Advice
RHS Help & Advice
Hardening off tender plants
Plants
raised in protected environments need to be carefully acclimatised
to cooler temperatures and increased air movement otherwise
the shock at planting out can kill them.
The time required for this hardening off varies, but generally takes between two to six weeks. It will be to the plants' detriment if this process is rushed over a few days.
Once hardened your plants can be planted outdoors in their final positions or potted up. Plants grown under warm conditions (i.e. a heated propagator or windowsill) will generally require the longest hardening off period, as they normally have very soft new growth. When moving plants out of propagators it is best to do so on an overcast dull day as this will help reduce wilting.
As plants begin to harden up, the natural waxes coating the leaves undergo changes in form and thickness to help reduce water loss. Stomatal pores on leaf and stem surfaces need to adapt to less favorable conditions. If plants are not hardened off, they may suffer a setback from which it can take several weeks to recover, or they may grow in ways that are not desirable; cauliflower, for example, is liable to develop premature heads. It should be remembered that even the best hardening-off process cannot make a frost-sensitive plant into a hardy one. Half-hardy and frost tender plants must be protected until the danger of frost has passed.
Timing
Place plants by a sheltered wall or hedge and cover with two layers of fleece. For the first week, leave outside during the day, but bring in at night.
In the second week, reduce to one layer of fleece but continue to bring them in at night. 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.
Cold frames are ideal for hardening off. Prop open slightly during the first week. 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 cold frame. If you do not have any of these ready built facilities place the containers near a sheltered wall or hedge preferably with a south-facing aspect.