Hardening off

This term is used to describe the process of acclimatising plants, which have been kept in a protected environment, to outdoor conditions. This is required when new plants have been propagated under cover or where tender plants have been brought indoors over the winter.

The period of hardening off is required to enable the natural waxes and stomata in the leaves to adapt to the new conditions, to reduce water loss. Effectively this ‘hardens’ the external layers of the leaf to withstand the wider temperature fluctuations, wind and sunlight which they will be subjected to outside.

Often a cold frame or cloche is used for hardening off plants, with the cover being lifted for gradually increasing periods (starting with daytime only). Alternatively the plant may be kept indoors but brought outside for gradually longer periods (starting in a partially shaded, sheltered spot, such as next to a hedge or wall).

When hardening off plants, keep an eye on the weather and provide plants with extra protection if heavy rain or frost is forecast, especially if this is early on in the hardening off process. It’s also advisable to put plants on tables or benches to begin with, while they’re very young and particularly vulnerable to the attentions of slugs and snails!

The process of hardening off will take between 1 and 6 weeks or more, depending on the type of plant and the extent of the difference between the protected and external environmental conditions.