Choose a sunny, sheltered spot

Almost every vegetable worth growing wants sun, and plenty of it. Aim for a spot that receives at least six hours of direct light a day in summer, ideally south or south-west facing. A little afternoon shade is a blessing in a heatwave, but a spot that sits in the shade of a fence or a mature tree until midday will leave you with leggy seedlings and sulking tomatoes.

Look up as well as around. Overhanging branches do more than cast shade — they drop leaves, suck moisture from the soil and harbour pests. If your only option is beneath a tree, consider moving the beds a metre or two clear of the canopy rather than giving up on the idea.

Shelter matters too. A bed on an exposed corner will have young beans snapped and tall crops flattened. A hedge, a fence or even a line of lavender upwind will take the worst of it. Finally, think about water. Siting beds within easy reach of a tap or a water butt saves an enormous amount of hauling in July, when a raised bed can lose a watering can's worth a day.

Work out the size and layout

The classic raised bed is 1.2m wide, because that is the distance most of us can comfortably reach across from either side without treading on the soil. Length is flexible — 2.4m suits most gardens and timber comes in convenient lengths — but anything much beyond 3m starts to feel like a trek. Keep beds to a width you can actually work.

Paths deserve more thought than they usually get. Allow 45cm between beds as a minimum, and 60–90cm on the main route so a wheelbarrow, a kneeling stool or a child on a scooter can get through. Grass paths are pleasant but need mowing; bark chip is cheap and quiet; gravel is tidy but weeds love it.

Height is a personal calculation. Fifteen to twenty-five centimetres is enough for salad, beans, courgettes and most herbs, and it saves a great deal of bending. Root crops such as carrots and parsnips want 30cm or more of loose soil, so either build deeper or accept shorter varieties. Deep beds cost more to fill, so build high only where you genuinely need it.

Build beds that will last

Sturdy beats pretty every time. Pressure-treated softwood is the sensible, affordable choice and will give you a decade or more; oak and larch last longer still; recycled plastic boards cost more but never rot. Avoid anything that has held creosote or engine oil, and if you use pallet wood, check it carries the HT (heat-treated) stamp rather than MB.

• Clear the ground of turf and perennial weeds first, then fork over the subsoil to break any pan.
• Level the site — a bed built on a slope will sit proud at one end and shallow at the other.
• Use corner posts of at least 7.5cm square, driven in with a mallet, and fix boards with galvanised screws or bolts.
• Line the base with overlapping cardboard if you are building over weedy ground; it suppresses growth and rots away.
• Check the frame is square before you fill it — once it is full of soil, it will not move.

Fill with a balanced mix

This is where raised beds succeed or fail. The temptation is to fill them with whatever is cheapest, but a poor growing medium will dog you for years. Aim for roughly 60 per cent topsoil, 30 per cent well-rotted compost or manure and 10 per cent sharp sand or grit, adjusting the grit upwards on heavy clay. A straight 50:50 topsoil and compost also works well if your soil is already decent.

Pure compost shrinks alarmingly as it decomposes, dries out to a crust and offers little anchorage for roots. Pure topsoil from a builder's yard is often lifeless and stony. The blend gives you structure, drainage and fertility in one go. As a guide, a 1.2m by 2.4m bed filled to 25cm deep needs around three-quarters of a cubic metre of material — enough to justify a bulk bag rather than dozens of plastic sacks.

Mix it roughly as you barrow it in, then leave the bed to settle for a week or two before sowing. It will drop by five centimetres or so. Top it up, firm it gently with your hands and rake the surface level.

What to sow, and when

Raised beds warm up earlier than open ground, which buys you a fortnight or three weeks at the start of the season. A sheet of fleece or a cloche stretched over the frame will extend that further.

• Easy first crops: salad leaves, radishes, chard, beetroot, spring onions and dwarf French beans.
• Worth the space: courgettes, runner beans, tomatoes and climbing peas along a south-facing edge.
• Sow little and often: a short row of salad or radish every two to three weeks gives you a continuous supply rather than a glut.

Respect the spacing on the packet. Crowded seedlings compete for water and light and are far more prone to mildew and bolting. Rotate your groups between beds each year — brassicas, roots, legumes and potatoes — to keep the soil healthy and pests guessing.

Keeping it productive

Raised beds drain faster than the ground beneath them, so water deeply and less often rather than little and often. A good soak twice a week in dry spells beats a daily sprinkle, and early morning is the best time. Mulch the surface with compost or straw to hold that moisture in.

Feed hungry crops — tomatoes, courgettes, beans and anything in a pot — from midsummer onwards. Weed little and often while the seedlings are tiny; a hoe on a dry morning deals with most of them. Net brassicas against pigeons, watch for slugs after rain, and pinch out the tips of runner beans when they reach the top of their supports.

Come autumn, clear spent crops, spread a layer of compost over the surface and let the worms work it in. Sow a green manure such as field beans or grazing rye on empty beds. Do that, and next spring you will start the season with soil that is better than the year before.