What to Put in the Bottom of a Raised Garden Bed
Cardboard, hardware cloth, logs or nothing at all? What to line the bottom of a raised bed with depending on your weeds, pests and bed height, and why gravel is a bad idea.
What goes in the bottom of a raised bed depends on two things: what’s under it (lawn, weeds, concrete, gophers) and how tall it is. For most ground-level beds, the answer is simple: a layer of cardboard, then soil.
Option 1: Cardboard (most beds)
For a bed built on lawn or weedy ground, lay plain brown cardboard over the ground inside the bed.
- It smothers grass and weeds by blocking light.
- It breaks down within a season, so roots and worms can move into the ground below.
- Overlap the edges by several inches so weeds can’t sneak through.
- Remove tape and glossy labels, and wet it down before adding soil.
Newspaper (several sheets thick) works too.
Option 2: Hardware cloth (gophers, moles and voles)
If burrowing animals are a problem, staple galvanized hardware cloth, a stiff wire mesh with small openings, across the bottom of the bed frame before adding soil. Chicken wire is too flimsy and rusts out quickly. You can lay cardboard on top of the mesh.
Option 3: Weed barrier fabric (persistent weeds)
For aggressive weeds like Bermuda grass or bindweed, landscape fabric adds a longer-lasting barrier. The downside: it can also block worms and deep roots. Many gardeners prefer cardboard and simply pull the occasional weed.
Option 4: Logs and branches (tall beds)
Filling the bottom of a very tall bed with untreated logs, branches and leaves saves soil and slowly breaks down into organic matter, a method often called hugelkultur.
- Expect the soil to settle as the wood rots. Top it off each year.
- Wood can tie up nitrogen as it decomposes. Keep it at the bottom, under at least 12 inches of soil.
- Avoid treated lumber, walnut and cedar chips.
What not to put in the bottom
- Gravel or rocks “for drainage.” It sounds logical, but water doesn’t move easily from fine soil into a coarse layer. It actually holds water in the soil above until it’s saturated, which can make drainage worse.
- Plastic sheeting. It traps water and can drown roots.
- Pressure-treated wood scraps, which can leach chemicals.
- Diseased plant material or weeds gone to seed.
On concrete or a patio
Beds on hard surfaces need drainage and depth. Add a layer of coarse organic material or a thin layer of wood chips to let water escape at the edges, and make sure the bed has at least 12 inches of soil. A bed with its own bottom, like an elevated planter, is easier.
Then fill it
Most vegetables do best in at least 10 to 12 inches of good soil, often a mix of topsoil, compost and something light like peat or coir. Here’s how much soil you need for a raised bed.
Still wondering?
Should I put cardboard in the bottom of my raised bed? For beds on grass or weeds, yes. It smothers them and breaks down in a season.
Do I need to dig up grass first? No. Cardboard on top does the work.
Does a raised bed need a bottom? Ground-level beds don’t, which lets roots go deeper. Elevated beds have a built-in bottom.
Shopping for a bed? See the best raised garden beds for vegetables.