What happens in a plant during hardening off?
Jack Foster, by email
Stefan says: Hardening off is the process of gradually accustoming a plant to outdoor conditions. Plants are made up of many tiny cells and, when these are examined, those of plants from a greenhouse are seen to be large with very thin walls. By contrast, plants that have been hardened off have smaller, thicker-walled cells. These smaller, tougher cells can resist having water withdrawn from them and the collapse of the tissues that this causes.
For hardening off, I adopt a threeweek cold frame routine. In the first week, after removing seed trays or plants from the greenhouse, I close up the frame at night and leave it half open in the daytime. In the second and third weeks, I leave it fully open in the daytime and half open at night.