You have raised strong, healthy young plants indoors or in the greenhouse. Then you plant them straight out into the garden — and within days they are pale, scorched or collapsed. Almost always, the culprit is a skipped step: hardening off.
What does "hardening off" mean?
Hardening off is the process of gradually acclimatising a plant that has been grown under cover to the harsher conditions outdoors — direct sun, wind, cooler nights and fluctuating temperatures. Indoor-raised plants have soft growth and thin leaf cuticles; thrown outside unprepared, they suffer transplant shock and wind burn. A week or two of gentle transition builds tougher tissue and a stronger plant.
When should you start?
Begin hardening off once the danger of hard frost has passed for your area and your plants are large enough to handle. As a rule of thumb, start the process about seven to fourteen days before you intend to plant out. Tender plants should never go out until nights are reliably above 10°C.
A simple week-by-week routine
- Days 1–2: Place plants outside in a sheltered, shaded spot for two to three hours, then bring them back in.
- Days 3–4: Extend to five or six hours, and let them have a little gentle morning sun.
- Days 5–6: Leave them out most of the day in a brighter position, sheltered from strong wind.
- Days 7–9: If nights are mild, leave them out overnight in a sheltered spot.
- Days 10–14: Full days and nights outdoors. They are now ready to plant.
Tips for success
Watch the water
Plants outdoors dry out far faster than those under cover, especially in wind. Check the compost daily — but avoid leaving them waterlogged in cold weather.
Mind the wind, not just the sun
Wind is the most underrated stress. A steady breeze thickens stems, but a cold gale will shred soft foliage. Start in a sheltered corner and move to open ground gradually.
Use a cold frame if you have one
A cold frame makes hardening off almost effortless: open the lid a little more each day and close it at night. A cloche or a sheltered spot against a south-facing wall works well too.
The payoff
It is tempting to rush — but a fortnight of patience now means plants that establish quickly, shrug off the weather and get straight to growing. Every plant we send out has been hardened off in season here at the nursery, so it arrives ready to face the garden. See what is ready to plant now.