How do I harden off seedlings?
Acclimatising indoor-grown plants to outdoor conditions over a week or two, and why skipping it kills them.
- Difficulty
- beginner
- Time
- 30 min
- Read
- 2 min
Short answer
Put them outside in a sheltered spot for a couple of hours on day one, increasing by an hour or two each day over one to two weeks, bringing them in at night until the last few days. Plants moved straight outside are often killed within a day.
Indoor-grown plants have thin leaves, no wind conditioning and no UV tolerance. Outdoors they meet all three at once. Hardening off gradually triggers the physical changes — thicker cuticles, stronger stems — that let them survive it.
What you'll need
- A sheltered outdoor spot
- Cold frame or cloche (optional)
- Fleece (optional)
Step by step
- Start about two weeks before you intend to plant out.Rushing this is the most common reason seedlings fail after transplanting. Two weeks is realistic; one is a minimum.
- Choose a sheltered spot out of direct midday sun and wind.Against a wall, under a table, or in the shade of a hedge. Both wind and full sun scorch soft indoor growth.
- Day one: two hours outside, then back in.A mild, still, overcast day is ideal for the first outing.
- Increase the time outdoors by an hour or two each day.Gradually introduce more sun and more exposure as the days progress.
- Bring them in every night for the first week.Night temperatures are the real risk. A cold night undoes a week of daytime acclimatisation.
- From about day eight, leave them out overnight if there is no frost forecast.Check the forecast rather than the calendar. A late frost is what kills hardened-off plants.
- Keep them well watered throughout.Outdoors they dry out far faster than indoors, particularly in small pots and on a windy day.
- Plant out on a dull, still day, ideally in the evening.It gives them a night to settle before facing sun and wind. Water in well after planting.
Tips
- A cold frame does this almost automatically — open it progressively wider each day and close it at night.
- Fleece over newly planted seedlings for the first week provides useful insurance against both wind and a late frost.
- If plants get scorched or wind-damaged, move them back to shelter for a few days. Most recover if caught early.
Common mistakes
- Planting straight out from a windowsill — The shock of sun, wind and cold together kills or badly checks most seedlings within a day.
- Starting in full midday sun — Soft indoor leaves scorch in hours. Start in shade and introduce sun gradually.
- Leaving them out overnight too early — Cold nights, not warm days, are what does the damage. Bring them in for the first week.
Questions people ask
How long does hardening off take?
One to two weeks. Start with a couple of hours in a sheltered spot and build up, bringing them in at night until the last few days.
What happens if I do not harden off plants?
They are usually badly checked or killed. Sudden exposure to sun, wind and cold scorches the leaves and collapses the stems, often within a single day.