How do I use horticultural fleece properly?
How much protection fleece actually gives, the two things people get wrong — laying it on the leaves and leaving it open at the bottom — and how to keep it usable for several seasons.
- Difficulty
- beginner
- Time
- 25 min
- Read
- 5 min
Short answer
Hold it clear of the foliage on hoops or canes, take it right down to the ground on every side and weight the whole edge, so the warmth rising out of the soil is trapped underneath. A single layer buys you a degree or two; doubling it buys more. Put it on in the early evening while the ground still holds heat, not at bedtime.
Fleece is the most useful few pounds a gardener spends and most of it is used badly. The material is a spun polypropylene web: it lets light, air and water through, so plants can live under it for weeks, and it works by intercepting the heat radiating away from the ground and the plant on a clear night. Understanding that mechanism explains every rule below.
What you'll need
- Horticultural fleece
- Hoops, canes or a frame (optional)
- Pegs, bricks, stones or soil to weight the edges
- Clothes pegs or clips
Step by step
- Choose a weight for the job.Light fleece is for pest exclusion and a very small amount of frost protection, and passes the most light. Heavier grades give more frost protection and pass less light. For serious frost protection a heavier grade or two light layers beats one light one.
- Build a frame first, then drape.Wire hoops, canes with an upturned pot on top, or a simple timber frame. The air gap between fabric and foliage is doing much of the work, and any leaf in contact with the fabric on a hard frost is the leaf that gets damaged.
- Take it to the ground on every side.This is the step that separates fleece that works from fleece that does not. Most of the warmth being retained came out of the soil, and if the bottom edge is open it escapes. Tuck the edge into a shallow trench, or weight it continuously with soil, bricks or pegs.
- Leave slack for growth and wind.Fleece stretched tight tears on the first gust and constricts a growing plant. Leave enough that it can billow slightly, and check the tension again after a fortnight of growth.
- Cover in the early evening.Put it on while the soil is still releasing the day's heat. Covering after dark on a clear night traps air that has already cooled and achieves substantially less.
- Leave it on for a settled cold spell rather than shuttling it daily.Fleece passes light, air and rain, so plants can stay under it for days or weeks. Take it off in a warm sunny spell, when pollinators need access to flowers, and once the risk period is over.
- Vent or remove it if the weather turns warm and bright.Under strong spring sun a doubled fleece can get uncomfortably warm, and plants under it grow soft and drawn. Watch for pale, elongated growth as the signal that it has been on too long.
- Dry it before you store it.Put it away damp and it comes out mouldy and weakened. Shake it out, dry it over a line, fold it and store it dry, out of light. Kept properly it lasts several seasons; left on a plant all year it degrades in ultraviolet and shreds.
Tips
- Two light layers with an air gap between them outperform a single layer of the same total weight, because the trapped air is the insulator.
- For individual plants, a fleece jacket or a bag made from a cut length and tied loosely at the base is quicker to fit and remove than a tunnel, and easier to get right.
- Clothes pegs are the cheapest way to join sheets and hold fleece against hoops without tearing it.
- Fleece is the same material used for pest exclusion. If you are covering brassicas or carrots against fly, use a fine mesh instead — it lasts longer, passes more light and does not tear.
- Mark the edges of a fleece tunnel with canes so you can find and lift it in the dark on the night you need it.
Common mistakes
- Laying fleece straight onto the plant — Any leaf pressed against the fabric is in contact with a cold surface and loses heat through it, so those leaves are damaged when the rest of the plant is not. The air gap is most of the protection.
- Leaving the bottom edge loose — Warm air rises out of the soil and straight out of the gap, and cold air flows in underneath. An unsealed cover gives a fraction of the protection of a sealed one, and blows away on the first windy night.
- Expecting fleece to save a plant from a hard frost — It buys a degree or two, sometimes a little more doubled. That is decisive on a marginal night and irrelevant in a severe freeze, and a plant that needs more than that needs to be somewhere else.
- Leaving it on all summer as a matter of course — Ultraviolet degrades it, plants grow soft and drawn under it, and flowering crops that need insects will not set.
Keeping on top of it
- Check tension and edge weighting after wind (After any windy night)
- Lift and inspect the plants underneath (Weekly while covered)
- Dry, fold and store out of sunlight (At the end of each risk period)
Questions people ask
How much frost protection does fleece actually give?
A single light layer typically raises the minimum under it by around a degree or two, and a double layer more. That is enough to save hardened-off bedding on a marginal night and not enough to save a tender plant in a hard freeze.
Can I use an old sheet or newspaper instead?
For a single night, yes — anything that intercepts radiation to the sky helps, and a sheet or several layers of newspaper over a frame works. Neither passes light or sheds water, so both have to come off in the morning.
Does fleece keep plants warm during the day?
It raises daytime temperature under it somewhat and reduces wind chill, which is part of why plants under fleece grow away faster in spring. In strong sun that becomes a problem rather than a benefit.
Will slugs live under it?
Yes — the conditions under fleece are ideal for them, and it is a common reason for mysterious losses under an otherwise successful cover. Check underneath when you lift it.