What to Put at the Bottom of a Raised Garden Bed

Before you shovel in a single bag of soil, there is a question worth settling: does the bottom of a raised bed need anything at all? The answer depends on what is under the bed and what you are trying to keep out.

Do you actually need a bottom layer?

A raised bed does not strictly require anything at the bottom. If your bed sits on open ground and you are happy for roots to grow down into the earth below, you can fill it and plant. But a bottom layer solves a few specific, common problems, and which one you have decides what - if anything - to add.

Weeds and grass pushing up from below. A smother layer stops the existing sod from growing through into your new bed.
Burrowing pests such as voles, gophers, or moles coming up into the roots. A metal barrier blocks them.
A hard surface underneath - a patio, deck, or gravel pad - where you need to protect the surface and let water escape.

If none of those apply, the honest answer is that you can leave the bottom open and let the bed connect directly to the ground, which is what most vegetables prefer anyway.

Cardboard, hardware cloth, and gravel

The trick is to match the material to the problem rather than layering in everything at once:

Cardboard or thick newspaper is the go-to for smothering grass and weeds. Lay plain, un-waxed cardboard with the tape removed, overlap the seams generously, and wet it down. It blocks light long enough to kill the sod beneath, then breaks down within a season so roots and worms pass freely into the soil below.
Hardware cloth - a galvanized metal mesh, usually 1/4 or 1/2 inch - is the answer for burrowing rodents. Staple it across the bottom of the frame before you fill. Ordinary plastic landscape fabric will not stop a determined gopher; the metal will.
A thin gravel or landscape-fabric layer makes sense mainly when the bed sits on a solid surface like a patio or balcony, where you want water to drain out without washing soil through the gaps.

You rarely need more than one of these. Piling on extra layers "just in case" mostly wastes material and can trap water where you do not want it.

The drainage myth worth unlearning

The most persistent piece of raised-bed advice is to add a layer of gravel or rocks at the bottom "for drainage." For a bed sitting on open ground, this usually does the opposite of what people expect. Water does not slide freely from fine soil into a coarse gravel layer; it tends to pool at the boundary until the soil above is saturated, an effect known as a perched water table. You can end up with soggier roots, not drier ones.

If drainage is your real worry, the better fixes are a well-structured soil mix and a bit more depth. A loose, airy blend like Mel's Mix drains and breathes far better than any rock layer, and simply building the bed deeper gives roots more well-drained room above whatever sits underneath.

How depth changes how much soil you buy

Whatever you decide about the bottom, it feeds straight into the one number that costs real money: how much soil to buy. Soil volume is length × width × depth, so depth is not a detail - going from 6 inches to 12 inches doubles the soil you need. A bottom layer that takes up space, like a few inches of gravel or a mound of logs in a hugelkultur-style fill, reduces the soil you have to purchase, while an open bottom means every inch of the frame gets filled.

It is easy to under-buy here, because a bed drinks more than it looks like it should. For the full formula and worked examples, see our guide on how much soil you need for a raised garden bed. When you would rather skip the arithmetic, the Raised Bed Soil Calculator turns your bed's length, width, and fill depth into an exact bag count with a settling buffer built in.

Sort out the bottom first - smother it, block it, or leave it open - then set your fill depth and let the Raised Bed Soil Calculator tell you exactly how many bags to bring home. One less thing to guess about on planting day.