You have raised healthy seedlings indoors — now comes the step too many beginners skip. Plants grown on a cosy windowsill are soft and unprepared for wind, strong sun and cool nights. Move them straight outside and they can scorch, wilt or stall. Hardening off is the gentle week-or-so of acclimatising that bridges the gap, and it makes all the difference to how well they take.
- Wind indoor plants have never felt it
- Direct sun far stronger than a windowsill
- Cool nights a shock after warm rooms
- Dry air lower humidity outdoors
What hardening off is, and why it matters
Hardening off means gradually exposing indoor-raised seedlings to outdoor conditions over about a week to ten days, so they toughen their leaves and stems before being planted out for good. Skip it and tender growth can be scorched by sun, battered by wind or checked by cold — undoing weeks of careful growing in a single day. A little patience here pays off in stronger, faster-establishing plants.
How to harden off, step by step
- Days 1–2 1–2 hrs, shade
- Days 3–4 longer, some sun
- Days 5–6 most of the day
- Days 7–8 out all day
- Days 9–10 overnight if mild
- Plant out after last frost
Start by setting plants outside in a sheltered, shady spot for an hour or two, then bring them back in. Each day, leave them out a little longer and in a little more sun and breeze. By the end of the week they can stay out all day, then overnight once nights are mild and frost has passed. Only then plant them in their final spot.
Avoiding transplant shock
When planting-out day comes, a few small habits stop plants sulking:
- Plant on a dull, still day or in the evening, not in blazing midday sun.
- Water the seedlings well before and after planting.
- Firm them in gently and keep the rootball intact — disturb roots as little as possible.
- Shelter new plants from strong wind and watch for slugs in the first weeks.
- Never rush plants out before your last frost — tender crops will not forgive it.
Common mistakes
- Going too fast — a full day out 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 outdoors dry quickly in wind and sun.
- Planting out too early — wait until frost has truly passed.
Frequently asked questions
How long does hardening off take?
Usually 7–10 days. Tender crops like tomatoes benefit from the full stretch; tougher plants need less.
Do I need to harden off direct-sown plants?
No — plants sown direct 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 makes it easier by doing some of the work for you.
What if frost is forecast?
Bring plants back in or cover them — do not plant tender seedlings out until the risk has passed.
Hardened off and ready? Browse all seeds.
