Raised Bed Planning Guide

Most raised bed disappointments are decided before a seed goes in: too wide to reach across, too shallow for carrots, shaded from two o'clock, or filled with cheap topsoil that sets like plaster. None of it is fixable once the bed is planted, so an evening of planning buys a decade of use.

Sizing a Bed You Can Reach Across

Width is the only dimension with a hard constraint: you must never need to stand in the bed, because compaction defeats the whole approach. Most adults have a comfortable bending reach of 60 to 70 cm (24 to 28 in), which gives three standard widths.

• 1.2 m (4 ft) with access from both sides. The default, for good reason.
• 60 to 75 cm (2 to 2.5 ft) against a wall or fence, with one-sided access.
• 90 cm (3 ft) if you garden seated or have limited reach.

Length is free within reason, but beds over about 4 m (13 ft) annoy you, because walking round the end becomes a real cost. Leave paths of at least 45 cm (18 in), and one main path of 75 to 90 cm if you use a wheelbarrow.

Depth: What Your Crops Actually Need

Depth decides your crop list, and raising a bed later means dismantling it, so be honest now. What counts is total rootable depth: the frame plus the soil beneath. Fork the base of a bed on open ground and a 20 cm frame behaves like a 40 cm one; over paving, the frame is all you get.

If you build only one bed, make it 30 cm deep over forked ground.

Siting: Sun, Water and Access

Sun comes first. Six to eight hours of direct summer sun allows fruiting crops; four to six restricts you to leaves, roots and alliums. Remember that the deciduous tree you judge in March is a solid wall by June, and run long beds north to south so tall crops do not shade what is behind them. More detail in how much sun vegetables need.

Water comes second. A 1.2 by 3 m bed in a dry July wants 60 to 100 litres a week, and carrying that from an indoor tap is a chore you abandon by August.

Access decides more than people expect. You visit a bed by the back door daily and one at the end of the garden weekly, and daily visits are how you catch slugs early. Avoid the base of slopes, which are frost pockets and waterlog.

Materials and How Long They Last

Untreated softwood is cheap and rots. Everything else trades cost against years.

• Untreated softwood, such as scaffold boards: 3 to 6 years. Fine if you accept rebuilding.
• Pressure-treated softwood: 10 to 15 years. Modern copper azole and ACQ treatments replaced arsenic-based CCA and are considered safe for food growing.
• Larch or Douglas fir: 10 to 15 years untreated, a good middle ground.
• Oak sleepers: 20 years plus. Avoid reclaimed railway sleepers, usually creosoted.
• Corrugated galvanised steel: 20 years plus and quick to build, though it warms soil early.

Use 38 mm (1.5 in) boards or thicker, stake the midpoint of anything over 3 m because wet soil pushes hard, and fix with galvanised screws.

Filling the Bed Without Buying a Lorry of Compost

A 1.2 by 3 by 0.3 m bed holds just over one cubic metre. Filling it entirely with bagged compost is expensive and, ironically, worse for plants than a mixed fill, because pure compost slumps by a third in the first year.

A good fill is roughly 50 percent decent topsoil, 30 percent well-rotted compost and 20 percent leaf mould, mixed rather than layered. For deeper beds, bulk out the bottom third and spend your money on the top.

1. Fork the base to break any pan and let roots and worms through.
2. Lay logs, prunings or cut turf, grass-side down, in the bottom third.
3. Add a green layer such as grass clippings, to offset nitrogen drawdown as the wood decays.
4. Top with 25 to 30 cm of the topsoil and compost mix.
5. Water heavily and leave two weeks to settle before planting.

Expect the level to drop 10 to 15 percent in the first season, and top up in autumn. Refuse cheap unseen topsoil; if it smells sour, send it back. For amending principles, see the soil and fertiliser basics guide.

Laying Out Crops: Spacing and Succession

Raised beds allow closer spacing than rows, because you are not leaving room to walk. Plant on a grid or in staggered triangles: lettuce 20 cm, beetroot 10 cm, carrots 5 cm, bush beans 20 cm, chard 25 cm, brassicas 45 cm, tomatoes 45 to 60 cm, courgettes 90 cm. Plants close canopy, shade the soil and suppress weeds, which is half the benefit.

Succession turns one bed into a season's food. Sow salad and radish in short runs every two to three weeks rather than one long row, follow early crops with late ones so garlic lifted in July gives way to autumn salad, and interplant radish between parsnips to mark the row.

Sketch the bed using each plant's mature spread, not the seedling. Nearly every first-year bed is overplanted, and crowded plants produce less food overall.

Rotation Across Two or Three Beds

Classic four-year rotation assumes four plots, but a simplified version still reduces club root, white rot and eelworm build-up.

• Legumes: peas, broad beans, French and runner beans. They fix nitrogen.
• Brassicas: cabbage, kale, broccoli, swede, radish. Heavy feeders, so follow legumes.
• Roots and alliums: carrots, parsnips, beetroot, onions, garlic, leeks. Follow brassicas.
• Everything else: potatoes, tomatoes, courgettes, salad. Flexible fillers.

With three beds, run legumes, then brassicas, then roots, moving one bed along each year. With two, alternate legumes and brassicas against roots and everything else. The rule is simple: never grow the same family in the same soil two years running, and leave three years between alliums.

Record what went where, because a rotation you cannot reconstruct is not a rotation. If a notebook has not stuck, a free Vegarden account keeps a bed-by-bed plan you can copy forward each season.

Year Two and Beyond

A bed improves for years if you leave the structure alone and feed the surface. Do not dig it. Each autumn, clear spent crops, spread 3 to 5 cm of compost, and leave it; worms incorporate it by spring, whereas digging inverts the profile and brings weed seed up. Keep it covered over winter, with a green manure such as field beans sown in September or with cardboard.

Watch three signals. A level dropping more than 5 cm a year means the organic fraction is burning off, so add bulkier material. Water pooling on top means it has capped, so fork the top 5 cm. Steadily worse yields from one family point at rotation, not fertility.

Build one good bed rather than three mediocre ones. The second is far easier once the first has taught you something.

Timber eventually needs replacing, which is your chance to correct the width or depth you got wrong.