Techniques
Hardening off
Gradually acclimatising indoor-raised seedlings to outdoor conditions over 7–10 days before planting them out, so the shock of wind, sun and cold does not check or kill them.
Seedlings raised indoors or in a heated propagator grow up soft. They have never felt wind, never had direct sun on their leaves, and have lived at a steady, cosy temperature. Take a plant straight from a warm windowsill and put it out into a breezy May garden and the change is brutal: leaves scorch, growth stalls, and tender crops like tomatoes, courgettes and beans can collapse altogether. Hardening off is simply the week or so of gentle exposure that toughens them up first, and skipping it is one of the most common reasons beginners lose plants at the transplanting stage.
The routine is easy and forgiving. Starting about 7–10 days before you plan to plant out, put your seedlings outside in a sheltered, lightly shaded spot during the day, then bring them back indoors (or into a porch or greenhouse) each night. On the first day, an hour or two is plenty. Each day after that, leave them out a little longer and in slightly more sun and breeze. By the end of the week they can stay out all day, and for the last night or two — frost permitting — you can leave them outside overnight. Then they are ready to go into the ground.
A cold frame makes this much simpler: you prop the lid open a bit more each day and close it at night, which does the acclimatising for you without all the carrying in and out. A cloche, or even a sunny wall you can tuck plants against, works well too. The key is to start gentle and build up — wind matters as much as cold, so don't leave young plants flapping in the open on day one.
Timing is everything in the UK, because hardening off is the bridge to planting out, and tender crops must not meet a frost. Watch your local last frost date: it can be late April in mild southern and coastal areas but mid-to-late May further north, inland, or up high. Keep an eye on the forecast and have some fleece ready to throw over plants if a cold night sneaks in. Hardy crops are more relaxed, but the same gradual approach still pays off and gets them growing away faster.
This whole step is well worth the patience. A seedling that has come through germination on your windowsill represents weeks of care, and a careless planting out can undo it in a single afternoon. For more on getting young plants from sowing to soil, see our guides to starting a vegetable garden and the easiest crops for beginners.
In a UK garden
In most of the UK you harden off through late April and May, timing planting out for after your local last frost — often mid-to-late May in the north, inland or up high.
Example
A tray of tomato seedlings raised on a windowsill goes outside for a couple of hours, then a little longer each day, before being planted out a week or so later.