At a glance
- Hardening off means: accustoming your pre-grown young plants slowly to sun, wind and cool over the last one to two weeks, before they go into the bed.
- Without this acclimatisation, shock threatens. Full sun burns the soft leaves, wind dries them out, cold slows the growth.
- The plan is simple: first a few hours in the shade, then more sun and time each day, bring them in at night at first, later leave them out.
- Plant out only after hardening off and after the last frosts, best on a cloudy day or in the evening.
For weeks you have raised your tomatoes on the windowsill, strong and green. And then, no sooner are they in the bed, the leaves hang limp or show white, burnt patches. The reason is almost always the same: the plants were not hardened off.
Hardening off is the last, often forgotten link of seed-raising. It bridges the gap between the sheltered room and the rough outdoors. Whoever takes these one to two weeks is thanked by the plants with a strong start, instead of days of ailing.
Why harden off?
In the warm, windless room, young plants form soft tissue and a thin protective layer on the leaves. They were never challenged by UV radiation, wind or cold. Set them into blazing sun without preparation and it is like a jump into cold water for them.
You soon see the consequences: the leaves get sunburn with pale, silvery patches, the wind makes them wilt, and a cold shock stops the growth for days. In hardening off, the plant builds a firmer leaf surface and a sturdier growth in peace. It is toughened up, quite literally.
The ten-day plan
You begin about ten to fourteen days before the planned planting out. Increase sun and time slowly and it works reliably.
Days 1 to 2: a gentle start
Put the plants outside for two to three hours in a sheltered, part-shady spot, then back in. No direct midday sun yet.
Days 3 to 5: more sun and time
Extend the time outdoors and give them some morning sun. Wind and changing weather are part of it now; they make the plants strong.
Days 6 to 8: almost the whole day
Now the plants may spend most of the day outdoors and in direct sun. If it is frost-free and mild, they can even stay out overnight in a sheltered spot.
Days 9 to 10: out day and night
Finally the plants stay out entirely, day and night, as long as no frost threatens. Now they are accustomed to the outdoors.
Plant out with care
Set the hardened plants into the bed after the last frosts, best on a cloudy day or in the evening. That way they do not get the full midday sun at once.
Take sunburn seriously
The commonest mistake is putting the plants into full sun too quickly. If the leaves show pale, silvery-white or bleached patches, that is sunburn. Move the plant back into part shade at once and go more slowly. Burnt leaves do not recover, but the plant produces new growth if you give it the acclimatisation.
Make hardening off easier
A cold frame or a sheltered corner by the house wall saves you the daily carrying. Slightly sparser watering during this time also hardens the plants without letting them wilt. Cold-tolerant crops like cabbage and lettuce you can harden and plant earlier, the warmth-lovers only after the last frosts.
Accustom slowly to sun, wind and cool. Two weeks of patience save you weeks of ailing in the bed.
The core rule for hardening off
Frequently asked questions
How long must I harden off young plants?
About one to two weeks. In that time you increase the time outdoors and the sunlight daily. More important than the exact duration is to go slowly and steadily and to watch the plants.
How do I recognise sunburn?
By pale, silvery-white or bleached patches on the leaves, usually on the upper side. They form when tender leaves get too much direct sun too fast. Back into part shade and carry on more slowly.
Can I skip the hardening off?
Better not. Unhardened plants often suffer a shock in the bed, lose leaves and do not grow for days. In the end you are slower that way than with the two weeks of hardening. Only robust, cold-hardy direct sowings do not need it.
What do I do if night frost still threatens?
Then the plants stay indoors at night or in the closed cold frame. Warmth-lovers like tomato and pepper may go out entirely only after the last frosts anyway. A sudden night frost can otherwise destroy the whole raising.
When is the best time to plant out?
After hardening off and after the last frosts in mid-May, on a cloudy day or towards evening. That way the plants start without the stress of blazing midday sun. How to get them that far in the first place is in Pre-growing on the windowsill.
Sources
- Bavarian State Institute for Viticulture and Horticulture (LWG): hardening off and planting out young plants
- Julius Kühn Institute: seed-raising and planting in the home and allotment garden
- Related in the magazine: Pre-growing on the windowsill and The garden in May
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