Preparing Raised Beds for Spring Planting

Why Raised Beds Warm Up First

Raised beds are the quiet heroes of the cold-weather garden. Because the soil sits 20–25cm above the surrounding ground, it is exposed to sun and moving air on all four sides, drains freely, and dries out far sooner after a wet spell than beds cut straight into the earth. In practice, that usually means raised beds are workable a fortnight or more before the open ground — a gift in a British spring, when the difference between sowing in early March and late March can decide whether you get a crop at all.

The most useful tool you can own right now is a soil thermometer. Push it 10cm down, leave it a few minutes, and read it. Most hardy seeds germinate from around 7°C, while beans and sweetcorn want 10–12°C. If your bed is still sitting at 4°C, be patient: seed sown into cold, soggy soil tends to rot rather than sprout. If your beds face south and sit against a wall or fence, they will be warmer still, while beds in a frost pocket at the bottom of the garden may lag a week behind.

Clear Weeds and Debris Before You Feed Anything

Before a single barrowload of compost goes on, deal with what is already growing. Winter weeds such as chickweed, hairy bittercress, annual meadow grass and groundsel germinate right through the cold months and will set seed by March if you let them. Slide a hand fork underneath and lift them out roots and all, rather than hoeing them off at soil level.

• Perennial roots — dandelion, bindweed, creeping buttercup — need removing in full. Every fragment left behind becomes next season's problem.
• Old crop debris should go, particularly brassica stalks and any foliage that showed signs of clubroot, whitefly or downy mildew.
• Legume roots are the exception: leave the roots of beans and peas in place and they will release a little nitrogen as they break down.
• Timber edges deserve a glance too. Tap loose boards back into place and replace anything that has rotted through, because a collapsed side means lost soil and a wobbly bed all summer.

Work from the paths, not the beds, so you are not compacting the very soil you have spent years improving.

Top Up with Compost — and Don't Dig It In

Beds settle by 5–10cm over a growing season, so expect to top them up every year. Aim for a layer of 5–8cm across the surface, which is roughly a barrowload per square metre. Well-rotted manure, homemade compost and leafmould are all ideal; spent mushroom compost is useful in moderation but is alkaline, so avoid it on beds where you plan to grow blueberries or potatoes.

Spread the material evenly and then simply leave it. Worms, beetles and the soil's fungal networks will pull it down over the coming weeks, and the mulch layer will suppress early weeds at the same time. If your soil is heavy clay, mixing in a little horticultural grit or extra leafmould will improve the texture. If you limed last autumn, check the pH before sowing anything that dislikes alkaline conditions.

Protecting Nutrients from Winter Rain

This is the step most gardeners skip, and it costs them. Heavy winter rainfall washes soluble nitrogen and potash straight down through the profile and out of reach of young roots. Covering beds over the wettest months keeps those nutrients where you want them and stops the surface panning and capping.

• Tarpaulin or black polythene laid over the bed is the simplest option. Weight it well against wind and remove it in late winter so you can top-dress and sow.
• A thick organic mulch of compost, leafmould or straw cushions the impact of rain and slowly feeds the soil as it breaks down.
• Never leave soil bare over winter. Bare earth loses structure, leaches nutrients and grows more weeds than any covered bed.

If a bed has been covered for weeks, peel back the sheet and check the moisture before sowing. Waterlogged soil should be left to drain for a few days, and if the surface is still sticky, wait a little longer.

Cloches, Fleece and Warming the Soil

Once the bed is clear and topped up, you can give it a head start. Horticultural fleece draped over hoops adds roughly 2–3°C, and a double layer more than that; it also keeps flea beetle and pigeons off young leaves. Cloches, low polytunnels and cold frames do the same job for individual rows. On bright days, open the ends for an hour or two so plants do not cook or damp off.

If you want to sow a fortnight earlier, lay black polythene over the bed for two to three weeks beforehand. It warms the top 10cm noticeably, and the difference in germination speed is often striking.

What to Sow First

With beds cleared, fed and covered, you can think about early crops. These will shrug off a late frost with a little protection:

• Broad beans sown in autumn or early spring, and garlic and onion sets, which need a cold spell to bulb up well.
• Early peas, radish and Perpetual spinach, all of which germinate in cooler soil.
• Winter lettuce, land cress, lamb's lettuce and chard for cut-and-come-again leaves.
• Kale, purple sprouting broccoli and parsley, planted out under fleece until the weather settles.

Hold back tender crops — courgettes, beans, tomatoes, sweetcorn — until the soil has reached 10–12°C and the risk of frost has passed. In most of the UK that means mid-May, and nearer the end of the month in northern areas. Prepare the beds now, and you will be sowing into warm, fertile, weed-free soil while everyone else is still waiting for the ground to dry.