Hardening off is simply the process of acclimatising a young plant or seedling that has been grown indoors to the outdoor environment.  The trick is to gradually acclimatise the seedling so that it doesn’t experience shock at the change of conditions.

Why harden off?

Many plants are prone to a check in growth if they are moved straight from a warm environment such as a greenhouse directly to an outdoor location.  With some varieties a cold snap after planting out can be sufficient to kill of your seedlings if they haven’t been hardened off.

How to harden off?

If you don’t own a cold frame, the traditional approach for hardening off is pretty laborious. On day one, the plant is placed outside for perhaps an hour and then brought back inside. You repeat this process each day, slowly increasing the amount of time spent outside until it is outside for most of the day and night. At which point it can then be transplanted to its permanent position outside.

If you have plenty of time, or you work from home then this method of hardening off is achievable.  Unfortunately many of us have work or other commitments that make this method of hardening off impractical.

Hardening off the easy way – A cold frame

One of the biggest advantages of a cold frame is that it becomes incredibly easy to harden off seedlings.  Hardening off with a cold frame is achieved by adjusting the position of the roof lights, with the seedlings spending the whole day in the cold frame.  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 open the light a little further each day. After a couple of weeks the roof light should be fully opened day and night.  The plant is hardened off and ready to be transplanted without its growth being stunted.