Guide
How deep should a raised bed be
The depth each crop uses, and what each depth costs in soil.
Short answer
Twelve inches suits most vegetables, and it takes about 35.8 cu ft of soil in a 4 × 8 ft bed (18 bags of 2 cu ft). Six to 8 in is enough for lettuce, greens and herbs; tomatoes, squash, potatoes and long carrots do better at 18 in or more, or in a 12 in bed over loosened ground.
Depth by vegetable
| Depth | Crops |
|---|---|
| 6 to 8 in | Lettuce, spinach, radish, spring onions, most herbs, strawberries |
| 12 in | Beans, peas, peppers, cucumbers, kale, chard, beetroot, onions, garlic, short carrots |
| 18 in and deeper | Tomatoes, squash and pumpkins, potatoes, parsnips, long carrots, asparagus, rhubarb |
Roots do not stop at the bottom of the bed. On open ground they carry on into the soil below, which is why a 12 in bed on loosened ground grows tomatoes well: loosen the ground with a fork before filling and the bed’s depth and the ground’s add together. On paving, a patio or compacted fill, the bed’s depth is all the root run there is, so build it to the full depth the crop needs.
Soil needed at each depth, for a 4 × 8 ft bed
| Depth | Soil | Cubic yards | 2 cu ft bags |
|---|---|---|---|
| 6 in | 17.9 cu ft | 0.66 | 9 |
| 8 in | 23.9 cu ft | 0.88 | 12 |
| 10 in | 29.9 cu ft | 1.11 | 15 |
| 12 in | 35.8 cu ft | 1.33 | 18 |
| 18 in | 53.8 cu ft | 1.99 | 27 |
| 24 in | 71.7 cu ft | 2.65 | 36 |
Past about 18 in, fill the bottom with logs and leaves rather than soil: see what to put in the bottom of a raised bed. Taller beds, 24 to 30 in, are also far easier on the back and knees, which is a good reason to build one even when the crops do not need it.
What the 70/30 rule means
It is a rule of thumb for filling a raised bed: about 70 percent topsoil and 30 percent compost, mixed together. It is cheaper than a bed of pure compost or potting mix and holds its structure for years. Sources give 60/40 as often as 70/30, and either works; what matters more is that the topsoil is screened and free of clay clods, and that you top up with compost every year.