Hardening off seedlings
You have raised healthy seedlings indoors. Then comes the step many beginners skip. Plants from a sheltered windowsill are soft and unprepared for wind, strong sun and cool nights. Move them straight outside and they scorch, wilt or stall. Hardening off is the gentle week or so that bridges the gap.
Where the shock comes from
Factor | Why it shocks them |
|---|---|
Wind | indoor plants have never felt it |
Direct sun | far stronger than through glass |
Cool nights | a shock after a warm room |
Dry air | humidity is lower outdoors |
The routine
Seven to ten days, leaving the plants out a little longer and in a little more sun each day.
Day | What you do |
|---|---|
1–2 | 1 to 2 hours, in shade |
3–4 | longer, with some sun |
5–6 | most of the day |
7–8 | out all day |
9–10 | overnight if mild |
Then | plant out, after the last frost |
Start with a sheltered, shady spot for an hour or two, then bring them back in. Each day increase the time and give them a little more sun and breeze. By the end of the week they can stay out all day, and then overnight once nights are mild and frost has passed. Only then do they go into their final spot.
Avoiding transplant shock
- Plant on a dull, still day or in the evening, not in blazing midday sun.
- Water the seedlings well both before and after planting.
- Firm them in gently and keep the rootball intact. Disturb the roots as little as possible.
- Shelter new plants from strong wind and watch for slugs in the first weeks.
- Never put tender crops out before the last frost has passed.
Common mistakes
- Going too fast. A full day outside on day one is too much, too soon.
- Full midday sun straight away. Start in shade and build up.
- Letting pots dry out. Small pots dry quickly outdoors in wind and sun.
- Planting out too early. Wait until frost has truly passed.
Frequently asked questions
How long does hardening off take? Usually 7 to 10 days. Tender crops such as tomatoes benefit from the full stretch; tougher plants need less.
Do I need to harden off direct-sown plants? No. Plants sown outdoors are already acclimatised. Hardening off is for indoor-raised seedlings.
Can I speed it up? Try not to. Rushing is exactly what causes shock. A cold frame does some of the work for you.
What if frost is forecast? Bring plants back in or cover them. Do not plant tender seedlings out while there is still a risk.
The full version of this guide is on seedschoice.com.
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Updated on: 05/10/2026
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