Question: How should we go about building sturdy raised beds for our school gardens? Are pressure-treated wood landscaping timbers safe to use?
Answer: Studies suggest that toxins from pressure-treated wood do leach into soils and can reach dangerous levels. To be on the safe side, avoid pressure-treated timbers in children's gardens. Instead of pressure-treated wood, use Eastern or Western red cedar, black locust, or imitation wood made of recycled plastic.
The key to sturdy, long-lasting beds is to anchor them not only to the ground but also to each other, because they tend to warp and shift over time. First level the area so the timbers will lay flat on the soil surface. Lay out the timbers, overlapping each layer at the corners like a log cabin. This will help strengthen the beds.
Next, drive large 6-inch spike nails into the timbers to anchor the second level to the first (this can be repeated for a third or fourth level if you want beds that high). This can be a tough procedure; the long nails tend to bend easily. To make it easier, drill pilot holes (slightly smaller than the nails) into the timbers before driving in each nail. Place nails in three places along the timber, but not at the overlapping corners.
After nailing together the bed timbers, drill 3/8-inch holes downward through the corners of the timbers where they overlap. With a sledgehammer, drive in 3/8-inch rebar rods (available from many hardware stores) cut long enough to go through the timbers and into the soil 1 foot deep. For safety, don't allow any rebar to extend above the top timber. Repeat the rebar anchoring along the length of the beds (once for beds one timber long and twice for beds two timbers long). Then fill the beds with soil mix and start your garden.