How to Protect Plants From Frost Overnight
A frost is forecast and you have tender plants in the ground. Here are seven proven ways to protect them — ranked from quickest to most thorough — plus the mistakes that ruin plants people thought they'd saved.
How to protect plants from frost: cover plants before dusk — while the ground is still warm — with floating row cover, old bedsheets, or light blankets that reach the soil on all sides and are anchored at the edges. Water the soil in the afternoon so it stores heat, and mulch around the roots. Move potted plants against a house wall or indoors. Cover whenever the forecast low is 2°C or below — tender plants like tomatoes, peppers, and basil are damaged at 0 to −2°C. Remove covers by mid-morning once it warms above freezing. Never drape bare plastic directly on foliage.
❄️ Frost Protection at a Glance
When Does Frost Actually Damage Plants?
Before reaching for a cover, it helps to know whether your plants are even at risk. Frost damage happens when water inside plant cells freezes, the ice crystals rupture the cell walls, and the tissue collapses — which is why frost-burned leaves look water-soaked and limp the next morning before turning brown and crisp. How much cold a plant can take depends entirely on what kind of plant it is.
One critical detail trips up most gardeners: the temperature on the forecast is not the temperature at your plants. Weather services measure air temperature in a shaded box about 1.5 m above the ground. On a clear, calm, dry night — classic frost conditions — heat radiates away from the ground into the open sky, and the soil surface and low foliage can sit 2–4°C colder than the official reading. A forecast low of 3°C can absolutely mean frost on your tomatoes. Cloud cover and wind both reduce the risk; clear and still is when you act.
| Plant group | Damaged at | Examples |
|---|---|---|
| Tender | 0 to −2°C | Tomatoes, peppers, eggplant, basil, cucumbers, squash, beans, melons, annual flowers |
| Semi-hardy | −2 to −3°C | Lettuce, beets, Swiss chard, celery, emerged potato shoots |
| Hardy | −4 to −8°C | Kale, spinach, peas, broccoli, cabbage, carrots, onions, garlic |
Thresholds are approximate — a brief dip is survivable, while hours below the listed temperature is not. Hardy crops need no spring frost protection at all; a light frost even sweetens kale and Brussels sprouts.
7 Ways to Protect Plants From Frost
These methods are ordered from the quickest emergency fixes to the most thorough protection. For a light frost (−1 to −2°C) any one of the first few will do; for a moderate frost (−2 to −4°C) combine covering with watering and mulch; for a hard frost below −4°C, tender plants really need to come indoors or under a cold frame.
1. Cover plants with row cover, sheets, or blankets
The single most effective method. A cover traps heat radiating up from the soil and stops the plant cooling to the air temperature. Drape it loosely before dusk so it reaches the ground on every side, and weight the edges with stones, bricks, or soil — an open gap lets the warm air escape. Use stakes, hoops, or tomato cages to hold heavy fabric off tender foliage. Floating row cover is the best choice (it breathes, passes light and rain, and can stay on for days); old bedsheets, light blankets, towels, and burlap all work for a one-night frost.
2. Water the soil in the afternoon
Moist soil absorbs and stores far more of the day's heat than dry soil, then releases it slowly overnight, lifting the temperature around your plants by 1–2°C. Water the ground — not the leaves — in the early afternoon so the surface isn't soaking at nightfall. This pairs well with covering: warm, damp soil under a cover is the ideal combination. (This is different from the orchard trick of spraying plants all night to coat them in ice — effective but impractical for a backyard.)
3. Mulch around the roots
A 5–10 cm layer of straw, shredded leaves, or bark around the base of plants insulates the soil and protects the roots and crown — the parts a plant can regrow from even if the top is nipped. Mulch won't save exposed foliage, but for perennials, strawberries, and recently set transplants it preserves the most important tissue. For low crops you can even mound straw loosely over the whole plant for a single hard night and rake it back the next day.
4. Move containers to shelter
Potted plants are far more exposed than the same plant in the ground — the root ball has cold air on all sides. The simplest protection is to move them: indoors, into a garage or shed, or at minimum against a south- or west-facing wall that radiates stored heat and blocks wind. Group pots together so they shelter one another, and raise them off cold concrete. If a pot can't be moved, wrap the container itself in bubble wrap or burlap to insulate the roots.
5. Use cloches over individual plants
For a handful of prized transplants, a cloche — a mini-greenhouse over a single plant — is quick and effective. An inverted plastic milk jug with the bottom cut off, a large yogurt tub, a cardboard box, an upturned bucket, or a purpose-made garden cloche all trap ground heat around one plant and protect to roughly −2°C. Set them in place before sunset and lift them off first thing in the morning so the plant doesn't cook or stay damp.
