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Hardening off is the process of gradually acclimatising seedlings, tender plants, or half-hardy species to the outdoor environment. Plants raised in the stable, humid, and warm conditions of a propagator, windowsill, or heated greenhouse are often too fragile to survive a sudden move to the garden. By slowly introducing them to cooler temperatures, increased air movement, and direct sunlight over a period of two to three weeks, you allow the plant to thicken its leaf structure and increase leaf waxiness, which reduces the risk of a severe check in growth. Adjust the pace if weather or plant condition requires it.
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Why seedlings require hardening off
Seedlings grown indoors exist in a protected bubble. When moved abruptly outdoors, they encounter three primary stressors: temperature fluctuations, lower humidity, and UV intensity. Without a transition period, the shock can severely check a plant’s growth. While many plants eventually recover, the process of hardening off ensures that new growth remains sturdy and the plant adapts its physiology to survive changeable weather.
It is important to note that hardening off is a process of acclimatisation, not a change in the plant’s genetic hardiness. For example, hardening off a frost-sensitive tropical plant will make it more resilient to wind and light, but it will not make that plant hardy enough to survive a freeze.
Determining the timing for your region
Work backwards from a suitable planting-out window for the actual crop. In many UK gardens this is in late spring or early summer for tender plants, with northern, elevated or exposed sites often later. A regional average is not a safe planting date for an individual garden.
Begin with short outdoor visits on mild days while retaining somewhere protected to return plants at night or during cold spells. Complete the transition when they are acclimatised and the forecast meets the crop’s temperature needs. Hardening off does not make tomatoes, peppers or basil frost-hardy, and warmth-loving crops can be checked by chilly weather above freezing.
A sheltered wall can alter conditions, but it does not justify subtracting a fixed number of weeks from the local season. Check the forecast, actual night temperatures and the seed packet or plant’s growing guidance. Delay permanent planting if frost or unsuitable cold is likely.
Step-by-step hardening off methods
The method used depends on the equipment available. Regardless of the tool, the goal is a gradual increase in exposure.
Using a cold frame or cloche
A cold frame is ideal for providing a controlled transition. During the first week, place plants inside and open the lid slightly during the day, closing it at night when conditions suit the crop. A cold frame is unheated; bring tender plants back to reliable protection if a cold night could exceed its limited protection. Over the following fortnight, gradually increase the amount the lid is raised each day until it can be removed entirely before final planting.
Using fleece and sheltered spots
If you lack a cold frame, use a sheltered position, such as against a south-facing wall. To prevent sun scorch and temperature shock, cover the plants with two layers of horticultural fleece.
- Week one: Leave plants outside during the day under two layers of fleece; bring them indoors at night.
- Week two: Reduce the cover to one layer of fleece. Towards the end of the week, remove the fleece during the day.
- Week three: If weather permits, leave plants outside at night but keep them covered. By the end of the third week, leave them uncovered before planting out.
Transitioning from heated glasshouses
Plants raised in heated environments should first move to a cold glasshouse if available. When moving plants out of a propagator, choose an overcast, dull day to reduce the risk of immediate wilting. After two weeks in cooler glasshouse conditions, they can move to a cold frame or the fleece method described above.
Diagnosing and solving common problems
Even with a careful schedule, issues can arise during the transition period. Recognising these early allows you to save the crop.
Cold damage and frost
If a late spring frost occurs, you may notice blackened or limp growth. Return plants to appropriate protection and assess surviving growth. Do not immediately cut every damaged-looking shoot off a tiny seedling; remove clearly dead material when its condition is apparent. If a cold night is forecast, moving tender pots indoors is more reliable than assuming an extra cover will keep them frost-free.
Sun scorch and wilting
Seedlings grown in low light or under glass can be burned by direct UV rays. This appears as bleached or brown patches on the leaves. To prevent this, always start plants in dappled shade or under fleece. If wilting occurs, move the plants back into a more sheltered area for a day before trying again.
Pest infestations
As plants move outdoors into warmer weather, they become accessible to pests. Greenfly (aphids) are common during the hardening off stage. Monitor the undersides of leaves and the growing tips regularly, as pests can multiply quickly in the sheltered environments of cold frames or under fleece.
Final checks before planting out
Before planting out, check the forecast against the specific crop’s minimum-temperature needs. Hardening off does not make frost-tender plants frost-hardy. Delay planting into waterlogged soil or unsuitable cold conditions.
Useful products for this job
The links below open Amazon UK product searches. Check the exact item, seller, current price and compatibility before buying.
- Horticultural fleece: Used to protect seedlings from sun scorch and temperature shock during the gradual outdoor transition. Check the grade, light transmission, fastening needs and maker’s protection guidance; fleece cannot guarantee tender plants survive a frost. Search Amazon for Horticultural fleece.
- Cold frame: Provides a controlled environment where the lid can be gradually opened to acclimatise plants. Ensure the lid is hinged or easily removable for daily ventilation adjustments. Search Amazon for Cold frame.
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
- 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.