How to harden off plants

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 by beans, courgettes and tomatoes for the veg garden. Wait another month to ensure tropical plants survive. Houseplants can be moved out for some air in the warmest months, while melons, squash and sweetcorn should be planted later when the ground is warmer.

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 in spring in case temperatures take a sudden dip:

You Will Need

• Plants
• Cold frame
• A heated propagator

Step 1

Over the course of a fortnight, take out plants grown in humid propagators during the day, but pop them back in again at night. Slowly lower a heated propagator’s temperature over a fortnight.

Step 2

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 3

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 4

Tender plants are particularly susceptible to pests that enjoy the soft, fresh growth. Slugs and snails are a real issue, so take steps to deter them.