Vegetable growers might be itching to get started, but the soil is still far too cold for sowing or planting. There are, however, ways to reduce the waiting time.
Seeds native to our temperate region will germinate at much lower temperatures than those from warmer areas. You can sow peas and brassicas when the soil is 7-10C, but tomatoes and cucumbers need more than double that.
So, before sowing or planting, buy a soil thermometer to check out your ground. Take the reading at the depth where young plant roots grow, roughly 4-5cm down. Like the sea, soil temperature changes very slowly and doesn’t fluctuate madly. From a low in February of 4-5C it rises to 15C in summer before sinking back.
Obviously, you can’t sow outdoors until all risk of ground frost is past, because low temperatures freeze the water on and just below the soil surface, breaking up the top crust – exactly where you sow seed – and damaging infant roots and shoots. But you can start some seedlings off over the next few weeks ready for planting out. If you don’t have a greenhouse or a propagator, use a warm, dark airing cupboard for germination, putting the seed tray in a clear plastic bag to keep the compost moist. Transfer to a windowsill once the first seedlings emerge.
Light levels are only half as high on a windowsill as in a greenhouse, so to avoid having leggy plants move them outside on sunny days and bring them indoors in the late afternoon. This increases light levels by 50 per cent.
Whether you prefer to buy plants or grow from seed, you can increase the temperature in beds for early crops. Ideally, you should start warming up the ground three or four weeks before planting out.
If the ground has been protected over winter with a mulch, muck, rough compost or green manure, start working over the area five or six weeks before planting. Then start warming it up.
Firstly, remove a mulch. Muck or compost should be dug in and the ground raked as finely as possible. Cut down a green manure with shears or a strimmer, allow it to wilt, then dig it in. Grazing rye exudes an allelopathic chemical that inhibits seed germination for a month after cutting back, so factor this into your planning.
The temperature always rises more quickly in raised beds, especially those with recycled plastic sides that readily warm up on sunny days. You can also increase soil temperature in raised beds and the open ground by covering with a cloche. There are several designs and some raised beds come with frames for supporting a membrane.
If you can, work the soil, remove weeds and dig and rake as usual. Once you’ve got a fine tilth, cover with the cloche. If the ground is still too claggy, lay down the cloche, let the soil dry out a bit, then work it over. Finely prepared soil will warm up much more quickly.
You must use cloche hoops to raise the membrane off the ground. This allows for air circulation and lets surplus water escape. The ground will remain soggy if a plastic membrane is simply spread on top.
Use either clear plastic or fleece for a cloche. The plastic raises the temperature much more efficiently, but, after planting, you’ll have to roll it back on sunny days to prevent scorching.
Fleece, on the other hand, raises ground temperature by little more than 2C after a few weeks. But you can leave the fleece cloche in place, whatever the weather. You’ll get best results by starting with plastic and switching to fleece after planting.
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