What is "hardening off"?
“Hardening off” is a term used for plant acclimatisation. It's basically just taking your plants outside for part of the day (before bringing them back in again later) so that they can gradually get used to the outside environment with it's strong sunlight, chilly nights and dry air.
Why is it important to harden off plants?
Hardening off ensures that your young tender plants thrive after being grown in a protected environment such as a greenhouse. It is important that they are gently acclimatised otherwise they may get a shock and die.
What should you harden off?
Any tender plants or new seedlings (for example, sweet pea seedlings). Also semi-hardy bedding plants and perennials.
When to harden off
Plants should not be moved out before the last frost.
Hardening off takes about 3 weeks, depending on the weather, so start around mid spring ready to have them out at the end of spring.
Can you do it wrong?
You can't go too badly off track, but be sure to introduce your plants to the outside world slowly and steadily. They need time to get used to direct sun and lower nighttime temperatures.
You can start by putting them out in a sheltered position where they'll get the sun for two or three hours.
Protect them from strong sunlight, wind, hard rain and cool temperatures.
Don't feed your plants while you are hardening them off as it encourages soft growth and this growth can scorch easily. (This is when tips of leaves go brown or yellow from the cold).
Ensure that you listen to the weather forecast and be adaptable. If you are worried it is better to take the plants back in for the night rather than lose them.