Building a Raised Garden Bed That Lasts More Than Five Years

Raised beds fail in three ways, and all three are decided when you build it: the timber rots, the long sides bow outward, or something burrows in from underneath.

None of them are fixed by spending more on soil.

Dimensions follow reach, not aesthetics

Width: 4 feet. That is two feet from each side, which most adults can reach. The entire purpose of a raised bed is soil you never stand on, so a bed you cannot reach across defeats itself.

Against a wall or fence, halve it — 2 to 3 feet, reachable from one side only.

Length: whatever suits the space, but brace anything over six feet.

Path width between beds: 18 inches minimum for feet, 24 to 30 if you want a wheelbarrow through. This is consistently underestimated and then regretted permanently.

Height:

Depth is where the money goes. Doubling the height doubles the fill, and fill costs more than timber.

Timber, and the pressure-treated question

The liner mistake

Plastic sheeting stapled to the inside faces feels like it protects the timber. It does the opposite: it holds water permanently against the wood, with no chance to dry, and rots it faster than plain soil contact would.

What is worth putting in:

• Hardware cloth across the base, stapled before filling, if you have gophers, voles or moles. Half-inch galvanised mesh. This is not optional where those animals are present.
• Cardboard over grass, which smothers the turf, then rots away and lets worms and roots through into the soil below.

And what to leave out: a drainage layer of gravel at the bottom. It does not improve drainage — it raises the saturated zone in the soil above it, for the same reason gravel in a plant pot makes things worse.

Building it

1. Level the ground. Dig the high side down rather than packing the low side up. A bed on a slope racks and spills.
2. Cut the boards and lay them out to check the footprint and the diagonals.
3. Corners: use posts or brackets. A 4×4 post inside each corner, screwed through the boards, is the simplest strong joint. Screwing into end grain alone will pull apart within a couple of years as the timber moves.
4. Exterior structural screws, pre-drilled in hardwood. Nails work loose.
5. Cross-brace anything over six feet — a timber across the middle, or a threaded rod through both sides with washers. Wet soil pushes outward with real force, and bowed sides are the single most common raised-bed photograph on the internet.
6. Staple the mesh or lay the cardboard, then fill.

Filling it

Roughly 60 percent topsoil, 30 percent compost, 10 percent aeration — bark fines, perlite, or coarse sand. That is a soil, not a potting mix, and it is the right thing for a volume this large.

Expect it to settle several inches in the first year as the organic matter breaks down. That is normal and is not a fault. Top up with compost every spring — a raised bed is a system you feed annually, not a container you fill once.

For deep beds, the bottom third can be filled with coarse organic material — logs, branches, turf stacked upside down — which reduces cost and rots into the bed over years. It settles a lot, so expect to top up more.

Where to put it

• Six hours of sun minimum for vegetables; eight is better.
• Level, or made level.
• Near water. You will carry more than you expect, and raised beds dry out faster than open ground.
• Not directly under trees — shade, root competition, and drips.
• Orient long beds north–south if you can, so plants shade each other less through the day.