Plants that have been kept indoors or in a greenhouse can’t be planted outside straightaway in spring – you need to acclimatise them first. This process is known as hardening off. If this doesn't happen, exposure to cold winds or a sudden drop in temperature can seriously weaken or even kill the plants.
You'll need to harden off young plants that have been raised on the windowsill or in the greenhouse, small veg or ornamental 'plug' plants ordered by mail order and tender plants that you've kept in the greenhouse over winter.
Caption: Hardening off will help prepare tender plants for life outdoors
How to harden off
Using a coldframe, makes hardening off a much easier task. Position your frame so it faces south or south-west to receive maximum sunlight. Ideally it will be protected by a wall or fence. Don’t stand it on low ground where cold air collects. Before using, wash down the glass and frame with a garden disinfectant. When Which? Gardening magazine trialled different methods of hardening off plants, the method that worked best was:
- Keep the coldframe lid half open during the day when sunny for the first six days and closed at night.
- Then keep the coldframe lid fully open during the day when sunny for the next six days and closed at night.
- Finally keep the coldframe lid fully open both day and night for the last six days.
When to harden off
Depending on the plant, you should be hardening off around the time of the last frost dates in spring. Seek local advice and check weather forecasts.
What to harden off, and when
When they’re large enough to handle, hardy pansies, cabbages and Brussels sprouts can be moved to the coldframe.
Alyssum and antirrhinum can cope with a few light frosts, so can be moved to the coldframe 4-5 weeks before the last frost date.
Lobelia, Phlox drummondii and mesembryanthemum should be hardened off before the last frost dates.
Other tender bedding (e.g. begonia, petunia, impatiens, salvia and pelargonium) and tender vegetables (e.g. courgettes cucumbers, sweetcorn, runner beans and tomatoes) should be hardened off around the last frost.
Coldframes offer great scope for DIY and improvisation – half of all Which? Gardening members who own a coldframe made their own.
Their advice is to:
- Make sure it’s big enough and tall enough for the plants you want to grow.
- Make sure the coldframe is easy to open, close and ventilate.
- Solid sides are generally preferable as these offer more insulation and reduce the Which? works for you risk of scorching on sunny spring days. Bricks, breeze blocks, old railway sleepers or other second-hand timbers are all suitable. If you can arrange it so the sides aren’t permanently fixed, you can dismantle the frame when you don’t need it any more. If you site your coldframe in a permanently shady position, or use it to raise summer crops such as melons, then it would be better to have transparent sides.
- The top needs to be transparent, easy to lift and remove, and capable of being propped in a semi-open position for ventilation. Glass is excellent for transparency and its weight reduces the risk of it blowing away. Second-hand windows are good for this. Plastic has the great benefit of being unbreakable – twinwalled polycarbonate is often used for packaging and may be available as off-cuts.
Coping without a coldframe
On a frosty morning, look round your garden and take a note of the places where there is little or no frost, for example, a sheltered, south-facing wall or fence, under a hedge or on the sheltered side of the house. These should prove ideal places for hardening off plants later in the spring.
Stand your plants in a sheltered spot outdoors during the day. To begin with, leave them for only a few hours when it’s mild and gradually increase the time until, after two or three weeks, they can be left out all night. However, be ready to cover them with newspaper or a double layer of horticultural fleece if frost is forecast.
What to do if you’re caught out by frosts
Even experienced gardeners can be caught out by an unusually late frost. If you find recently hardened-off plants have been frosted, try to make sure they thaw out as slowly as possible.
If possible, move the plants into permanent shade until they thaw. Otherwise, you could try draping newspapers or netting over them.