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Use a cold frame to harden off young plants by gradually increasing their exposure to outdoor light, air movement and temperature. Allow roughly two to three weeks, adjusting to the plants and weather. Hardening off helps plants adapt, but does not make frost-sensitive plants hardy.
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Choose a suitable starting point
Check the forecast and the requirements of each crop. A cold frame can be much colder overnight than a heated greenhouse or windowsill. If it cannot maintain a suitable minimum temperature, move plants into it for part of the day and return them to warmer shelter at night.
Group plants with similar needs and choose a mild, overcast day for the first move where possible. Keep newly exposed plants out of intense direct sun at first. The frame should be sheltered, well drained and easy to reach for daily checks.
Increase exposure in stages
- Begin with short periods in the frame and modest ventilation in suitable weather. Use the lid’s secure stays rather than an improvised prop.
- Over the following days, increase ventilation and time outside as plants tolerate it. Check compost moisture as wind and sun increase water loss.
- Progress towards an open or removed lid during the day only where the design and weather permit. Store a removable lid safely.
- Leave plants exposed at night only when temperatures are suitable for the crop and frost is not forecast.
- Plant out after the transition when both the forecast and the final site are suitable.
These stages are not a rigid weekly timetable. Slow down after a cold spell, strong wind or signs of stress. On a bright day, a nearly closed frame can overheat, so do not leave the lid barely open simply because it is the first week.
Use crop-specific temperature guidance
There is no single 7°C or 10°C night threshold for every tender plant. Tomatoes, basil, peppers and ornamental seedlings can have different preferences. Check the seed packet, supplier or crop guide and use a thermometer to understand the frame’s actual overnight conditions.
Regional planting dates are only planning guides. Altitude, frost pockets and the current forecast matter more than moving planting day forward by a fixed number of weeks because a wall faces south.
Respond to stress without trapping more heat
If leaves wilt, inspect the compost and temperature. Dry roots may need water; wet roots do not. Reduce harsh sunlight with suitable temporary shade and keep ventilation adequate. Closing the lid on a hot frame can make wilting worse.
Bleached or damaged leaves may reflect scorch, cold or another problem. Move vulnerable plants to appropriate shelter, assess the cause and resume acclimatisation when they recover. Do not assume a closed lid or fleece will safely protect them through every frost.
Check pests and prepare the final position
Inspect leaf undersides and new growth while checking moisture. Identify pests before choosing a control; ventilation alone will not remove an aphid colony. Keep plants spaced so that foliage and roots can be examined.
Prepare the final bed or container before planting out, water the rootball as needed and choose calm conditions. Continue watching weather and watering after planting. The gradual transition reduces avoidable stress, but young plants still need care in their first weeks outdoors.
Useful products for this job
The links below open Amazon UK product searches. Check the exact item, seller, current price and compatibility before buying.
- Aluminium and glass cold frame: Provides adjustable shelter for gradually acclimatising suitable young plants. Check lid restraint, ventilation, usable height and anchoring. Plan warmer shelter when the frame cannot maintain the crop’s minimum temperature. Search Amazon for Aluminium and glass cold frame.
- Horticultural fleece: Can provide temporary extra protection within its limited frost performance. Check the material and crop needs. Fleece does not guarantee protection in a sharp frost; move tender plants to suitable shelter when necessary. Search Amazon for Horticultural fleece.
Related garden guides
- Growing winter salad in a cold frame
- Overwintering plants in an unheated greenhouse
- More greenhouse & growing guides
Sources and further guidance
- Hardening off tender plants | RHS Advice
- Coldframes and Mini-Greenhouses | RHS Advice
- When to plant out tender plants to avoid frosts | 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.