Plants grown in warm indoor conditions will need to be acclimatised very slowly to cooler outside conditions or they may suffer shock, which can check growth and damage foliage. This toughening-up process is termed 'hardening off'.
This is particularly true of half-hard and frost tender plants, which should not be placed outside until all danger of frost has passed. Usually around May or early June.
Hardy plants still need to be hardened off but will acclimatise faster than tender kinds.
Seedlings and cuttings grown in a warm greenhouse, propagator or a warm windowsill need to be slowly acclimatised to the cooler, less humid and windy conditions outside. When removing plants from propagator it is best done on an overcast day, keeping them out of direct sunlight, which will help reduce wilting.
Hardening off should be done steadily and progressively.
The best method is to transfer the plants to a coldframe and keep the top closed for a week. Raising the plants up on a sheet of polystyrene, bubble wrap or cardboard will also help to insulate them from the cold ground. If frost is forecast, cover the frame overnight with a blanket or old carpet, removing it the next day.
Start ventilating the frame on mild days, by raising the lid a little. Gradually open the lid up further over the course of a few weeks, until it is left off completely. At this point the plants should be ready to plant out into the garden.
A mini plastic PVC greenhouse can be used as an alternative if you don't have a coldframe or if space is limited.
Alternatively, stand the plants outdoors on warm days in a sheltered, south-facing, spot and bring them back in at night for the first two weeks. Initially placing them outside for a few hours and cover them with horticultural fleece to prevent temperature shock and sun scorch. Slowly increase the length of time they are outside, eventually leaving them out on warm nights until they are fully used to outside conditions.
If the plants suffer frost or cold damage, cut out the damaged growth and continue the hardening off process. Keep a watch out for pests, such as slugs, snails and greenfly, which will be attracted to the fresh young foliage once the weather warms.