A soft, fibrous, translucent material laid over or around plants hastening their growth and protecting them against weather and pests. Heavier grades of fleece give about 2°C of protection from frost. It is usually used without supporting hoops.

fleece

• Plastic-free options include fleece made from plant-based materials and sheep’s wool
• Polypropylene ‘fleece’, sometimes referred to as horticultural fleece, is a non-woven plastic-based material

The concentration of water vapour in the atmosphere. This affects the rate at which plants transpire (lose water from their leaf pores), which in turn affects how much water their roots absorb from the soil. If air humidity is low, plants transpire quickly and become at risk of wilting if water is lost from the leaves faster than it can be absorbed by the roots. Different plants require different levels of humidity to thrive. Many tropical houseplants need high humidity and are best grown in a steamy bathroom, misted regularly or placed on a saucer of damp pebbles. In a greenhouse, humidity can be raised in hot weather by damping down (wetting) the floor or overhead misting.

humidity

• Plastic-free or
  Anything that can be decomposed by fungi, bacteria and micro-organisms, so it doesn’t persist indefinitely in the environment. Biodegradable plastics are usually made from plants, such as bamboo, rather than fossil fuels. Ideally, products that claim to be biodegradable should break down quickly and leave nothing harmful behind. However, they may need specific conditions in order to biodegrade successfully or may produce greenhouse gases such as methane in the process.

  biodegradable options are starting to become available
• Different grades of crop cover give different levels of warmth and protection

Fleece and crop covers can be used in gardens to cover plants. These materials provide a small but significant amount of protection from cold or windy weather, and can be used to exclude some pests.

Fleece is white, extremely light, soft and warm to the touch. For this reason it is often used to wrap slightly tender plants against winter weather. Its porosity makes it much less prone to over-heating and wet conditions than polythene covers.

Crop covers play a similar role to cloches but can cover a large area and are easy to store. They can be laid over plants to encourage faster growth, particularly in spring, and earlier crops.

Raising temperatures

Crop covers work by warmth from the sun raising temperatures by a few degrees, 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 fleece give different levels of cold 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 protection

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).

Hail

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, unperforated 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.

Pollination problems

Insect-pollinated crops such as strawberries and courgettes are unsuited to growing under crop covers during their flowering period.

Sustainability

Fleece made from polypropylene and crop covers made from polythene have a useful but short life and councils don’t currently offer recycling to home gardeners for these products. Choosing eco-friendly protective coverings made from natural and biodegradable materials is a better option. Bear in mind that covers cut to specific/small sizes may not be so easy to re-use. 

Fleece: Both plastic-free options and polypropylene fleece can be used for tunnel cloches and floating films. These trap warmth, allow rain through and, being porous, allow excess heat and humidity to escape.

They are especially useful for covering seeds, transplants and rooted cuttings. Fleece-covered crops can be harvested two weeks ahead of crops in the open. Unlike polythene, fleece can be left on all summer (to exclude pests, for example). Fleece can also be double-layered for extra frost protection, and can be used beneath glass or plastic cloches and in cold frames and greenhouses for extra insulation.

• Plastic-free options, such as sheep’s wool or BioFleece made from plant materials, are the most sustainable choices. They do handle differently to traditional fleece, the sheep’s wool being a heavier material, but still do the same job. 
• Polypropylene fleece is not durable, especially the 17g fleece, which may only last a year and quickly break down into small pieces. It is also is vulnerable to fox damage.

Woven meshes: Woven polythene covers such as Envirotect are durable – lasting at least five years – and resist foxes to a considerable extent. It can be rinsed to clean it. It is made of very soft polythene and claims to have similar properties to 17g fleece.

Perforated or ‘aerated’ polythene: Clear polythene with numerous holes cut into the sheet were the first crop cover that provided pest protection. Temperatures and humidity can also rise to damaging levels from late spring, but on the other hand they capture more warmth and prompt more growth in early to mid-spring. This is plastic-based, so if you can use cloches instead, these are a more sustainable choice.