Raised Bed Depth: How Deep Is Deep Enough
Depth is the first decision in any raised bed build, and it cuts both ways: too shallow and roots hit the bottom and stall; too deep and you pay for soil nobody’s roots will ever reach. Here is how deep each crop actually needs, and what depth costs you in soil.
8–12 inches of soil suits almost every vegetable; 6 inches is the minimum for salads and herbs; give carrots, parsnips, and tomatoes 12–18 inches. Open-bottom beds borrow depth from the ground below, so 8 inches of good mix over decent soil grows nearly anything.
Depth by crop
Roots need uncompacted soil to at least the depths below. Shallow crops are happy in 6 inches; deep-rooted crops keep growing well past 12.
| Crop group | Minimum depth | Notes |
|---|---|---|
| Salad greens, rocket | 6 inches | Shallow roots; the classic 6-inch bed crop |
| Herbs | 6–8 inches | Deeper for rosemary and dill taproots |
| Garlic, onions | 8–10 inches | Bulbs form in the top few inches |
| Bush beans, peas | 8–10 inches | Fibrous, moderately deep roots |
| Peppers | 10–12 inches | Deeper soil = sturdier plants |
| Sweetcorn | 10–12 inches | Needs depth for anchorage |
| Tomatoes | 12 inches | Plant deep; stems root along their length |
| Potatoes | 12+ inches | Hill soil or mulch up around stems |
| Carrots, parsnips | 12–18 inches | Root must grow straight and unobstructed |
What depth costs in soil
Every extra inch of depth is extra soil to buy. For a rectangular bed:
Cubic feet = length (ft) × width (ft) × (depth in inches ÷ 12)| Depth | Soil for a 4 × 8 ft bed | In cubic yards |
|---|---|---|
| 6 inches | 16 cu ft | ≈ 0.6 cu yd |
| 8 inches | ≈ 21 cu ft | ≈ 0.8 cu yd |
| 12 inches | 32 cu ft | ≈ 1.2 cu yd |
| 18 inches | 48 cu ft | ≈ 1.8 cu yd |
Doubling depth from 6 to 12 inches doubles the soil bill — so build deep only where your crops need it. One money-saving trick: fill the bottom half of a deep bed with bulky organic matter (logs, branches, straw) and put quality mix in the top 8–12 inches where most roots feed.
Open bottom vs closed base
Open bottom (frame sitting on soil) is best whenever the ground beneath is decent: roots grow straight through into it, drainage is natural, and earthworms move in on their own. Loosen the ground with a fork before setting the frame.
Closed base is needed on concrete, contaminated ground, or balconies. Then the bed must supply the full root depth itself — build at least 12 inches deep — and drainage becomes your job: leave gaps between base boards or drill holes, and add a 2-inch layer of coarse gravel or bark at the bottom.
On either base, skip the weed fabric if you can: it blocks deep roots and worms. A 3–4 inch mulch layer on top suppresses weeds just as well.
Know the depth? Price the soil
Deeper beds need a lot more soil. Work out exactly how many cubic feet and bags your bed takes before you buy.
Frequently asked questions
Is a 6-inch-deep bed enough?
For salads, herbs, radishes, and spring onions — yes. For fruiting crops like tomatoes and peppers, and all root vegetables, step up to 10–12 inches minimum. When in doubt, build 12 inches: it grows everything.
Can I put a raised bed on concrete?
Yes. Use a closed base at least 12 inches deep, ensure drainage gaps or holes, and expect to water more often — concrete beds dry faster and can’t draw moisture from below.
Should I put landscape fabric under my bed?
Usually not. On an open-bottom bed it blocks the very roots and worms you want moving between bed and ground. Use cardboard instead if weeds are the worry — it smothers them and rots away within a season.
How deep does soil need to be for tomatoes?
Twelve inches of good mix is the practical minimum, and deeper is better. Plant transplants deep — burying the stem up to the first true leaves — since tomatoes root all along the buried stem.
Related guides
How Much Soil Do I Need for a Raised Bed?
Turn your bed dimensions into cubic feet and bag counts.
How to Grow Tomatoes in a Raised Bed
Deep roots, big harvests — tomatoes done right.
Square Foot Gardening: Spacing for Small Beds
Plan what goes into the bed once it’s built.
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