How to harden plants off
All plants are hardened off in gradual stages.
Plants raised in heated glasshouses and on windowsills should go first into a cold glasshouse if available. When moving plants out of propagator it is best to do so on an overcast dull day as this will help reduce wilting. After two weeks in these cooler conditions, plants should be moved into a well-ventilated cold frame.
If you don't have a greenhouse, move plants into a cold frame, with the lid open slightly during the days of the first week and closed at night. Gradually raise the lid during the next fortnight until removing it entirely prior to planting. A cloche may be used but this does not give as much frost protection as a coldframe.
If there are no specialist facilities available, place plants in a sheltered position in front of a south-facing wall or hedge and cover with two layers of fleece to prevent sun scorch and temperature shock.
For the first week, leave outside during the day, but bring in at night. In the second week reduce to one layer of fleece. Towards the end of the fortnight remove the fleece during the day. If the weather is suitable leave the plants outside at night but ensure they are covered. Towards the end of the third week leave them uncovered before planting out.
Covering with an old curtain or extra fleece can protect from sudden sharp night frosts that occassionally occur in late spring.