Vegetables in particular benefit from pre-warming of soils, helping crops to germinate earlier than usual, increasing the length of the cropping season. Covering the soil will warm it during the day and slow down heat loss at night.

Early crops that benefit from sowing in late winter or early spring into pre-warmed soil include:

• Beetroot (bolt-resistant cultivars)
• Broad beans
• Calabrese
• Carrots (early cultivars)
• Kohl rabi
• Leafy salads including rocket
• Leeks
• Lettuce
• Onions
• Parsley
• Parsnips
• Peas
• Radishes
• Spinach
• Turnip (early forcing cultivars)

Tender crops sown in late spring or early summer into pre-warmed soil include:

• Courgettes
• French beans
• Outdoor cucumbers
• Runner beans
• Squash
• Sweetcorn

Seedbeds for ornamental plants can also be pre-warmed.

Clay soils are slow to warm up in spring, due to their retention of water after the winter. Warming can help on such soils but where soils are waterlogged it may not be very efficient. In fact, raising young plants in pots and cell trays is a more practical way to get early crops from clay soils.

Pre-warming is not difficult but does require a little planning, as late winter and early spring sun is weak:

• Covering from early January until early April is ideal for early crops, but for tender later crops, covering can be effective even if delayed until mid-spring
• Allow soil moisture to be replenished by winter rains before covering
• Cloches can be left on for a little longer after planting to speed up growth

Just a few simple steps are needed to pre-warm soil:

1. Prepare soil by removing weeds from the surface to make a seedbed, adding a natural-based general fertiliser if required at the same time
2. Cover with cloches or biodegradable or re-used fleece.
3. Clay soil must lose water by evaporation before it will warm significantly and for this reason cloches are preferred for pre-warming clay soils
4. Leave covered until sowing, ideally for at least six weeks if covered in winter, but four weeks may be sufficient if covered later
5. Remove cover and any weeds growing underneath
6. Sow or plant and, ideally, re-cover with the cloches or fleece

Fleece and cloches can blow around in the wind if not securely fitted, negating any warming effect. To avoid this, weigh down fleece around the edges with bricks, or tuck the edges into the soil with a spade.

Glass cloches can be heavy and easily broken so care is needed when using them.