How to harden off plants

How to harden off plants

Find out how to acclimatise tender plants to outside conditions, in our step-by-step guide.

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 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 stay prepared in case temperatures take a sudden dip:

Advertisement

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.

How to harden off plants - covering plants with a propagator lid
How to harden off plants – covering plants with a propagator lid

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.

How to harden off plants - plants hardening off in a cold frame
How to harden off plants – plants hardening off in a cold frame

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.

How to harden off plants - covering plants with fleece
How to harden off plants – covering plants with fleece

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.

How to harden off plants - protect plants from slugs and snails
How to harden off plants – protect plants from slugs and snails
Advertisement