Britain's growing season is short and its summers are unreliable, and cover is how you deal with both. It brings crops forward in spring, holds them later in autumn, produces salad through the winter, and makes tomatoes, peppers and cucumbers realistic when outdoors they often are not.
The commonest mistake is not underestimating the cold. It is forgetting to open the door.
What each one buys you
Cover does not only extend the season. It changes what you can grow: tomatoes, peppers, chillies, aubergines, cucumbers and melons all become realistic under cover in Britain and are marginal to impossible outdoors.
It also lets you grow through the winter, which is the part that matters most for this app. Winter salad, land cress, mizuna and spinach under cover crop in February when nothing outdoors does. See salad and fast crops.
A polytunnel is the best value if you have the space: far more covered ground per pound than a greenhouse, quick to put up, and the cover is replaced every several years rather than the whole structure.
Fleece and cloches
The cheapest end, and worth having even if you have a tunnel.
Horticultural fleece is a light spun material laid directly over a crop or over hoops.
- Two or three degrees of frost protection, which is often the difference in April and October.
- Excellent as an insect barrier: carrot fly, cabbage white, flea beetle, leek moth. See pests without chemicals.
- Lets rain and light through, so it can stay on.
- Weight the edges properly or the wind takes it.
- Remove it for pollination on anything that needs insects.
Cloches are rigid covers: bought tunnels, home-made hoops with polythene, or a cut-down plastic bottle over a single plant.
- Warm the soil for a fortnight before sowing, which does more than covering afterwards.
- A cut bottle over a transplant protects from slugs, wind and cold at once, and costs nothing.
- Ventilate them. A closed cloche on a sunny March day cooks what is under it.
Cold frames
The most underrated structure, and the easiest to build from salvage.
A box with a sloping glazed lid, ideally facing south. Old windows, a pallet frame, offcuts of polycarbonate. See salvage.
What it is for:
- Hardening off. Moving plants from indoors to outdoors in stages, which is the step people skip and which loses more plants than frost does.
- Overwintering salad, herbs, and tender plants.
- Early sowings weeks before the open ground is workable.
- Late crops held on into November.
- Raising plants for gaps, which is the technique behind continuous production.
Open it on sunny days and close it in the late afternoon, which is the whole management. A prop stick with notches gives you settings.
Insulate it in hard weather with a piece of old carpet, bubble wrap or sacking over the lid overnight.
Polytunnels and greenhouses
Siting matters more than the structure.
- Full sun, and not overshadowed in winter when the sun is low.
- Sheltered from the prevailing wind. Wind is what destroys polytunnels, at the ends and the doors.
- Level ground, and near water.
- Orientation: for a tunnel used year round, running north to south generally gives more even light. East to west catches more winter sun and is better if winter cropping is the priority.
Building a tunnel:
- Anchor it properly. Ground tubes concreted in, or long anchor plates. Every tunnel that has blown away was inadequately anchored.
- Bury or batten the cover into a trench so wind cannot get under it.
- Fit the cover on a warm still day. Polythene stretches in warmth and goes drum tight as it cools, and fitting it cold and slack gives you a flapping cover that fails early.
- Doors at both ends, which is the ventilation.
- Expect to replace the cover every several years. Buy the better grade with the longer guarantee.
A greenhouse is longer lived, better for propagation, and easier to heat a corner of. It is also more expensive per square metre and breaks.
Ventilation, which is the thing that kills
Humidity is the other half. Still damp air is what botrytis, mildew and damping off need. Air movement is the control, and it is free.
- Both doors open on a tunnel whenever it is not freezing.
- Prop the cold frame even on cool bright days.
- Do not water in the evening under cover, which leaves everything damp overnight.
- Space plants so air moves between them.
- Remove dead and yellowing leaves promptly.
In winter, ventilate on any mild dry day. A tunnel shut for a fortnight of damp weather grows mould on everything in it.
Watering under cover
Rain does not get in. Obvious, and routinely forgotten in spring when everything outside is wet.
- Everything under cover depends entirely on you, from the day you put the cover on.
- Water the soil, not the leaves, and water in the morning.
- Deep and rare still applies, and containers still need frequent watering. See watering and drought.
- A water butt beside the tunnel fed from its own gutter, or from a nearby roof, saves a great deal of carrying.
- Soil under cover dries out much faster than open ground, and it also accumulates salts over years if only watered from the mains. Occasionally leaving the cover off, or flooding it, helps.
Pests and disease under cover
A warm sheltered space with no rain and no predators is excellent for pests as well as for plants.
- Red spider mite thrives in hot dry air. Damping down the floor and keeping humidity up suppresses it, which is a direct trade against the ventilation advice and has to be balanced.
- Whitefly on tomatoes and brassicas. Sticky traps, and biological controls are effective in an enclosed space.
- Aphids, without the predators that would find them outdoors.
- Slugs, which love the sheltered damp conditions.
- Mice, which move in over winter and eat seed and stored crops.
- Fungal disease: botrytis, mildew, damping off. All are humidity problems and all are reduced by ventilation and spacing.
Biological controls work well under cover precisely because it is enclosed, which is the one advantage of the environment.
Clean the structure annually. Wash the glazing or the polythene, which is what your winter light depends on, and clear out debris where pests overwinter.
Rotate. Growing tomatoes in the same tunnel soil year after year builds up disease. Change what goes where, or change the soil, or grow in containers.
The winter limit is light
The thing that surprises people who put up a tunnel expecting winter production.
From about November to late January, growth nearly stops, and it is day length and light intensity that stop it, not temperature. Heating a structure in December does not produce growth, because there is not enough light for the plant to use.
The consequence:
- Plants must reach a usable size before the light goes. Sow winter crops in August and September. What they have grown by early November is roughly what you have to work with.
- Pick sparingly through the darkest weeks, a few leaves at a time.
- Growth restarts in February, which is when a well-stocked tunnel earns its keep, right in the hungry gap.
- Do not bother heating for winter production. Heat is for frost protection and for propagation in late winter, not for growing.
Windowsills
No structure, no cost, and genuinely useful.
- Propagation in February and March, weeks before anything can go outside. A bright windowsill starts tomatoes, chillies, brassicas and leeks.
- Herbs all year, which transform stored food.
- Pea shoots and microgreens in trays, ten days from sowing to cutting.
- Sprouted seeds in a jar, needing no light at all. See sprouting seeds.
- Turn seedlings daily, because a windowsill lights them from one side and they lean badly.
- Leggy pale seedlings mean too little light, not too little heat, which is the commonest indoor propagation error.