HOW TO HARDEN OFF TENDER PLANTS
Best techniques for toughening up your plants
IF your greenhouse and windowsills are groaning under the weight of cuttings and seedlings, you might be tempted to create more space by moving some outside.
But hold fire – rising temperatures may seem plant-friendly but nights are cold and the last frosts are still some way off, so moving young plants into the garden now could severely stunt their growth and kill them. The last frosts usually occur in late spring in the south, and slightly later further north.
Wait for a few more weeks before starting the process of hardening them off, which is how you prepare tender and overwintered plants for life outside.
It takes two or three weeks, and during this time the plant’s leaf structure thickens and becomes waxier so it is more able to cope with lower temperatures. However, it will not make frost-sensitive plants hardy.
Plants should be hardened off in gradual stages. Those raised in a heated greenhouse or indoors should start by being moved into a cold greenhouse for a couple of weeks before being placed in a cold frame.
If you don’t have a greenhouse, go
straight to a cold frame, with the lid open during the day and closed at night. Keep the lid open for longer during the next two weeks until the plants are left totally exposed just before planting.
If you don’t have a cold frame, place plants against a sheltered sunny wall and cover with two layers of fleece. Bring them in at night for the first week, and in the second week take away one layer of fleece. After this, remove the fleece altogether during the day, replacing it only as dusk falls.
By the end of the third week the plants can remain uncovered the whole time and should be ready to plant out.
Acclimatise seedlings in a cold frame A cold frame and fleece are two important features of the hardening-off process