What frost cloth actually does
Frost cloth does not warm anything. It is not insulation in the way a wall is insulation, and it has no heat of its own. What it does is trap the heat the soil radiates upward at night and slow the rate at which it escapes to the sky. That is the entire mechanism, and understanding it tells you nearly everything about how to buy and use the stuff.
Three consequences follow immediately. First, the cloth has to reach the ground on all sides, because the heat source is the soil, not the plant — a sheet draped over a plant like a tent with open ends does a fraction of the job. Second, it works best on still, clear nights, which is exactly when radiation frost happens. Third, it does nothing whatsoever if you put it on after the temperature has already dropped, because by then the heat you were trying to trap is gone.
Inside a greenhouse the same logic applies with a bonus: the structure has already cut the radiative loss, so a fleece layer over the bed is a second barrier working on air that is already a few degrees warmer than outside. Two thin layers consistently outperform one thick one, which is the main reason we recommend buying two weights rather than one.
Weight classes, and what each one buys you
Row cover is sold by weight per unit area — grams per square metre, or ounces per square yard. That single number determines both how much protection you get and how much light you give up, and the two move in opposite directions. There is no weight that is good at everything.
| Class | Rough weight | Light passed | Best for |
|---|---|---|---|
| Ultra-light / insect | ~17–19 gsm | Around 85–90% | Pest exclusion, wind protection, minimal frost value |
| Light | ~30 gsm | Around 70–80% | Leaving on for weeks; a couple of degrees of protection |
| Medium | ~40–50 gsm | Around 60–70% | The general-purpose fall choice inside a greenhouse |
| Heavy | ~60–70 gsm | Around 30–50% | Overnight deployment on hard-frost warnings only |
Manufacturers advertise degrees of frost protection for each weight. Treat those numbers as directional rather than literal, because the result depends on wind, cloud cover, how completely the cloth is sealed at the ground, and whether the soil went into the night warm or cold. A heavy cloth badly deployed protects less than a light cloth deployed properly.
The buying decision in one line.
Buy a medium weight to live on the bed through fall, and a heavy weight to throw over the top on the three or four nights a year that actually threaten. That combination costs less than one premium heavy roll and protects better, because the heavy layer never has to sit there blocking light on ordinary days.
Materials: spunbond, woven, and the things that are not row cover
Most horticultural fleece is spunbond polypropylene — non-woven, breathable, permeable to water and light, and cheap enough to be semi-disposable. It is the default and it is the right default. Its weakness is tear strength: it snags on anything sharp and shreds along the tear.
Woven and reinforced covers cost more, last several seasons rather than one or two, and stand up to repeated deployment and clips. If you are covering the same beds every fall for years, the woven material earns its price. If you are covering a small greenhouse bed occasionally, spunbond is fine.
What does not work: clear plastic sheeting laid on plants. Plastic is not breathable, it holds condensation directly against leaf surfaces, and where it touches a leaf it conducts cold straight into it — often producing worse damage than no cover at all. If you use plastic at all, it goes over hoops with a gap, never in contact, and it comes off in the morning without fail or you will cook whatever is underneath.
Sizing and coverage without waste
Measure the bed, then add generously. You need enough width to reach the ground on both sides plus enough to weigh down, and enough length to close both ends. For a bed a metre wide with plants a foot tall, a cover two metres wide is comfortable and a 1.5 metre one is a constant fight.
Buy by the roll rather than in pre-cut sheets if you have more than one bed — the price per unit area is substantially better and you can cut to your actual dimensions. Cut with a hot knife or seal the cut edge if you can, because raw cut edges on spunbond fray steadily.
Label your cut pieces with a marker: bed name, weight class, and which way round they go. In October, in the dark, with a frost warning, you want to grab the right piece first time.
The hardware matters more than the cloth
Almost every frost-cloth failure is a fixing failure. The cloth blows off, or a corner lifts and vents all the trapped heat, or it sits directly on the foliage and the leaves in contact freeze anyway.
- Hoops keep the cloth off the foliage. Anything in contact with the fabric loses most of the protection at the contact point, and on a heavy cloth the weight itself can crush young plants. Low wire hoops or lengths of alkathene pipe both work.
- Clips hold the cloth to the hoops without tearing it. Snap clips or purpose-made fleece clips beat clothespins, which chew the fabric.
- Ground pins or staples seal the edges. Inside a greenhouse where there is no wind, a length of timber or a row of bricks along each edge does the same job and is faster to lift.
- Sandbags or fabric weights for the ends. The ends are where heat escapes and where people get lazy.
Using it inside a greenhouse specifically
Two adjustments matter under glass. First, ventilation discipline gets stricter, not looser. A covered bed in a sealed greenhouse is a still, damp micro-environment, which is ideal conditions for gray mold on the very seedlings you are protecting. Lift the cover on mild days, keep air moving, and read autumn condensation and gray mold before you leave a cover down for a week straight.
