How to harden off plants

Nick Moyle and Rich Hood

When reading growing guides, one phrase you will repeatedly come across goes something like this: “harden off your plants before planting them outside.” But what is hardening off, and how do you go about it? Let us explain…

Any plant that has been kept in a warm house or greenhouse that is destined for a life outdoors—whether it’s something that has been sown indoors, bought from the heated part of a garden centre, or brought inside over winter—will be so used to its cosy interior conditions that suddenly moving it to a colder location will send it into shock. 

Plants kept inside will be more tender and not ready for the range of weather conditions and variable temperatures they will experience outside, so before they’re moved permanently they need to be toughened up first, a process known as “hardening off”.

To do this you need to gradually introduce plants to the outside world, steadily increasing their exposure until they’re tough enough to move out on a full-time basis. This will allow their stems and leaves to thicken and slow down growth, so they can strengthen ready for new growth in their new environment

Ideally you want to manage this process over the course of around three weeks, and make sure that any plants sensitive to frost make the permanent move once all icy danger has passed.

 

Week 1

For the first week, only put your plants outside during the day, bringing them in at night before the temperatures start to plunge, placing them somewhere that is a little cooler than where they’re coming from. 

If they’ve been in a heated house or greenhouse, then an unheated greenhouse will make a suitable next step. If you have lots of plants to transfer, then it might be worth investing in a cold frame—a glass or plastic windowed structure with a fully removable lid specially designed for the process. 

Anyone without such luxuries will need to wrap their plants up in something warm instead. Horticultural fleece or bubble wrap are both suitable options. Placing them by a wall of the house will also be a touch warmer and more sheltered than the middle of the garden.

 

Week 2

As your plants get used to being outside during the day you can gradually expose them to more cold and wind. 

Start opening the doors and windows of your greenhouse; raise the lid of your cold frame; or gradually remove layers of fleece and bubble wrap. 

 

Week 3

As you move into the third week you can start leaving your plants outside for longer, eventually keeping them out all night. However, as with those first few days, increase their nighttime temperature gradually by first placing them into a cold frame or wrapping with fleece before nestling yourself into your own cosy duvet. 

At the end of this third week they should be ready for a few days and nights all on their own with no protection, although if the temperature plunges to considerably lower than they’ve been used to, tuck them back in until the cold snap has passed. 

Once they’ve proved they can handle a few chilly nights out then they’re ready to plant out full time—stronger, weather hardened and ready to grow into healthy, outdoor plants.

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