Details Landscape Art is a Sonoma County landscape contractor, designing and building fine gardens since 1991. Several times a year we are asked by clients to construct above ground vegetable boxes.
We build our boxes out of surfaced redwood. Rough redwood can be used, but to minimize splinters, smooth S4S redwood is a better choice. For our purposes here we will assume that the box to be built will be seven and a half feet long, by three and a half feet wide, and two feet tall. The posts will be three feet long to allow one foot to be buried in a hole, which will NOT be filled with concrete.
Starting with the posts, we will need to cut six 4×4 posts three feet long, three posts along each side. So there will be one post at each corner, and a center post on each side. Next we will cut 2×12 surfaced redwood boards to length. We will need to cut four pieces seven and a half feet long, and four cut to three and a half feet for the ends. Two long pieces are stacked (total of 24 inches when two 2x12s are stacked) and screwed to the posts that have been laid out on a hard surface. This makes one side panel. This is repeated to build the other side panel. Next, the two side panels are held vertically (with the posts on the INSIDE) as the shorter end pieces are screwed to the corner posts, creating the end panel. Repeat for the other end panel. The basic rectangular box is now complete.
Place the box in the desired location and mark the holes for the posts. Move the box out of the way and dig the holes one foot deep. We recommend applying ‘Copper Green to the one foot section of post below the box to retard the deterioration of the posts due to moisture and earth to wood contact. Move the box into the holes and backfill the holes. Be sure and level the box on all sides (hole depths may need some adjusting).
Next, we like to line the inside of the 2×12 side panels with sheeting plastic, to prevent tannins from the wood leaching into the vegetables. Since the 24 inch deep box doesn’t need to be filled with soil (vegetables don’t have deep roots), we suggest filling the box halfway with gravel for fast drainage. Then cover the gravel with porous weed cloth, and fill almost to the top with veggie mix soil. The veggie soil mix ingredients will give the veggies a good nutritious start, but after a full planting cycle, new veggie mix must be added before planting the next crop.
Lastly, we like to finish off the top edge of the box with a nice 2×6 redwood cap, with 45-degree corners. If more than one box is to be built, be sure to leave room between the boxes to have ample access to all sides of each box.
One more thing…plant your veggies and enjoy!