preservative mentioned in Step 2) if you're using pressure-treated lumber.
Remember to sneak up on the cut—it’s easier to cut off a little more wood than it is to add it back on. Someday, I’ll invent a board stretcher and be rich beyond my wildest dreams! But until then, I’ll go slow. Repeat for each side, then attach with 3" screws.
I went with pressure-treated lumber on this project for several reasons, but cedar and juniper would work just as well. For more on how to choose which wood type is best for you, check out this blog, which highlights the differences and benefits of the three.
Now you've got a raised bed frame! Check out part 2 of this series to make it self-watering by adding irrigation and an overflow—which keeps the soil moist while draining excess water.