Why Bother With a Raised Bed?

A raised bed does three useful jobs at once. It gives you deep, loose soil that roots can push through easily, it lifts the growing area above cold, heavy ground, and it means you can work standing up or perched on a stool rather than kneeling on wet clay. For anyone with a bad back, a small garden, or soil that has been compacted by years of being walked on, a timber frame filled with good compost is a genuine upgrade.

The real beauty of it is that you control the soil. Fill the bed with a decent mix and you can grow carrots, parsnips and beetroot in ground that would otherwise produce forked, stunted roots. You also get better drainage in winter and warmer soil in spring, which means earlier sowings and a longer season.

Choose the Right Timber

For vegetables, stick to untreated timber. Pressure-treated and tanalised boards contain preservatives you would rather not have leaching into the soil your lettuce is growing in, and old railway sleepers are often soaked in creosote. Untreated boards will rot eventually — expect five to eight years from softwood, considerably longer from oak, larch or Douglas fir — but that slow decay is harmless and adds organic matter to the ground beneath.

Good options include:

• Untreated scaffold boards — roughly 225 mm wide, sturdy, and often available second-hand.
• New untreated timber in 200 x 50 mm sections, which is easy to cut and screw.
• Reclaimed boards from a local yard, provided they have not been treated with anything dubious.
• Oak or sweet chestnut if your budget stretches — they weather beautifully and last for years.

Whatever you use, avoid anything that has held chemicals, oil or paint. If you are unsure of a board's history, line the inside with a sheet of cardboard or a breathable membrane before filling.

Getting the Size Right

This is the detail people most often get wrong. Keep the bed no wider than 1.2 metres. That is the distance an average adult can reach across from either side without stepping onto the soil. Stepping on a bed compresses it, and compacted soil is the enemy of good roots.

Aim for these dimensions:

• Width: 1.2 m maximum, or 0.9 m if you only have access from one side.
• Length: 2.4 m is a sensible standard, matching the length of most timber without waste.
• Depth: 400 mm is generous but lets you grow carrots and maincrop potatoes. Two courses of 200 mm boards will do it; 250 mm is workable if your ground beneath is decent.

Site the bed in full sun — six hours or more — and within reach of a tap or water butt. Leave at least 600 mm between beds so you can move a wheelbarrow down the path.

Building the Frame

Clear the ground of perennial weeds first, then lay the boards out dry to check the fit. You will need four corner posts of 50 x 50 mm timber, cut to about 600 mm long.

• Cut two boards to 2.4 m and two to 1.2 m. If you are stacking two courses, cut four of each.
• Drive the corner posts 300 mm into the soil with a mallet, checking with a spirit level as you go.
• Screw the boards to the posts using 75 mm exterior-grade screws, pre-drilling to avoid splitting. Two screws per board end is plenty.
• Add a mid-post along each long side if the boards bow, and line the inside with cardboard to suppress weeds underneath.

Leave the base open. A solid bottom stops worms and drainage, and there is no advantage to it.

Filling and Planting Up

Fill the bed with a mix that will hold moisture but drain freely. A reliable blend is roughly 60 per cent topsoil, 30 per cent garden compost and 10 per cent well-rotted manure, though you can happily use all compost if you have a good supply. Fork it in loosely and do not tread it down — the whole point is to keep the structure open.

Leave the surface 20-30 mm below the rim so that watering and mulching do not spill over the edge. Then plant. Salad leaves, radishes and beetroot will crop within weeks, while courgettes, runner beans and tomatoes will fill the space by midsummer. Tall crops such as sweetcorn or climbing beans belong on the northern side so they do not shade their neighbours.

Keeping It Productive

A raised bed dries out faster than open ground, so water thoroughly rather than little and often, and spread a 50 mm mulch of compost or straw each spring to hold moisture in. Top up the soil level annually as it settles — an inch or two of compost every year keeps fertility ticking over.

Rotate your crops between beds or between halves of a single bed: roots, then brassicas, then legumes, then potatoes. It interrupts pest cycles and stops any one set of nutrients being stripped out. Finally, check the timber each spring. If a board has softened at the base, replace it in autumn when the bed is emptiest, and the frame will outlast the garden that surrounds it.