Should a raised bed be open to the ground?

Yes. An open base lets worms and roots move down and gives natural drainage. Line with cardboard, not plastic.

How much soil does a raised bed need?

Volume = length x width x depth. A 1.2m x 2.4m bed at 30cm depth needs about 0.86 cubic metres, roughly 30 bags of 30 litres.

Herbal Home Remedies

How Deep Should a Raised Bed Be?

Q: How deep should a raised bed be?

30cm (12 inches) suits most vegetables. Use 15cm only for lettuce, herbs and radish; use 40cm or more for tomatoes, carrots, potatoes and squash.

Q: Is 15cm deep enough for a raised bed?

For shallow-rooted crops like lettuce, herbs and radish, yes. For carrots, tomatoes or potatoes it is not enough.

Most raised beds are built 15 cm deep because that is the height of a standard timber. That works for salad and herbs, and fails for carrots and tomatoes.

Depth by crop

Depth	What grows well	Notes
15 cm / 6 in	Lettuce, spinach, radish, herbs, spring onions	Minimum viable bed. Dries out fast.
25 cm / 10 in	Bush beans, peas, onions, beetroot, chard, most herbs	The practical default for most gardeners.
30 cm / 12 in	Carrots, peppers, determinate tomatoes, broccoli	Carrots need this to grow straight.
40 cm+ / 16 in+	Tomatoes, cucumbers, squash, potatoes, parsnips	Best water retention and root room.

Why deeper is almost always better

Water — a 15 cm bed holds roughly half the water of a 30 cm bed, so it needs watering about twice as often
Temperature — deep soil buffers heat and cold; shallow soil swings with the weather
Root room — carrots fork and tomatoes struggle in shallow beds
Weeds — a deep bed smothers what is underneath far more reliably

If your bed is already shallow

You do not have to rebuild it. Three options:

Add a second tier — bolt another 15 cm board on top. Cheapest fix.
Choose shallow-rooted crops — lettuce, herbs, radish and beans all crop well in 15 cm.
Mound the soil — a 10 cm mound over a 15 cm bed gives 25 cm effective depth for the price of compost.

Does a raised bed need a base?

No. Open to the ground is better — worms and roots can move down, and drainage is natural. Line the bottom with cardboard to suppress weeds, not plastic, which creates a swamp.

How much soil does that mean?

Volume = length × width × depth. A 1.2 m × 2.4 m bed at 30 cm needs about 0.86 m³ — roughly 30 bags of 30 litres, or one bulk bag.

A simple mix: 60% topsoil, 30% compost, 10% coarse sand or perlite for drainage.

Common questions

Is 15 cm deep enough for a raised bed?

For lettuce, herbs, radish and spring onions, yes. For carrots, tomatoes or potatoes, no — they need 30-40cm.

Should a raised bed sit on soil or concrete?

On soil is far better. On concrete you need at least 30cm of depth because roots cannot go down, and drainage must be built in.

How deep for carrots?

At least 30cm of loose soil, with no stones or fresh manure, or they fork and split.

Can a raised bed be too deep?

Practically no, though beyond 45cm you are paying for soil you will not use. 30cm suits most gardens.

Bed layouts, soil mix ratios and a simple rotation: 5 Minute Garden.

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