How to harden off plants
Find out how to acclimatise tender plants to outside conditions, in our step-by-step guide.
This article has been checked for horticultural accuracy by Oliver Parsons.
Towards the end of May, all risk of frost has passed in most areas of the country, and it's therefore safe to put tender plants outside. But if you take them straight outside from a warm greenhouse, cool night temperatures can still scorch leaves and winds may tear them.
So, by gradually getting plants used to outside temperatures by putting them out during the day, then protecting them at night, you can help acclimatise them to the new environment. Do this over a week to a fortnight, and use a max-min thermometer to monitor temperatures.
Take fuchsias, flowering annuals and pelargoniums outside first, followed a little later by beans, courgettes, squash, sweetcorn and tomatoes for the veg garden. Always delay planting if the soil is not yet warm enough, as the roots will not expand out into cold, wet soil. Wait another month to ensure tropical plants survive. Houseplants can be moved out for some air in the warmest months.
Be aware of regional differences – frost-free nights occur from mid-May in the south to mid-June in the north, but late frosts may still occur so it's important to check the weather forecast.
In this video clip from Gardeners' World, Monty Don shows you how to protect tender plants, like 'Lady's Slipper' in spring in case temperatures take a sudden dip:
You will need
- Plants
- Cold frame
- A heated propagator
Step 1
Shut the lid of a covered cold frame at night and open it during the day for about a fortnight. This protects plants from wind while acclimatising them.
Step 2
Be aware of local weather forecasts and if a cold night is due, use fleece or a cover that will give some protection to plants that are already out in the garden.
Step 3
Tender plants are particularly susceptible to pests that enjoy soft, fresh growth. Slugs and snails are a real issue, so take steps to deter them.
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