Hardening Off

Sunday, April 12, 2009


A lot is written with reference to "hardening off". In fact you can hardly read any gardening book without seeing the term. When I first started gardening (only a few years ago) it was a term that confused me and didn't seem to be explained anywhere.

What is hardening off?

In essence hardening off is simply the process of acclimatising a young plant or seedling that has been grown under glass or indoors to cooler outside temperatures.

Why harden off?

Many plants are prone to significant checks in growth if they are moved straight from a warm cosseted environment (such as a greenhouse or propagator) directly to their outside location. If the weather is kind to you, you’ll probably get away with it, however if you have even a minor cold snap then you could be in trouble.

Cauliflower in particular is prone to these checks in growth and can frequently cause the plant never to develop full sized heads, leaving you with golf ball sized cauliflowers.

Last year my sweet corn suffered because I didn’t follow my own advice and planted directly from a greenhouse to outside. I stupidly thought early June would be okay weather wise. What happened was of course that we then had about two weeks of solid rain and cold weather. The result was that only about half the crop grew to a useable size with some of the plants dying and rotting where they stood.

How to harden off?

Without a cold frame, the standard procedure for hardening off is pretty laborious. You place the plant outside for perhaps an hour on the first day and then bring it back inside. You repeat this slowly increasing the amount of time it spends outside until it is outside for most of the day. At which point it can then be transplanted to its permanent position outside.

This process is fine if you have plenty of time on your hands, however if you work this process can be nearly impossible.

Hardening off the easy way - A cold frame

If you have a cold frame, it becomes incredibly easy to harden off your seedlings. The basics are simple, on day one start with the roof light of the cold frame just slightly ajar and over the course of the next couple of weeks raise the light a little higher each day. After a couple of weeks the roof light should be fully opened and the plant is now hardened off and ready to be transplanted.

The added advantage of using a cold frame to harden off your plants is that the plants remain much more protected from the elements, such as heavy rain and strong winds while they are getting used to the change in temperature.