How do you build a raised vegetable bed?
A raised vegetable bed is a frame of soil under 1.5 m wide, so you can reach the centre without standing on it. Fill it mainly with topsoil (7 parts, plus 3 of well-rotted organic matter and 2 of sharp sand) and expect to water it more often in summer than a bed at ground level.
By region
| North | Centre | South | |
|---|---|---|---|
| When it helps | On cold, heavy or waterlogged soil: raised soil warms earlier in spring. | If the ground drains badly, or if you want to work without bending to the floor. | Drainage helps soil that puddles. In summer, though, the bed dries out fast. |
| Water | Even in the north it dries faster than a bed at ground level. Check the moisture and keep a mulch on it. | Same as in the north: less often than in the south, but more than a bed level with the soil. | In the hot months you need mulch and water at the base, not on the leaves. |
A raised bed does not change the frost calendar: the soil warms a little earlier, and it needs more water in hot weather.
The RHS describes two effects that land differently across Italy: raised soil warms earlier in spring, and in summer it dries out faster than soil at ground level. Sowing dates stay those of the planting calendar: a frame does not cancel a frost.
Size
Keep the width under 1.5 m (5 ft) so you can reach the centre from the sides. About 1.2 m (4 ft) is comfortable from both sides: that is the width the RHS uses for beds you work without standing on. Run the long side north to south if you can, in the sun and out of the shade of trees. Paths at least 45 cm (18 in), and at least 1 m if a wheelchair has to pass.
Depth: 30 cm (1 ft) is enough for salad leaves. For most vegetables, tomatoes included, go towards 45 cm (18 in). A taller bed means less bending, and also more soil and more water. Tomato timing and spacing are in when to plant tomatoes.
How to build it
Timber, brick, stone or recycled plastic. The RHS lists pressure-treated timber as a durable, widely used choice; engineering bricks cope with frost better than soft domestic ones. Creosote-treated railway sleepers are not for the garden: the RHS says so because of the risk from frequent skin contact. If you paint untreated wood with a preservative, line the inside so the product does not reach the soil.
You can build in winter, when there is time, but not while the soil is frozen or sodden. Mark the rectangle, level the first course and screw it together. Lift the turf on the base and loosen the soil so roots can go down. On a bed deeper than 50 cm (20 in), a coarse layer can go at the bottom and the good soil on top: it drains, and you use less topsoil.
A brick or stone wall higher than 20 cm (8 in) needs mortar and a footing, about 15 cm (6 in) of concrete on 15 cm of hardcore, roughly 30 cm (1 ft) wide. On concrete or tarmac, drill drainage holes in the sides and use the sandier mix described below. A bed on a balcony weighs as much as the wet soil inside it: check what the floor can carry before you build. For pots, rather than beds, see the container garden guide.
What soil to use
The mix the RHS gives for most beds is 7 parts topsoil, 3 parts well-rotted organic matter (garden compost, leafmould or well-rotted manure) and 2 parts horticultural sharp sand, not builders' sand. On a hard surface the sand goes up to 4 parts, still with 7 of topsoil and 3 of organic matter, and water has to be able to leave.
Compost on its own slumps within a few seasons. Fill to about 5 cm (2 in) below the rim, firm it and leave it to settle for a couple of weeks, then top up if it has sunk. Each year, while the bed is empty, add a few centimetres of well-rotted compost. Don't fill it with soil from a bed that had diseased plants: why is under pests and diseases.
Pros and cons
Pros, from the RHS: you choose the soil, you drain ground that stays wet, the soil is a little warmer in spring, you don't walk on the bed, and covering or rotating crops is simpler. Rotation still matters in a small bed: families are in crop rotation and pairings in companion planting.
Cons: it costs money and has to be built, untreated timber rots and needs repair, and the level sinks and has to be topped up. It dries out faster than open ground. The RHS suggests feeling the moisture a finger's depth down, and mulching if the soil is often dry: materials and depth are in mulching. Water at the base, under the mulch, is the job described in watering and drip irrigation.
If your soil already drains and you don't tread on it, a bed at ground level will grow as much as a raised one. A raised bed fixes a problem. It does not stand in for soil that was already good.
In the app
Every plant in the catalogue has its own sowing calendar, and weather alerts for your area help you judge whether to sow now or hold off.
Frequently asked questions
How wide should a raised vegetable bed be?
Under 1.5 m (5 ft), so you can reach the centre without stepping on it. About 1.2 m (4 ft) is comfortable. Paths at least 45 cm (18 in).
What soil goes in it?
Mostly topsoil: 7 parts, plus 3 of well-rotted compost or manure and 2 of horticultural sharp sand. Compost on its own slumps within a few seasons.
How deep should it be?
30 cm (1 ft) is enough for salad leaves. For most vegetables, aim for about 45 cm (18 in). The taller it is, the more soil it takes and the faster it dries.
Can I put it on a balcony or on concrete?
On concrete, yes, with drainage holes and a sandier mix (4 parts sand instead of 2). On a balcony, wet soil is heavy: check what the floor can carry before you build.
Do I actually need one?
No. It earns its place where the soil is poor, heavy or waterlogged, or if you want to work higher up. On soil that already drains, a bed at ground level grows just as well.