Why a raised bed earns its place

A raised bed is simply a box of soil sitting above the ground, and that small change solves a surprising number of problems. You start with soil you've chosen rather than soil you've inherited, so heavy clay, old builder's rubble or a thin layer over chalk stops being a barrier. Drainage improves, the soil warms up faster in spring, and weeds are far easier to keep on top of because you're not walking on the growing area and compacting it.

There's a practical bonus too. Raising the surface by 25–30cm means less bending and kneeling, and a bed roughly 1.2m wide can be reached from both sides without ever stepping on the soil. Even a small courtyard, a strip of gravel or a tired patch of lawn can become productive.

Pick the spot and measure up

Most vegetables, herbs and flowering annuals want at least six hours of direct sun a day, so watch your garden through a sunny morning before deciding. Avoid sitting a bed directly under a tree or large shrub — the roots will quietly steal moisture and nutrients from below, and the canopy will shade your crops.

Next, measure. You need the length, the width and the depth you'd like, because those three figures decide how much timber and soil you'll buy.

• Width: keep it to 1.2m maximum so you can reach the centre from either side. If the bed sits against a wall or fence, 60cm is more realistic.
• Length: whatever suits the space. 2.4m is a common, manageable size and matches standard timber lengths.
• Paths: leave 60–75cm between beds so you can kneel down and wheel a barrow through.

If you're building over grass, cut the turf off and stack it upside down in a corner to rot down into lovely loam. Over paving, you can build straight on top — just check the surface is reasonably level.

Choose untreated timber and the right size

For a bed growing food, use untreated timber. Pressure-treated wood (often called tanalised) and old railway sleepers contain preservatives you'd rather keep out of your soil and your carrots. Untreated boards will gradually rot, typically lasting five to eight years, which is a fair trade for peace of mind.

Good options include untreated scaffold boards, usually around 225mm wide, or standard 150mm x 50mm timber. Two boards stacked gives you a 30cm deep bed, which suits most salads, beans, courgettes and herbs. Root crops such as carrots and parsnips prefer 30–40cm, so go deeper if those are your priority. A 75mm x 75mm post at each corner holds everything rigid.

Assemble the frame

Cut four corner posts about 45cm long. Mark out the rectangle, then drive each post roughly 15cm into the ground at the corners, checking with a spirit level as you go. Screw the boards to the posts using galvanised screws — they'll last far longer than plain steel in damp soil.

Before you fix the final side, measure the two diagonals of the frame. If they match, your bed is square; if not, nudge a corner until they do. On hard standing, skip the buried posts and join the boards with metal angle brackets instead. If you fancy a comfortable perch while you work, screw a wide plank along the top edge of one side.

Fill it properly

Work out your volume first: length x width x depth. A bed of 2.4m x 1.2m x 0.3m comes to about 0.86 cubic metres, which is roughly 25 bags of 35 litres — so buying a bulk bag of topsoil and a bulk bag of compost usually works out cheaper.

Aim for a mix of roughly 60 per cent topsoil and 40 per cent well-rotted compost or manure. On heavy clay, stir through a few handfuls of horticultural grit per barrowload to improve drainage. Fill in layers, firming gently with your hands as you go — you want the soil settled, not squashed flat. Then water it thoroughly and leave it a week or two to settle before planting, topping up any dips.

Your first season and beyond

Start with forgiving crops. Salad leaves, radish, beetroot, chard, spring onions and dwarf French beans all crop quickly and reward you early. Add a courgette plant, a few tomatoes or a wigwam of climbing beans, and don't forget flowers — cosmos, calendula, nasturtiums and sweet peas grow happily alongside vegetables and bring in the pollinators.

Water regularly through dry spells, mulch the surface with compost to hold moisture, and feed hungry crops such as tomatoes once they start flowering. Each spring, top the bed up with a 5cm layer of fresh compost and rotate what you grow where, which keeps pests and diseases guessing. Treat the timber as a consumable, not a permanent structure, and you'll get a decade of good growing from a weekend's work.