Why a raised bed earns its keep

If your soil is heavy clay, thin and stony, or you simply cannot bend quite so far as you once did, a raised bed is the kindest thing you can do for your back and your brassicas. You control the growing medium rather than arguing with whatever nature left you, drainage improves out of all recognition, and weeds are far easier to pull because you never walk on the soil and compact it. Twenty minutes with a hoe on a Saturday morning keeps on top of most of it.

There is a practical bonus too: a raised bed warms up earlier in spring, so you can sow a fortnight or three weeks ahead of open ground, which matters a great deal if you garden in the north or on a chilly, exposed plot.

Siting and sizing the bed

Choose the sunniest spot you have. Vegetables want six hours of direct light a day, and more for tomatoes, courgettes and beans. Avoid the rain shadow of a fence or hedge, and keep well clear of overhanging trees whose roots will steal both water and nutrients from below.

Width matters more than length. 1.2 metres is the sweet spot, because you can reach the centre from either side without treading on the soil. Length is entirely up to you and the space available. Leave paths of at least 60 centimetres — 90 if you want to wheel a barrow round — and remember to allow for the thickness of the timber when you measure.

For depth, think about what you actually want to grow:

• 20–25cm for salads, herbs, radish and spring onions
• 30cm for carrots, beetroot and most brassicas
• 40cm or more for maincrop potatoes, parsnips and other long-rooted crops

Building the frame

Timber is the traditional choice and the simplest to work with. Untreated timber will last a good decade if it is not sitting in permanently wet ground. If you want a longer life, use pressure-treated timber and line the inside faces with a permeable membrane so roots and soil never come into direct contact with the preservative. Avoid old railway sleepers unless you know their history — they were once soaked in creosote.

Cut four sides, then fix them with galvanised screws or coach bolts driven through corner posts set inside the frame. Two or three courses of timber stacked and screwed through the face is ample for a 25cm bed. Sit the frame on a bed of sand or a shallow gravel trench so it stays level and does not sink at one end, and check it with a spirit level before you fill. A wonky bed is a permanent annoyance.

If the bed sits on soil, loosen the ground beneath with a fork to help drainage and root penetration. On paving or gravel, you will need a permeable membrane across the base to stop your expensive topsoil washing away down the joints.

Lining and filling

Line the inside of the frame with a woven, permeable membrane — never solid polythene, which turns the bed into a bath and drowns roots in a wet British winter. Staple it to the timber with the fabric running up the sides and, on hard standing, across the base. Trim it just below the top edge so it is hidden by the soil.

The fill is where most people go wrong. A good working mix is roughly:

• Six parts topsoil — decent loam, not subsoil scraped off a building site
• Three parts well-rotted compost or manure, at least a year old
• One part sharp sand or horticultural grit if your soil is heavy clay

Mix it in a barrow or tip it in layers and fork it together, then firm gently as you go. Fill to within 3–4cm of the top so there is room to water without it running straight off. Finish with a scattering of blood, fish and bone or a general-purpose organic fertiliser, and let the bed settle for a week before sowing.

Planting through the year

A raised bed will crop almost all year round if you keep it working hard.

• March to May: sow radish, lettuce, spinach, chard, beetroot and carrots in short rows, and plant out onion sets and early brassicas.
• May to July: runner and French beans, courgettes, sweetcorn and tomatoes — a bed against a sunny wall is ideal for cordon tomatoes.
• August to October: follow early crops with winter salad leaves, kale, winter lettuce and spring cabbage.
• October to November: plant garlic and overwintering broad beans for a flying start next season.

Sow little and often rather than one enormous row, and slot quick crops into the gaps between slower ones — radish between parsnips, for example.

Keeping the bed productive

Raised beds drain faster than open ground, so they need watering more often in dry spells. A soaker hose laid along the surface under a mulch saves a great deal of standing about with a watering can. Mulch with two to three centimetres of compost each autumn: it feeds the soil, suppresses weeds and slows evaporation through the summer.

Rotate your crops between beds where you can — legumes, then brassicas, then roots, then potatoes — to keep pests and diseases guessing. Top up the soil level each spring as the contents settle, and repair any timber that has begun to soften before it fails completely.

Look after the soil and it will look after you, season after season.