Tender young plants raised indoors or underglass, need to be 'gently' introduced to the outside climate found in the garden, this process is known as 'hardening off' and normally takes about 2 week period.
Hardening off is especially important if the plants have been grown from seed in peat compost as this tends to produce tender, soft, lush growth - plants grown in soil based compost tend to be more hardy but a degree of hardening off is still necessary.
Whether the plants are flowers or vegetables, the general principles of hardening off are similar. Keep a careful watch on the plants during the hardening off period; if the plants seem to be suffering, reduce the daily period of exposure or even return the plants indoors or underglass until the weather warms up a bit.
Method 1 - just put outside
The simplest method of hardening off requiring no special equipment (i.e. cloches or a cold frame), is to just put the boxes or pots of plants outdoors in a sheltered position every day during the warmest part of the day for increasing periods. After about 10 days, if there is no risk of frosts, the plants can be left outside all the time.
Method 2 - using cloches
Place the boxes or pots of young plants in the garden initially covered with cloches with the ends covered. Depending on the daytime temperature, remove the end covers during the day and move the cloches to uncover the plants during the warmest part of the day for increasing periods of time - eventually leaving the plants uncovered all day but covered at night. When there is no risk of frosts, the plants can then be left uncovered all the time.
Method 3 - using a cold frame
Place the boxes or pots of young plants in a Cold Frame, start ventilating the cold frame for a short period of time during the warmest part of the each day. Gradually increase the period of ventilation until the frame is left open all day but is closed at night. When there is no risk of frosts, the cold frame can be left open all the time.