Hardening off plants is a gradual process
Avoid losing your stock to fluctuating temperatures and sunlight
One of the problems with starting plants off under glass is that eventually, they need to be hardened off. Even hardy plants get used to a life of luxury – regular watering, still air and stable temperatures.
Putting them outside to survive in widely fluctuating spring temperatures, much stronger sunlight and winds will lead to a check in growth, even death if they are caught by frost.
The effect of hardening off is to thicken and alter the plant’s leaf structure and increase leaf waxiness.
It ensures new growth is sturdy although much slower.
You need to harden off gradually, over a period of a couple of weeks, depending on weather. On a mild day, start with two to three hours of sun in a sheltered location.
Protect seedlings from strong sun, wind, hard rain and cool temperatures. Hardy plants acclimatise faster than half-hardy or tender kinds.
Don’t plant out tender plants before the date of the last frost, which is usually the end of May/beginning of June in North East England – but have horticultural fleece ready.
If you don’t have a cold frame, place plants in a sheltered position in front of a south-facing wall or hedge and cover with fleece to prevent sun scorch and temperature shock.
I make a small lean-to out of poles and bubble wrap – it’s not pretty, but it does the job. It’s also sheltered against a west-facing wall, which slowly releases its heat at night.
For the first week, leave outside during the day, but bring in at night. In the second week, leave outside at night, but keep covered (unless there’s a frost forecast).
Towards the end of the fortnight, remove the bubble wrap during the day (it has plently of airflow).
If the weather is suitable, leave the plants outside at night but ensure they are covered. After this, leave them uncovered before planting out.
Covering with an old curtain or extra fleece can protect from sudden sharp night frosts.