Best frost protective covers

Fleece and crop covers promise to trap the sun's warmth and keep off pests from your young veg so you can get off to an early start in spring.

They can also be used to wrap around tender plants that would be damaged by chilly weather or cold wind.

Which? Gardening trialled different frost protective cover and shares the best ones to use. We alos explain how to use them in your garden to get the best results.

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Best frost protection covers

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We found this product easy to use; it allows rain to pass through (as well as protecting the soil from wind and sun, so the soil retains moisture) cutting down on the need for watering, and because air can pass through, the plants stay well ventilated if there is a burst of strong sunshine. All our crops got off to a flying start and put on lots of healthy growth. 

One disadvantage is the lack of visibility. It was difficult to check on germination.   We chose to peg down the edges which left holes. These didn’t tear wider during use but did make it trickier to reuse. It didn’t get too dirty during use, and it is washable, though it doesn’t come up like new. 

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This product allowed more water through than others so the young plants looked just a little healthier and we didn’t have to water as often. The light fleece didn’t catch the wind so didn’t threaten to blow away in blustery spring weather. The lightness of the fabric allowed the young plants to push up the product as they grew and we tried just weighting the edges of the fleece, rather than using pegs and found this a simple solution to rips (that allow in insect pests) and allowed us to easily check on the young plants and roll out more cover if necessary. Though a lot lighter than other products, the crops didn’t suffer any frost damage.

Other covers on test

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This product is has a looser weave than some other covers but is strong and  quite heavy despite this it still didn’t crush the young growing plants – both the lettuce and beetroot grew well though the potatoes looked a little cramped. You could use supporting hoops if there were any concerns. Germination was slightly slower but the plants showed no signs of frost damage and water could get through the material. The product was tough and resisted any tears so it was difficult to peg down but it was easily weighted down. It could be used for another season but it got quite a bit of debris (leaves and twigs) caught up  during use so it was tricky to clean. 

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This product is often used to prevent insect attack but the fine mesh can also act as a frost protective blanket. It is quite heavy so though we didn’t see any real problems your veg might prefer it to be supported by hoops rather than laid directly on top. It’s a tough material but is air and rain permeable so allowed rain through to the young plants and air to circulate. Because it’s a tough material it is best held down with stones or lumps of wood rather than pegged down which rips the mesh. This product didn’t keep things quite as warm as other covers but we didn’t see any frost damage on the young plants. 

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This was the least successful cover. It didn’t let any rain through so though the young plants germinated more quickly than other covers, they needed extra watering. There was little protection from the frost so the tips of the leaves burnt and in hot spells, the plants got very sweaty. Despite this the young plants did get off to a reasonable start but swiftly became thin and weedy.  We kept this cover over the plants until the end of May to be consistent with the other covers in the trial but if you were using it,  it would have to be removed before the last frosts have passed to allow for ventilation and to reduce the amount of watering needed.

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We planted the beetroot and lettuce seeds, and a seed potatoes under these covers. They came with pegs to keep them sturdy and vents at the top. Our veg germinated quickly and developed well as they had good ventilation and decent frost protection, but they were not easy to use for starting seeds. It was tricky to reposition the covers after watering underneath and the covers were brittle and actually snapped around the holes for the pegs meaning they could only be used for one season. 

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When does frost occur?

There are two types of frost that can affect our young plants in the spring. Air frost is when the ambient air temperature is 0°C or lower. Ground frost is often seen on parked cars and grass when temperatures approach zero but remain positive. Even tender plants can be little affected by ground frost because often only the outside of leaves is coated with frost. The effect of air frost is much more marked. Principally two things might happen. First, the water inside the leaf cells might freeze, rupturing the cell walls. Badly affected foliage darkens and appears to have been cooked, then finally blackens. Second, if the soil freezes, roots are unable to take up water (and they might themselves be damaged) and the plant above wilts.

The last frost generally occurs between mid-April and mid-May in central England. Final frost dates are usually earlier in coastal areas and later in upland areas. Experienced gardeners tend to err on the side of caution, leaving tender plants under cover until towards the end of May. A good compromise is to plant out tender bedding, container plants and vegetables as soon as the soil is at least 5°C overnight and not too wet in April. But make sure you have some frost protection on standby. Use the local weather forecast from the Met Office website, but bear in mind they give air temperature, and ground frosts can occur even if the air temperature is up to 2°C. Late frosts can harm relatively hardy perennials too, especially if they have been prompted into growth by a mild early spring. Flower buds are particularly vulnerable to frost damage, which will affect flower displays and fruit crops later in the season.

Frost tips

Gradually acclimatise tender plants raised in a greenhouse or bought from a garden centre to outdoor conditions for a fortnight - this process is called hardening off.   Plants raised from seed in a greenhouse or bought from a garden centre will be ‘soft’ and therefore vulnerable to a late frost. Hardening them off gradually acclimatises them to life outdoors, and increases their chance of thriving. Keep them indoors on cold nights or if a frost is predicted, but stand them outside in a sheltered spot during the day for a week or so. After this, leave them outside on frost-free nights, until you can safely leave them outside all the time but keep some frost protective covers in case of unexpected late frosts. 

Flower buds can be damaged by late frosts. In frost-prone gardens choose late-flowering apple or pear varieties or wrap early flowering shrubs such as camellias with a frost protective cover - remember the thawing process can be just as damaging to flower buds so protect them from the early morning sun too.  

How we test frost protective covers

• The researchers for Which? Gardening magazine selected six different crop covers. 
• We sowed a row of lettuce, potatoes and beetroot seeds under each protection cover without any supporting hoops.
• We monitored the growth of the veg through the spring recording any problems and then recorded the how well the crop turned out. 
• We also measured the temperature above and below the covers through the spring to see how steady they kept the temperature and when the trial ended, we assessed if the covers could be cleaned and reused in a following season. , such