Tips on How to Harden Off Plants

If your plants have been raised indoors or in a greenhouse, they’ll probably need to go through a ‘toughening up’ process known in the gardening world as hardening off before they can cope with the cooler temperatures outside. However, if you have little gardening experience, as many people do, you might not know where to begin with this process. This guide should help you.

Why harden off plants?

This process means that plants can adapt from being protected and cared for in the safety of a greenhouse, to the changeable weather conditions of outside. If tender plants were to be moved outside without being hardened off, their growth could be significantly affected. This process helps to change the plants’ structure and make them more sturdy. You should be aware, however, that this process will not make them resilient against frost, ice and snow.

When should you harden off plants?

The process isn’t a very long one, taking between two and three weeks, on average. However, the warmer the initial conditions the plant was grown in, the longer the process will take as the plant will have more adapting to do. To give your plants the best chance of surviving, you should aim to have the process finish after the last frost. This is usually late spring for southern parts of England, but for the north and Scotland it may be later in the year.

How to harden off the plants

This process is comprised of different stages:

  1. Move plants raised in heated greenhouses or on windowsills into non-heated greenhouses. It is best to do this on an overcast, dull day to help prevent the plants from wilting in the sun.
  2. After two weeks in these conditions, you should move the plants into a cold-frame that it well ventilated. Over the next week, you should gradually raise the lid of the cold-frame until it is completely removed prior to planting. You could use a cloche instead of cold-frame, but it won’t be as effective in preventing frost-damage.
  3. You can now move the plant fully outside under two layers of fleece. For the first week, it should still be brought inside at night to prevent frost damage. In the second week, you can reduce to just one layer of fleece.
  4. Now the fleece can be removed completely during the day. Plants can now, weather permitting, be left outside at night but should be covered with fleece.
  5. In the third week, plants can be left completely uncovered. However, you should make this decision based on the weather and the likelihood of frost. If frost is likely, you should ideally cover to reduce damage.

Do you have any tips and advice on how to harden off tender plants? Share your ideas in the comments section below!

[Photo Credit: Marj Joly ]