Crop covers work by warmth from the sun raising temperatures by a few degree, typically 2°C, compared to uncovered plants. This can advance maturity or flowering by about two weeks.
Spring: Covered tender plants can be sown or planted out earlier than if uncovered by about two weeks in spring. Crucially crop covers prevent overheating by allowing heat to escape through holes built into them during manufacture. Unpierced transparent polythene sheeting or bubblewrap, for example, would not only lead to excess moisture but damage plants by excess heat on sunny days even in mid-spring.
Late summer and autumn: In late summer and autumn crop covers can allow tender crops such as cucumbers and bush tomatoes to ripen. Autumn sowings can be protected over winter to allow earlier crops than spring sowing; carrots and chard for example.
Winter: Fleece is often used to insulate plants against winter cold. In fact without heat from the sun (solar gain) the temperature around covered plants will fall quite quickly to the ambient temperature. However, fleece does protect from other aspects of harsh weather – wind and hail, for example – and is often used in conjunction with straw and similar materials where the combined effect of fleece and insulation is very effective.
Different grades of crop cover give different levels of warmth and protection; light fleece (17g per sq m) for instance, gives less protection but better light penetration (and is cheaper) than heavy duty fleece (35g per sq m). The latter might be used to protect tender plants and the former to advance young crops of seedlings.
Wind often retards growth of plants in British gardens as much as temperature. Crop covers, if well anchored, enhance growth rates by avoiding stress from wind (mechanical stress and water loss).
Crop covers offer some protection against the small hailstones generally encountered in Britain.
Managing difficult soils
Crop covers break the impact of heavy rain and protect the soil to some extent from mechanical damage that can lead to significant surface compaction limiting seed emergence and growth of young plants.
Pest and disease protection
Used as a physical barrier, crop covers can be highly effective in excluding pests. Insect-proof meshes are a variant of crop covers that give protection against insects often without significant increases in temperature but good protection against wind and hail.