Three ways to warm soil for sowing

Polythene cloche over a raised bed

Soil in spring can take a while to warm up, especially heavier soils such as clay. So, if you’re sowing seed in March or April, it’s a good idea to warm the soil before you sow. If seeds are sown in cold soil, there’s a risk that they won’t germinate and will rot.

Read about sowing seeds outdoors.

Different soil types warm up at different rates – open sandy loam warms up more quickly than clay. The warmth of the soil also depends on the local weather, particularly how cold the winter was and how sunny conditions are when you’re sowing. Hardy varieties can cope better with lower temperatures than half-hardy and tender plants – use a soil thermometer to check the temperature of the soil before you plant, soil with a temperature lower than 5ºC should be warmed before sowing.

Here are few tricks to help warm your soil in spring.


Cover with polythene

Covering a raised bed with polythene
Covering a raised bed with polythene

Cover the surface of the soil with a sheet of polythene, mulch fabric or roofing felt. The protective cover will keep out cold rain and late snow. Don’t worry about the colour of the material – dark colours will absorb heat from the sun and transfer it to the soil, but clear polythene will enable the sun to warm the soil directly.


Cover with a cloche

Polythene cloche over a raised bed
Polythene cloche over a raised bed

Often used to cover seeds and young plants, cloches are even more beneficial if you put them in place two to three weeks before sowing. They trap air, allowing it to warm in the sun, and this gradually percolates into the soil surface.


Cover with mulch

Measuring soil temperature through a layer of straw mulch
Measuring soil temperature through a layer of straw mulch

Mulch works in two ways. Garden compost, straw or manure acts as a layer of insulation and protects the soil from frost. It’s especially useful when applied in autumn, as it can prevent the ground from freezing (or lessen its impact). Simply removing this mulch when air temperatures increase in spring will enable the sun to warm the soil quickly.

If you’re applying a mulch in spring then choose straw. Its coarse structure traps pockets of air, which warm up and insulate the soil from cold winds. For best results, add a 10cm layer of straw on the soil surface and weight it down using planks, bricks or ridge tiles.

 

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