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A cold frame can become too hot on a sunny spring day even when the air outside feels cool. Open the lid before heat builds up, check a thermometer at plant height and add light, removable shading when ventilation alone is insufficient. How much protection is needed depends on the plants: an unheated frame is neither frost-proof nor safe to leave closed in sunshine.
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Check the plants and temperature
A maximum-minimum thermometer helps reveal the hottest part of the day. Position its sensor out of direct sun so it measures air temperature rather than the heat of the glazing. The RHS warns that greenhouse temperatures above about 27°C can cause damage; treat this as a prompt to act, not a universal safe limit for every seedling or crop.
Drooping leaves despite moist compost, scorched patches and shrivelled young shoots can indicate heat stress. These symptoms can have other causes, so check root moisture and drainage as well. Do not automatically add water to already wet compost.
Open the lid safely
Use the frame’s proper stays to hold the lid securely. Start ventilating early on bright days and increase the opening as conditions warm. A loose stick or an unsecured lid can slip or catch the wind. Follow the frame maker’s instructions for windy conditions; if it cannot be ventilated safely, move vulnerable pots to a sheltered, suitably bright position.
In mild weather, a larger opening or removal of a lid designed to be removable may be appropriate. Close or reduce ventilation before a cold night when the plants need protection, while remembering that a closed frame cannot guarantee frost-free conditions.
Add removable shade
External shade intercepts sunlight before it passes through the glazing. Secure shade netting so it cannot blow away or obstruct the lid, and use the minimum shading that controls heat. Remove it in dull weather to avoid keeping seedlings unnecessarily dark.
Check both the shading material and the frame instructions. Shade paint may leave marks on acrylic or polycarbonate, so removable netting is often a simpler option for a small frame. Shading supplements ventilation; it does not replace it.
Water the roots according to need
Small pots and seed trays can dry rapidly in warm weather. Check the compost directly and water as appropriate for the crop, allowing excess water to drain. Keep drainage holes clear and avoid leaving sensitive plants in standing water.
Damping hard surfaces can temporarily increase humidity in a greenhouse, but it is a secondary measure for a small cold frame. There is no useful fixed watering or damping-down schedule for every frame. Avoid persistently wet foliage and a humid, closed environment, which can encourage disease.
Choose a summer cover for the actual purpose
| Cover | Useful for | Watch for |
|---|---|---|
| Solid glazed lid | Cool-weather shelter and extra warmth | Rapid overheating when closed in sun |
| Shade netting | Reducing strong sunlight | Too much shade and obstructed ventilation |
| Insect mesh | Excluding specified pests | Fine mesh can still raise temperature and humidity |
| Horticultural fleece | Extra warmth or limited frost protection | It is not a cooling material |
If insects are the only reason for covering a summer crop, suitable mesh may be more useful than a closed lid. Check mesh size against the pest and allow access for pollinators when required. Keep all covers secure and inspect conditions beneath them.
Useful products for this job
The links below open Amazon UK product searches. Check the exact item, seller, current price and compatibility before buying.
- Maximum-minimum thermometer: Essential for diagnosing if a cold frame is overheating by tracking peak daily temperatures. Ensure it has a clear display and is suitable for outdoor/high-humidity environments. Search Amazon for Maximum-minimum thermometer.
- Horticultural shade netting: Used to reduce solar gain and prevent leaf scorch in cold frames during summer. Check the percentage of shade provided; use the minimum needed to avoid restricting plant growth. Search Amazon for Horticultural shade netting.
Related garden guides
- Greenhouse thermometers: tracking minimum and maximum temperatures
- Growing winter salad in a cold frame
- More greenhouse & growing guides
Sources and further guidance
- Greenhouse: Ventilation and Shading Tips | RHS Advice
- Coldframes and Mini-Greenhouses | RHS Advice
- Fleece and crop covers | RHS Advice
Guidance researched in September 2026. The feature image is an AI-generated illustration. Product guidance is based on published information, not hands-on testing.