What is a cloche for, and when should I use one?

The value of a cloche in Britain is less about frost than about soil. Cold, wet ground in March is what stops seed germinating, and a sheet of anything transparent over the bed for a fortnight beforehand changes that measurably. The frost protection is a bonus rather than the main event.

Safety

What you'll need

• Cloches — tunnel, barn, bell or improvised
• Pegs, hoops or bricks to anchor them
• Horticultural fleece (optional)
• Clear polythene for soil warming (optional)

Step by step

1. Use it first to warm and dry the soil.Put a cloche or a sheet of clear polythene over the bed two to three weeks before you intend to sow. It raises the soil temperature by several degrees and, just as importantly, keeps the rain off so the surface becomes workable.
2. Then use it to protect the sowing.Early carrots, beetroot, salads, spinach and peas sown under a cloche in March come up faster and more evenly than the same sowing left open, and they are safe from the heavy rain that caps a seedbed.
3. Use it as a pest barrier.It keeps pigeons off brassicas and seedlings, and a fine mesh version keeps carrot fly and cabbage white butterflies out. For pest exclusion the edges have to be sealed to the ground, or it achieves nothing.
4. Use it to keep winter crops dry.A cloche over lettuce, chard, parsley or a row of alpines through winter does more good by keeping rain off than by keeping cold out. Most winter losses are rot rather than freezing.
5. Ventilate as soon as the weather turns.Open the ends of a tunnel cloche on mild days from about April, or lift bell cloches during the day. Sealed plastic in spring sun overheats quickly and produces soft, drawn plants that scorch when it comes off.
6. Anchor everything properly.Hoops through the plastic, pegs into the ground, bricks along the edges. A lightweight cloche in a March gale ends up in a neighbour's garden and takes the crop with it.
7. Take it off before the crop touches the top.Leaves pressed against cold plastic scorch and rot. Once the plants fill the space, either the cloche goes or the crop needs a taller one.
8. Know the different types.Tunnel cloches of polythene over hoops are cheap and cover a row. Rigid polycarbonate barn cloches last longer, ventilate better and cost more. Bell cloches suit individual plants. Fleece is the lightest and most forgiving, letting rain and air through while giving a degree or two.
9. Improvise where it makes sense.A cut-off plastic bottle over a single seedling protects it from slugs, wind and cold and costs nothing. Trim the edges cleanly, take the cap off for ventilation, and push it into the soil so slugs cannot get under.
10. Use fleece where ventilation is the problem.Fleece breathes, so it does not cook plants the way sealed plastic can, and it lets rain through. It gives less warmth in return. On an unattended plot it is often the safer choice.
11. Watch for slugs and mice underneath.A cloche is a warm sheltered slug hotel with a food supply. Check under it regularly, particularly after rain, and use control appropriate to the site and the current product label.

Tips

• Two or three weeks of clear polythene on a bed before sowing is one of the highest-value, lowest-effort things you can do in early spring.
• Cloche ends matter: an open-ended tunnel becomes a wind tunnel and loses much of its benefit.
• Fleece for forgiveness, polythene for warmth, mesh for pests. Choosing by what you actually need beats buying one of everything.

Common mistakes

• Leaving a sealed cloche closed through a warm spell — It cooks the crop, and the soft growth that results scorches the moment the cover comes off. Ventilate the ends from April.
• Not anchoring it — Wind gets underneath and lifts the whole thing, usually taking the seedlings with it.
• Using a cloche as a pest barrier with gaps at the ground — Carrot fly and cabbage white butterflies walk or fly straight in. Pest exclusion needs the edges buried or weighted down all round.
• Using glass cloches in a bed you kneel in — Broken glass in a vegetable bed is a lasting hazard. Polycarbonate does the same job.

Questions people ask