It turns out the construction of such beds is ridiculously easy and can be done in an hour or less if you have a clear plan. We decided to make one for both our front and back decks, and do make them six inches deep. The shallow depth limits our ability to grow carrots, but we didn't want to test the weight bearing abilities of our third-floor decks too much! (And we can grow carrots in other, deeper pots.)
To make two 4' x 4' blocks, you'll need:
For the bed:
1 sheet of plywood, 4' x 8'x 7/16"
4 boards, 1" x 8" x 8'
40 screws of at least 1.5" length
24 small nails or screws
Small pieces of scrap wood to raise the bed above whatever surface will support the bed
For the trellis:
4 pieces of 1/2" aluminum electrical conduit, 10' long
8 semicircle brackets
4 electrical conduit pass through 90° elbows
4 electrical conduit 90° elbows
Tools: saw, drill, screwdriver, pipe cutter. Some home improvement stores may be willing to cut the conduit to your specifications if you ask them. In that case, you wouldn't need the pipe cutter. Similarly, if you can get them to cut the plywood in half, it will be easier to fit in your vehicle (or bike trailer) and you get to skip to step two below! Oh boy!
(Thanks to Google Sketchup for making it possible to show what we forgot to take pictures of last year when we were building these! Some of the photos below are from our SFG on the front deck, and some from the back deck, so apologies if the changing background is confusing.)
1. Cut the plywood in half the short way to generate two 4' x 4' panels. These are the bottoms of the raised beds.
2. From each 1" x 8" x 8' board, cut two 1" x 8" x 3'11.25" pieces (ok, the first two dimensions will actually be 0.75" x 7.25" if you're using standard dimensional lumber, but don't panic. It will be ok! The length is what's important.) Use these pieces to form a box and make sure they fit your piece of plywood properly, like so:
It might not be super important for the shallow depth of soil here, but forming the boards in sort of a pinwheel configuration around the edge is supposed to make a stronger box since outward forces of the dirt always have to fight a set of perpendicular screws. (If you made two longer and two shorter ones, outward forces would have to fight two sets of perpendicular screws in one direction, but only two sets of parallel screws in the opposite direction, making it easier for the boards to pull away from each other.)
3. Screw the sections of 1" x 8" together with two screws in the end of each board. Drill pilot holes for the screws first to avoid splitting the boards. Then flip the whole thing over and screw the plywood onto the box you just made with the boards, adding three screws to each side. While it's upside down, drill several holes in the bottom for drainage. Try a 0.25" drill bit--big enough not to get clogged too easily, but small enough that all your dirt won't run out with a hard rain! Then you can flip it back over, putting your wood scraps underneath to further help the drainage and keep your wooden deck from rotting (if that's where you're putting the SFG).
4. If you want to grow climbing veggies like cukes or pole beans, or unwieldy veggies like tomatoes, you can add a trellis. Cut the conduit pieces into a section of about 6'3" (leaving the remainder about 3'9") for the trellis. Use two 10-foot sections of conduit per block, connecting with the elbows and tees.
5. Attach the trellis to the box with the clamps as shown. It helps to have another person hold the trellis up while you screw in the clamps.
6. Fill the box with dirt, or compost, or whatever your favorite growing medium is. Four feet by four feet by six inches deep works out to eight cubic feet if you're going to buy dirt (e.g., if you're desperate like us and had to). Adjust the box to it's final position before adding the dirt, because it is much harder to move when full. We used high-organic matter potting soil to keep the weight down, but it tends to dry out quickly.
7. On the outside or top of the boards, measure one foot increments from the edges and pound a nail partway in at each increment (or sink in a screw partway), then make a grid with string, twine, jute, or whatever you prefer. Tie six strings across the nails/screws marking the one-foot increments (three one way, three perpendicular) to make a grid of 16 one-square-foot patches.
On the side with the trellis, it would be better to put the grid nails/screws on the outside rather than the top. That way it's easier to anchor any strings you tie to the trellis to help support the plants.
Also tie strings from the top of your trellis to the nails/screws you added on that side to complete your trellis and give your peas and cukes a leg up!
9. Plant all the stuff you meticulously planned out with your garden spreadsheet and keep an eye on it. One thing about having such a thin layer of soil is that it constantly needs watering. We set up a gravity-fed drip irrigation system that we'll talk about in a future post.
10. Wait for the veggies to grow up, then gobble them down. Yum yum!
Do you have experience with square foot gardening? Do you prefer this type of set up or deeper containers for growing vegetables on decks and patios? Tell us about it in the comments section below!