Preparing raised beds for spring sowing

Why raised beds give you a head start

One of the quiet pleasures of a raised bed is that it dries out and warms up weeks before the ground around it. Because the soil sits above the surrounding level, it catches more sun on its sides, drains freely and is not compacted by feet. In a chilly UK spring, that difference of two or three degrees in the top 10cm can mean the difference between seeds sulking in cold, wet soil and seedlings racing away.

So before you think about sowing, spend a morning getting the bed into the best possible condition. It is the single most useful job you can do in March, and it pays you back all season with healthier plants, fewer weeds and far less watering.

Clear the bed before anything else

Start by taking off anything still standing from last year — spent brassica stems, old bean poles, bolted salad leaves — and add the healthy material to the compost heap. Leave anything that looks diseased or carries white rot well away from your compost.

Next, deal with the weeds by hand. In a raised bed this is genuinely manageable, so do it properly rather than skimming the surface.

• Perennial weeds: couch grass, bindweed, dock and dandelion all regrow from fragments. Fork gently under each clump and lift the whole root run out, following it as far as it goes.
• Annual weeds: chickweed, bittercress and groundsel come away easily with a hoe on a dry day. Leave them on the surface to wilt, then collect them.
• Never rotavate a weedy bed. Chopping roots into pieces simply multiplies the problem across the whole bed.
• Edges and corners: weeds hide where the timber meets the soil. Run a hand fork along the inside face of the boards.

If you can, cover the cleared bed with a sheet of black polythene or a thick layer of cardboard for a fortnight. It smothers the first flush of weed seedlings and stops the soil being leached by spring rain.

Top up with compost and organic matter

Raised beds sink. Between settling, cropping and the worms pulling organic matter downwards, the level can drop by 5cm or more over a season, so plan to top up every spring.

Spread a layer of well-rotted garden compost or manure 5–8cm deep across the surface — roughly two or three barrowloads per square metre. If you are filling a new bed, mix roughly one part compost to two parts topsoil rather than filling it entirely with compost, which can be too rich and holds water like a sponge.

Do not dig this in. Leave it on the surface as a mulch and let the worms and weather work it into the soil. For hungry crops such as courgettes, brassicas and potatoes, you can also tickle a handful of blood, fish and bone per square metre into the top few centimetres a week or so before planting. If you grow brassicas on the same ground year after year, a light dusting of lime in late winter helps keep clubroot at bay.

Loosen the soil without digging it over

A raised bed should never be walked on, so in most cases the soil structure is already good. All you need is a fork pushed in to its full depth, rocked gently back and forth, and lifted. Work backwards across the bed so you never stand on ground you have just loosened.

You are aiming to open the soil, not turn it upside down. Inverting the layers buries the aerobic life near the surface and drags weed seeds up into the light. Remove any stones bigger than a golf ball and break up obvious clods with the back of the fork, but leave the crumbs slightly rough — a fine tilth too early just slumps in the first heavy shower.

If the soil sticks to your boots and compresses into a shiny ball in your hand, it is still too wet. Wait a few days; working wet soil on a heavy clay does damage that takes years to undo.

Warm the soil before you sow

This is the step most gardeners skip, and it is the one that makes the biggest difference in an unpredictable spring. A cheap soil thermometer is the best small investment you can make.

• Hardy crops — broad beans, peas, spinach, early carrots and salad leaves — will germinate once the soil holds steady at about 7°C.
• Half-hardy crops — beetroot, chard, onions and lettuce — want 10°C or so.
• Tender crops — French and runner beans, sweetcorn, courgettes and squash — need 12°C and are usually best left until late May in most of the UK.

To speed things up, lay black polythene over the bed for two or three weeks before sowing. It traps warmth and keeps the rain off. Cloches, fleece and low hoops do the same job for individual rows, and a fleece cover also keeps the worst of the wind and any late frost off young seedlings. Take fleece off on mild, sunny days so pollinators and air can get in.

Plan your first sowings

With the bed cleared, fed, loosened and warmed, you can start sowing in earnest. Mark out rows with a taut line or a bamboo cane, sow thinly at the depth on the packet, and label everything — spring is exactly when you forget which row was which.

Consider intercropping: a quick row of radishes or cut-and-come-again lettuce between slower crops makes good use of the space while the main plants fill out. Keep the soil moist but not soaked, and thin seedlings early so the ones you keep have room to grow.

Get the preparation right and the rest of the season becomes a pleasure: strong roots, fewer weeds and a bed that gives you something to harvest from early summer right through to autumn.