6. Plant in the warmest microclimate
The best protection is chosen weeks earlier, when you decide where to plant. A spot against a south-facing brick or stone wall stays several degrees warmer overnight as the wall releases stored heat. Raised beds drain and warm faster and sit above the pooled cold air. Avoid low-lying corners and the bottom of slopes — cold air is dense and flows downhill like water, settling in “frost pockets.” Slatted fences that let cold air drain through are better than solid walls that dam it up behind them.
7. Wait for the right planting date
The frost you never have to fight is the one you planted around. Most lost tomatoes are the result of transplanting too early in a warm spell. Know your local last frost date, hold tender crops until it has reliably passed (in much of Canada, the Victoria Day long weekend is the traditional anchor), and harden off indoor-grown seedlings for 7–10 days first. A plant set out on the right date with a row cover on standby almost never needs rescuing.
A lightweight floating row cover you drape straight over seedlings and beds when a frost threatens — method #1 above, in a roll you can keep by the back door all season. It breathes, lets light and rain through, and buys several degrees of protection on both ends of the growing year.
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Frost Protection Mistakes That Ruin Plants
Plenty of plants are lost not because they were left uncovered, but because they were “protected” the wrong way. These are the errors that catch gardeners out most often.
Draping bare plastic on foliage
Plastic does not insulate — wherever it touches a leaf it conducts cold straight into the tissue, often causing worse frost burn than no cover at all. Only ever use plastic over a frame that keeps it off the plant, and pull it off early in the morning before the sun bakes everything underneath.
Covering only the top of the plant
A cover works by trapping heat rising from the soil. A sheet thrown over just the top, with the sides open, lets that warm air escape and does almost nothing. The cover must reach the ground on every side and be weighted down at the edges.
Covering too late — after dark
A cover traps existing warmth; it does not generate any. Put it on in the late afternoon or at dusk while the soil is still radiating the day's heat. Throwing a sheet over a plant at 11 p.m. when it's already cold seals in cold air, not warm.
Leaving covers on through a sunny day
An opaque blanket or sheet left on blocks light, and trapped heat under plastic can cook plants within hours. Uncover by mid-morning once the air is back above freezing. Breathable row cover is more forgiving and can ride out a multi-day cold spell.
Pruning frost-damaged plants too soon
A frost-hit plant looks worse the morning after than it really is. Wait several days — even a week or two for shrubs — to see how much tissue genuinely recovers before cutting anything away. Damaged leaves also give a little shelter to the buds below.
Common Questions About Frost Protection
At what temperature should I cover my plants?
Cover tender plants whenever the forecast overnight low is 2°C or below. Forecasts measure air temperature about 1.5 m up; on a clear, calm night the ground and low foliage can be 2–4°C colder, so a forecast of 3–4°C can still bring frost at plant level. Tender plants (tomatoes, peppers, basil) are damaged at 0 to −2°C. When in doubt, cover — a row cover on a mild night does no harm.
Will a bedsheet protect plants from frost?
Yes — an old cotton sheet, light blanket, or towel handles a light-to-moderate frost down to about −2 to −3°C. It traps heat radiating up from the soil. The cover must reach the ground on all sides; a sheet draped only over the top does little. Drape it before dusk, weight the edges, and use stakes or cages to keep heavy wet fabric off tender foliage.
Should I use plastic to cover plants for frost?
Only as a last resort, and never touching the plant. Plastic doesn't insulate — where it contacts a leaf it conducts cold straight in and causes worse frost burn. If plastic is all you have, build a frame with stakes or hoops so the sheeting stays off the foliage, and remove it first thing in the morning before the sun cooks the plants underneath. Fabric covers are always the better choice.
My plants got frosted overnight — can they be saved?
Often, yes — don't write them off in the morning. Frost damage looks worse than it is at first. Water the soil gently and give the plant shade from direct morning sun, which can make thawing damage worse. Wait several days before pruning to see exactly how much tissue recovers; plants frequently push new growth from the base or from undamaged buds. A tomato or pepper that loses its leaves but keeps a green stem will usually regrow, just behind schedule.
How do I know when frost season is over where I live?
Check your local last spring frost date — the historical average after which frost becomes unlikely. It ranges from mid-March on the BC coast to late May on the Prairies. Our frost date calculator gives the dates for your area, and the last frost dates for 36 Canadian cities page lists them city by city. Even after the average date, keep a row cover handy — roughly one year in five sees a frost a week or two late.
📍 Related Frost & Planting Resources
Know Your Frost Dates, Skip the Panic
The surest frost protection is timing. Check the average last and first frost dates for your area, then plant tender crops with a comfortable margin — and keep a row cover by the door for the rare late cold snap.