Warming the Soil

Information sheet

By warming the soil in spring you can sow seeds and harvest crops earlier in the year.

  • School term: Early Spring
  • Level of experience: No experience needed
  • Subject(s):

Benefits of soil warming

School gardening benefits from an early start in the spring term as the harvesting deadline at the end of the summer term arrives all too soon. Hardy vegetables such as broad beans, carrots, lettuce, peas, radish and hardy salad leaves can be sown once the soil temperature is above 7oC. This can be as early as February or March, especially if you cover the soil to help it warm up more quickly.

Covering the soil also encourages weed seeds to germinate earlier. This means you can hoe them off after two or three weeks, before you start sowing and planting. This ‘sterile seed bed’ treatment reduces the weed seed bank, so fewer weeds will grow among your crops later in the year.

Cloches and tunnels

Flat sheets are fine for soil warming, but to keep the young crops warm you will need to use cloches or make tunnels, supporting the covers with metal or plastic hoops. Commercial cloches are widely available and are worth the investment. They either have a built-in frame or are made of rigid materials. They should be stored away when not in use to prolong their life. Some fit over and attach to raised bed kits such as Linkabord.

DIY soil-warming tunnel

To make your own soil warming tunnel you will need:

  • Blue alkathene water pipe (available from builders merchants) cut to 2m lengths for a 1.2m wide bed, or metal wire 5mm diameter cut into 3m lengths
  • Bamboo canes or similar, 60cm long
  • Plastic pegs or tent pegs
  • String
  • Plastic sheeting, 3m wide, available from good garden centres
  1. Cut the pipe to length and insert a cane in each end. The canes will anchor the pipe in the soil. If you are using wire, bend it roughly to shape first.
  2. Insert the hoops at 1m intervals along the bed, pushing the canes or wire down well into the ground.
  3. Put a peg in the centre of each end of the bed.
  4. Tie string to one end peg, then loop it once or twice round the centre of each hoop, pulling tight each time, then tie to the peg at the other end of the bed. This will help hold the hoops upright.
  5. Cut the plastic sheet to size and lay it over the hoops.
  6. Push a peg into the ground at each end of each hoop
  7. Tie string to the pegs, looping it over the sheet close to the hoop and tying to the peg at the other side. This will hold the sheet in place.
  8. Gather the ends of the sheet together and either tie to the peg which is already there or weigh down with bricks.

Other uses for hoops

Once the crops are growing well, and the weather has improved you can remove the plastic or fleece. The hoops can then be used to support netting to keep out birds and butterflies, or insect-proof mesh to exclude smaller pests including carrot fly, cabbage root fly and aphids.

Soil warming basics

Sandy soils are free draining, so dry out easily and warm up quickly in the spring. Heavy clay soils hold on to moisture and so they are slow to warm up. Both benefit from covering to speed up warming.

First you should prepare the soil by removing any existing weeds, then lightly fork the surface of the soil and rake level. The simplest way to warm the soil is to lay a clear plastic sheet over the surface and either tuck the edges under the soil, or weigh it down. This dries out the soil and traps in warm air.

When you see weeds germinate this indicates that the soil is warm enough for crops to grow. Hoe off any weeds that appear and level off the soil, mark your rows and sow your seeds.

Plastic or fleece?

Covers can be made from plastic sheet or horticultural fleece. Plastic will warm the soil more than fleece but the crops have to be inspected and watered if necessary.
Fleece is a permeable material allowing light and water to pass through. It will warm the soil, especially if used double, but to a lesser degree. It is good at protecting the growing crop and since it allows rain to come through watering is less of a problem.

Suppliers of cloches

Harrod Horticultural (www.harrodhorticultural.com)
The Organic Gardening Catalogue (www.organiccatalog.com)
Linkabord (www.linkabord.co.uk)