Which Cloches Work Best for Frost Protection and How Do I Use Them?

Cloches are individual covers placed over single plants or short rows to create a protected microclimate. They are one of the oldest season-extension tools in gardening, and with good reason — a well-placed cloche can give you two to three weeks' extra growing time at each end of the season.

Types of Cloches and Their Strengths

Bell cloches are the classic single-plant cover, originally glass, now mostly clear plastic or polycarbonate. They trap heat efficiently and are ideal for protecting individual transplants or a few rows of seedlings. Tent cloches — a sheet of glass or rigid plastic bent into an A-shape over a row — cover longer stretches quickly and are easy to move. Tunnel cloches are wire hoops with polythene stretched over them; inexpensive and versatile, they can cover an entire bed. Barn cloches are larger, ridged versions with side panels, offering more height for taller plants.

How Much Protection Do Cloches Provide?

A single-layer glass or polycarbonate cloche typically raises temperature by 3–6 °C compared to outside ambient. On a night forecast at −2 °C, plants under a cloche may experience only −0 °C or slightly above. Adding a layer of fleece inside or over the cloche can push protection up to 8–10 °C gain for the most tender plants on very cold nights.

Ventilation — the Forgotten Step

The biggest mistake with cloches is forgetting to ventilate on mild or sunny days. A closed cloche in spring sunshine can heat to 40 °C or higher internally, scorching leaves and killing plants faster than frost would. Open the ends of tunnel cloches, prop bell cloches slightly to one side, or use cloches with built-in ventilation slots during any day the temperature outside rises above 10 °C. Check daily during spring — the temperature swings are large and fast.

Making Cheap DIY Cloches

You do not need to buy commercial cloches. Large clear plastic bottles with the bottom cut off and the cap removed make excellent individual cloches — push the neck slightly into the soil so they do not blow over. Plastic storage boxes inverted over plants work for larger individuals. Old wire coat hangers bent into half-circles and draped with polythene make serviceable row tunnels. The key with any DIY cloche is to ensure it is firmly anchored (wind will catch anything light), allows ventilation, and is clear enough to admit adequate light.

Warming the Soil Before Planting

One underused trick: place cloches over bare soil one to two weeks before transplanting. The sun heats the covered soil more quickly than open ground. Transplanting into warm soil gives seedlings a much stronger start — roots establish faster and the plant is less stressed. This pre-warming effect is one reason professional market gardeners use cloches even before a single plant goes in the ground.