Guide
What to put in the bottom of a raised bed
Cardboard, logs, leaves or nothing at all, depending on what the bed sits on.
Short answer
On open ground, nothing but a layer of plain cardboard to smother the grass. In a bed deeper than about 18 in, fill the bottom with logs, branches and leaves: in a 4 × 8 ft bed 24 in deep, a 12 in layer of them saves about 35.8 cu ft of soil, 18 bags of 2 cu ft. Skip the gravel layer: in a bed on soil it does not improve drainage.
It depends on what the bed sits on
- Grass or weeds: lay plain brown cardboard, tape and labels removed, overlapping by a hand’s width, and wet it. It smothers the grass, rots down within a season, and worms come up through it. It does a better job than landscape fabric, which also blocks roots and worms from the soil below.
- Bare soil in good condition: nothing. Loosen the top few inches with a fork so the bed’s soil and the ground knit together, and roots will carry on into it.
- Paving, a patio or a roof: a few inches of coarse bark, gravel or broken pots over a membrane, so the water has somewhere to go and the soil does not wash out through the joints.
- Gophers or voles: galvanised hardware cloth across the whole base, stapled to the frame, under anything else.
Fill a deep bed with wood and leaves, not all soil
Tall beds are expensive to fill, and roots rarely use the bottom foot. The cheap way is the old hugelkultur approach: rotten logs and branches first, then leaves, grass clippings and spent plants, then 12 in of good soil on top. The wood soaks up water and releases it slowly, and everything below breaks down into compost over a few years.
| Fill | Soil | Bags of 2 cu ft |
|---|---|---|
| Soil all the way down | 71.7 cu ft | 36 |
| 12 in of logs and leaves, 12 in of soil | 35.8 cu ft | 18 |
Expect the bed to sink a few inches over the first two years as the bottom layer rots, and top it up with compost each spring. Use hardwood or well-rotted wood; fresh pine and cedar work but break down slowly, and avoid treated, painted or walnut wood.
Why not gravel?
A layer of gravel under soil does not drain a bed faster. Water will not move from fine soil into coarse gravel until the soil above it is saturated, so the gravel layer holds a band of wet soil right where the roots are, the opposite of what it is there for. Drainage in a raised bed comes from the soil mix and from the open ground under it. The exception is a bed on paving, where the gravel is a drain, not an improvement.
The cheapest way to fill a raised bed
- Fill the bottom third to half with free material: logs, branches, leaves, grass clippings.
- Buy the top 12 in as soil, in bulk if you need more than about a cubic yard (27 cu ft), which is usually a single 4 × 8 ft bed 12 in deep. Bulk soil costs roughly half as much per cubic foot as bags.
- Blend in your own compost where you can, and top up with compost every year rather than buying new soil.