How to Build a Raised Garden Bed

For a 4 x 8 ft bed at 12 inches deep you need six 8-foot boards of 2x6 untreated cedar or Douglas fir, four 12-inch corner posts, exterior screws, a drill, a saw and a level. Total build time is an afternoon, most of which is leveling the ground.

1. Sun. Six or more hours of direct summer sun for fruiting crops. Watch the spot through a day before you commit -- shade moves, and a bed is not easily relocated once filled.
2. Water. Within reach of a hose. A bed you have to carry water to is a bed that gets watered less than it needs.
3. Level. Water runs downhill inside a bed too. A noticeably sloped bed ends up dry at one end and waterlogged at the other.
4. Access. Leave at least 24 inches of path around it, and more if you use a wheelbarrow.

Two courses of 2x6 gives about 11 inches of depth, which is a practical working height. The short sides are cut at 45 inches rather than 48 so that, with the long boards overlapping the ends, the finished external width is 4 feet.

1. 1

   Clear and level the ground

   Strip or flatten the turf and get the footprint level. This is the step people rush and the one that determines whether the bed drains evenly. Check with a level across both diagonals.

2. 2

   Assemble the frame

   Screw the first course of boards to the corner posts with the posts on the inside. Add the second course on top of the first, screwed to the same posts. Pre-drill near board ends so the timber does not split.

3. 3

   Fit the center brace

   Run a 2x4 across the middle of the long sides and screw it through from outside. On an 8-foot bed this is what stops wet soil pushing the sides outward.

4. 4

   Lay cardboard on the ground

   Plain corrugated cardboard, overlapped generously, with tape and labels removed. It smothers the grass and breaks down within a season, leaving the bed connected to the soil below.

5. 5

   Fill in layers

   Coarse woody material at the bottom if the bed is deep, then bulk soil and compost, then 10 to 12 inches of good growing mix on top. Water each layer as you go so it settles rather than sinking later.

6. 6

   Mulch the surface

   Two to three inches of straw or compost. It cuts evaporation substantially, suppresses weeds and steadies soil moisture through the season.

• Do not line the bed with plastic. It traps water against the timber and blocks drainage. Cardboard on the ground is all you need.
• Do not add gravel in the bottom for drainage. It creates a perched water table that leaves the soil above it wetter, not drier.
• Do not use old pressure-treated lumber of unknown age. Residential treatments moved away from arsenic-based CCA in 2003; anything older is not worth the doubt in a food bed.
• Do not build wider than you can reach. Four feet with access on both sides, 2 to 3 against a wall.

If you want steel, buy a kit -- fabricating coated steel panels at home is not realistic. If you want depth beyond two courses of timber, buy a kit. And if you price cedar boards at retail, a dovetail-corner cedar kit is frequently cheaper than the lumber for the same bed. Building is the right answer when you want a non-standard size or already have the timber.

Whatever you build or buy, the next step is the fill: how to fill a raised garden bed and the soil calculator.