Second, light is scarcer than you think. Your glazing already costs you a share of the incoming light, your bubble insulation costs more, and a heavy fleece on top of both can put a fall sowing into functional darkness. This is the argument for the light-plus-heavy pairing rather than one permanent heavy layer.
Frost cloth is also not a substitute for a heater, and it is not doing the same job as bubble insulation on the structure. We compare the two directly in frost cloth vs bubble insulation, because buying the wrong one for the job is the most common mistake in the category.
What to buy, by situation
Medium-weight row cover, by the roll
$ to $$ · The default fall buy · around 40–50 gsm
This is the weight that earns its place on the bed for weeks at a time: enough protection to matter on ordinary fall nights, enough light through it that a young sowing keeps growing underneath. Buy a roll rather than sheets and cut to your beds. If you only buy one weight, buy this one.
Medium-weight row cover on AmazonMedium-weight row cover on eBay
Heavy frost blanket for warning nights
$$ · Deploy overnight only · around 60–70 gsm
The emergency layer that goes on top of the medium cover when the forecast turns. It blocks too much light to live on the bed, so buy only as much as covers your most vulnerable plants and keep it folded by the door. Heavier is not automatically better here — deployment quality matters more than weight.
Ultra-light insect and wind netting
$ · Late summer into fall · around 17–19 gsm
Barely any frost value, but it is the cover you want on brassica seedlings and fall salad in the warm end of the season, when the threat is cabbage white butterflies and drying wind rather than cold. Very high light transmission means it can stay on continuously.
Insect netting and ultra-light cover on AmazonInsect netting and ultra-light cover on eBay
Low hoops and support wire
$ · Buy with the cloth, not after · keeps fabric off foliage
The single upgrade that makes frost cloth work as advertised. Contact between fabric and leaf transfers cold straight into the plant and negates the protection at exactly the points you were protecting. Wire hoops for small beds, flexible pipe for anything wider.
Fleece clips, ground pins and weights
$ · The fixing kit · where most failures happen
Clips to hold cover to hoop, pins or staples to seal the edges to the soil, and weights for the ends. Cheap, endlessly reused, and the difference between a cover that traps heat and a cover that vents it through a lifted corner all night.
Row cover clips and ground pins on AmazonRow cover clips and ground pins on eBay
Reinforced woven cover for repeat seasons
$$ to $$$ · Multi-year option · if you cover the same beds annually
Woven and reinforced covers survive repeated clipping, folding and deployment for several seasons where spunbond shreds after one or two. The economics only work if you are genuinely covering the same beds every year — occasional users should stay with spunbond.
Reinforced garden covers on AmazonReinforced garden covers on eBay
Making it last a second season
Spunbond fleece is treated as disposable mostly because people store it badly. Dry it before it goes away — folded damp, it grows mold and comes out of the shed stained and weak. Shake off soil, fold rather than stuff, and store it somewhere rodents cannot reach, because mice will shred it for nesting material given one winter of access.
Repair tears early with a strip of packing tape on both faces. A small tear becomes a metre-long split the first time you pull the cover taut.
At end of season, take covers off as soon as the frost risk passes. Left on into spring they cut the light exactly when the light is finally worth having, and they hold humidity around plants that are starting into rapid growth.
Greenhouse Guide earns commission on qualifying purchases made through the links above, at no additional cost to you. We link to product categories rather than single items because stock and pricing in this category change week to week — compare current listings and buy what fits your structure.
Common questions
How many degrees of protection does frost cloth give?
Manufacturers quote figures by weight, but the real result depends on wind, cloud, how well the edges are sealed to the ground and how warm the soil was going into the night. Treat quoted figures as directional. A medium cloth sealed properly outperforms a heavy cloth with a lifted corner.
Can I use bed sheets or plastic instead?
Old sheets and blankets work in a pinch for a single night if kept off the foliage and removed in the morning, but they absorb water and get heavy. Clear plastic laid directly on plants is worse than nothing — it conducts cold at every contact point and traps condensation against leaves.
Should frost cloth touch the plants?
No. Anything in contact with the fabric loses most of the protection at that point, and heavier cloths can crush young growth. Use low hoops or wire supports to hold the cover clear, and let it reach the ground on all sides so it traps the soil's heat.
Do I still need frost cloth inside a greenhouse?
For young sowings and tender plants, yes. Glass alone buys a few degrees; a fleece layer over the bed is a second barrier working on air that is already warmer than outside. Two light layers beat one heavy one, and the inner layer costs you very little light.
When should I put frost cloth on?
Late afternoon, before the temperature starts falling, so you trap the day's heat. Putting it on after dark once the air has already cooled is largely pointless, because the heat you were trying to hold has already gone.