All half-hardy plants that have been raised in a controlled environment must be acclimatised gradually to the natural conditions outdoors before being planted out. This will harden the plants, reducing their dependence on artificial heat and protection without subjecting them to any sudden changes that might cause damage.

This process should not be rushed; the natural waxes that coat the leaves of the young plants need to adapt their form and thickness in order to reduce water loss. The leaf pores, through which oxygen and carbon dioxide pass in and out and which control water loss, also need time to adapt to the harsher conditions outside.

Method

The easiest way to harden off plants is by using a greenhouse and a cold frame.

Step 1

Move the seedlings to a cooler part of the greenhouse or turn off the propagator heat 6 or 7 weeks before planting them outdoors.

Step 2

After a week, transfer them to a closed cold frame, gradually increasing the ventilation during the day as the weather improves. The frame should be closed at night to begin with, graduating to leaving it partially open at night, except when frost is anticipated. In the final few days before planting, the lid can be removed completely.

Alternative Methods

Other equipment, such as glazed frames, cloches or polytunnels can also be used for hardening off, or alternatively, the young plants may be placed outdoors in sheltered positions and protected at night with plastic on temporary wood or bamboo frames. The plants should be left uncovered during the day to ensure that they receive enough light and ventilation; however, they should be covered if the weather conditions are poor.