Hardening off takes a week, and skipping it costs you a month

Hardening off is a week of getting indoor seedlings used to the outdoors before you plant them. Put them outside for a couple of hours in the shade on the first day, a little longer and in a little more sun each day after, and by the end of the week they can stay out overnight.

Skip it and a plant that’s never felt wind, direct sun or a cold night can scorch, wilt or simply stop growing for weeks. A week of hardening off gets them going faster.

Why it matters

Seedlings raised on a windowsill or under lights have soft, thin leaves with a weak waxy coating. Outdoors, the sun is far stronger, wind dries and bends them, and the nights are colder. Hardening off thickens the leaves and toughens the stems, so the plant can cope.

A week-by-week plan

Start about a week before you plan to plant out. For tender crops, that’s usually around your last frost date, so they’re ready to go out a week or two after it.

Day	Where	How long
1	Sheltered, shady spot	1 to 2 hours
2	Shade	3 to 4 hours
3	Morning sun, then shade	Half the day
4	Mostly sun	Most of the day
5	Full sun	All day
6	Full sun	All day, and out overnight if mild
7	Out all the time	Plant out

If a day is very windy, wet or cold, skip it and carry on the next day. The plan stretches easily; it shouldn’t be squeezed into two or three days.

Easier ways to do it

• Cold frame. Put the plants in a cold frame and open the lid a little more each day. Close it at night at first.
• Greenhouse. Move plants from a heated greenhouse to an unheated one, then outside.
• Fleece. Put plants out under horticultural fleece for the first few days, then take it off during the day.

If you’re out at work, a cold frame or a sheltered porch is the most practical option, as you don’t need to move plants in and out.

Which crops need it most

All seedlings raised indoors benefit. It matters most for:

• Tender crops: tomatoes, peppers, chillies, aubergines, courgettes, squash, cucumbers, sweetcorn, beans and basil.
• Plants raised in warmth: anything grown under lights or on a heat mat.

Hardy crops such as cabbages, kale, lettuce, leeks and onions harden off quicker and can go out before the last frost once they’re used to the cold.

Planting out

Plant on a calm, cloudy day, or in the evening, so the plants have a night to settle before full sun. Water them well in their pots an hour before, and again after planting. For the first few days, water every day or two until the roots grow out into the soil.

Signs of trouble

White or pale patches on leaves. Sun scorch. Shade for a few days; new leaves will be fine.

Purple leaves. The plant is cold. It usually recovers as nights warm.

Wilting. Too much sun and wind too soon. Move it back into shade and slow down.

For when to sow the seedlings in the first place, see starting seeds indoors, and for tender crops, the last frost in the UK explains why the date matters.