How to Harden Off Seedlings
Hardening off is the gradual move from protected indoor conditions to the wind, brighter light and cooler temperatures of a British garden. It takes one to two weeks and greatly reduces transplant shock. Start only when seedlings are sturdy, the soil is workable and you can protect tender plants if an unexpected cold spell arrives.
Begin hardening off when seedlings have several true leaves, firm stems and a well-held root ball. Check the weather forecast before starting, as a period of fierce wind, hail or near-frost is not ideal. Tender plants such as tomatoes, courgettes, French beans and sweetcorn need particular care. In many southern areas, this process starts during May, while cooler northern or Scottish gardens may need to wait longer. Hardy salads and brassicas tolerate lower temperatures but still benefit from gradual exposure. Water plants well before taking them outside, then place them in a sheltered spot with bright, indirect light. A cold frame, unheated greenhouse, porch or protected wall can provide a useful first stage.
For the first two or three days, take plants outside for a short daytime visit, around two or three hours, then bring them back indoors or under glass. Avoid strong midday sunshine because leaves raised behind glass can scorch quickly. Choose a calm morning or late afternoon and protect trays from slugs, pets and curious wildlife. Keep compost evenly moist, as small modules dry fast in a breeze. Do not stand pots in a saucer of water outdoors, where roots can become waterlogged in rain. If leaves bleach, curl badly or plants flop, return them to shade and shorten the next exposure. Mild temporary drooping can occur, but a plant should recover promptly after watering and shelter.
Over days four to seven, extend outdoor time each day and introduce a little more sun and air movement. Move plants from deep shelter to a site that receives gentle morning sun, then later to their intended light level. Bring them in before evenings become chilly, particularly if temperatures are forecast to fall sharply. Open greenhouse vents for longer periods on mild days so seedlings experience a gradual temperature change. Keep labels secure, because wind can mix similar trays. Check each plant daily for pale leaves, damaged stems or pests. Remove severely weak seedlings rather than investing space in them. Good spacing between pots improves air movement and makes it easier to spot dry compost before plants suffer.
During the second week, leave hardy seedlings outside for longer days and, if the forecast is settled, overnight in a protected cold frame or sheltered position. Tender plants should stay under glass overnight until the last spring frost has generally passed. Fleece or a cloche gives useful temporary protection, but it is not a substitute for sensible timing. Close a cold frame on cold nights and open it gradually each morning to prevent sudden overheating. Water early in the day so foliage dries before evening. Avoid feeding stressed seedlings while hardening off; focus on moisture, shelter and light first. A strong plant with a little slower growth is preferable to a soft, lush plant rushed into exposed ground.
Prepare the final planting site while seedlings harden off. Weed the bed, spread garden compost where appropriate and water dry soil a day before planting. For containers, use fresh peat-free multipurpose compost and ensure there are drainage holes. Compare the site with the plant's needs: tomatoes need full sun and shelter, while many brassicas cope with more open conditions. On planting day, choose calm, overcast weather if possible. Set plants out at their proper spacing, remove them carefully from modules and keep the root ball intact. Water each plant into its planting hole, then shade tender seedlings briefly if bright sun follows. Label rows so later watering, feeding and protection are easier to organise.
Keep observing plants for the first week after planting out. Water at the roots when the soil begins to dry, especially in containers and during windy weather. Protect young plants from late cold with fleece, a cloche or temporary return to the greenhouse if they are still in pots. Check that covers do not trap excessive heat during sunny days. A few lower leaves may yellow after transplanting, but new growth should soon look healthy. Do not replace every plant at the first sign of a setback; allow several days for recovery. Once seedlings are growing strongly in the garden, remove routine protection gradually so they adapt fully to normal conditions.
Frequently asked questions
Can I harden off seedlings in one weekend?
A weekend is usually too short for seedlings raised indoors. Aim for one to two weeks, beginning with sheltered daytime exposure and gradually adding sun, wind and cooler conditions. Hardy plants may adapt faster, but tender tomatoes, courgettes and beans need the full gradual process to avoid scorching, wilting and stalled growth.
Should seedlings stay outside overnight while hardening off?
Only after they have had several days of daytime exposure and nights are reliably mild. Hardy seedlings can use a sheltered cold frame sooner. Tender crops should remain indoors, in a greenhouse or under other frost-free protection until the local risk of late spring frost has generally passed.
What should I do if a cold night is forecast?
Bring tender plants back inside or into a greenhouse if they are still in pots. For planted seedlings, cover them with fleece or a cloche before evening and remove or vent the cover during warmer daylight. Water soil earlier if it is dry, but do not soak plants immediately before cold conditions.
General guidance for UK home gardens and allotments. Conditions vary by region, soil and season; the RHS or a local garden centre can give site-specific advice. Page updated 19 Sep 2026.