Late frost is most damaging in still, clear conditions when the ground radiates heat upward into a cold sky. Covers trap that heat and keep air around the plant above freezing. But covers only work if they reach the ground and do not touch the foliage — two rules most first-time gardeners get wrong.
Quick Answer
Best method for most situations: horticultural fleece over wire hoops, anchored at the edges. Fleece lets light and moisture through, can be left on for days, and gives approximately 2–4°C of protection against radiation frost.
For individual valuable plants: a cut-off plastic bottle cloche over each plant — free, reusable, and effective down to around –2°C on still nights. Remove the cap on warm days.
Crops that need no protection: spinach, kale, broad beans, peas, carrots, radishes, onions, and leeks all tolerate light frost. Do not waste covers on these — use them for tomatoes, courgettes, cucumbers, basil, and other tender transplants.
According to Iowa State University Extension’s frost protection guide (updated April 2025), cool-season vegetables like spinach, kale, carrots, and lettuce can often survive down to 28°F (–2°C) or lower with little damage. The crops that need protection are warm-season transplants: tomatoes, peppers, courgettes, cucumbers, and beans. Most spring frost damage in home gardens happens because gardeners leave tender transplants uncovered while worrying about crops that would have been fine anyway.
The Two Types of Frost — and Why It Matters
UF/IFAS Extension explains the critical distinction: covers protect from radiation frost but not from advective frost. Radiation frost occurs on still, clear nights when the ground loses heat upward and air near ground level cools below freezing — even when the official temperature reads above it. Covers trap that radiated heat close to the plant. Advective frost — driven by cold wind — replaces the air under covers as fast as it warms, making covers largely ineffective. When the forecast shows wind alongside cold, the primary protection is shelter and a windbreak, not a cover.
Four Methods — What Each Protects Against
Horticultural Fleece
- 2–4°C protection against radiation frost
- Permeable — lets moisture and light through
- Can stay on for days; no daily removal needed
- Must be anchored at all edges — fleece that lifts does nothing
Best for: rows of transplants, whole beds. Use over wire hoops to keep fleece off foliage.
Plastic or Glass Cloche
- 2–3°C protection; glass slightly better than plastic
- Traps warmth and humidity around individual plants
- Must be ventilated on warm days — remove or open ends
- Ends must be open on mild days to prevent overheating
Best for: individual courgette or tomato transplants; tunnel cloches for rows.
DIY Bottle Cloche
- 2-litre bottle with base removed, placed over plant
- Effective down to around –2°C on still nights
- Remove the cap on days above 10°C to ventilate
- Free and reusable season after season
Best for: individual seedlings; valuable transplants not yet established.
DIY Cold Frame
- Old window or sheet of perspex over a wooden or brick surround
- Temperatures inside 5–10°C warmer than outside air
- Prop lid open during the day — overheating kills as effectively as frost
- Best semi-permanent solution for repeated late frosts
Best for: hardening off transplants; protecting an entire bed through variable spring weather.
Which Crops Need Protection — and Which Don’t
| Crop | Frost Tolerance | Minimum Surviving Temp | Cover Needed? |
|---|---|---|---|
| Tomatoes, peppers, aubergines | Tender — frost kills | 32°F (0°C) | Yes — protect at any frost forecast |
| Courgettes, cucumbers, squash | Tender — frost kills | 32°F (0°C) | Yes — even light frost is fatal |
| French and runner beans | Tender | 32°F (0°C) | Yes — do not transplant until frost risk has passed |
| Spinach, kale, chard | Hardy | 28°F (–2°C) or lower | No — generally fine without protection |
| Lettuce (young transplants) | Semi-hardy | 28–30°F (–2°C) | Fleece on nights below 28°F |
| Peas, broad beans | Hardy once established | 28°F (–2°C) | No — established plants tolerate light frost |
| Basil | Extremely tender | Above 50°F (10°C) to thrive | Yes — bring inside or cover at any cool night |
✗ Mistakes That Cause Frost Damage
- Cover touching foliage directly—UF/IFAS Extension warnsthat foliage in contact with plastic covers is often injured because the cover draws heat away from the plant. Always use hoops or supports to keep covers clear of leaves
- Cover not reaching the ground— the trapped warm air escapes from the bottom. Bury or weight the edges of fleece all the way around to seal the heat inside
- Leaving plastic covers on during a sunny day— temperatures inside a sealed plastic cloche can reach 40°C+ on a sunny spring morning. Remove or ventilate before 9am
- Using covers on an advective frost night (wind + cold)— fleece and cloches offer little benefit when cold wind replaces the air underneath continuously. Prioritise shelter and delay planting
When Frost Is Forecast
- Cover tender transplants before sunset — not after frost starts
- Use hoops to keep fleece off foliage
- Anchor all edges to the ground
- Move potted tender plants to a sheltered wall or indoors
Morning After Frost
- Remove or ventilate plastic covers before 9am
- Leave fleece on if more frost is forecast that night
- Do not water frost-damaged foliage until midday — allow slow thaw
- Check hardened-off plants — wilting recovers; black foliage does not