What is ‘hardening off’ of plants?

Why harden off?

When you grow from seed or plugs then you spend a lot of time and energy looking after your babies, making sure they have the correct compost, enough warmth, light, and water. You pot them up from their seed trays into larger pots and fill with a good peat free compost such as Miracle-Gro Peat Free Premium All Purpose Compost.

These plants are used to being warm, in a calm environment and out of harsh sunlight so the worst thing you can do is to suddenly plant them out in your garden, exposed to all the elements. They will go into shock and no matter how warm the day is, the temperatures at night will be too low for these tender youngsters to cope with.

Steps to successful hardening off

1. Wait until all risk of frost has passed, this will differ between the north and south of the country so always check the local forecast.
2. Place your plants outside during the day, but out of direct sunlight as this will scorch their delicate leaves.
3. This will be their first exposure to wind so just be careful that it’s not too blowy.
4. Watch out for pests! Slugs and snails absolutely love soft young foliage to munch so ideally place them somewhere high and away from other plants where slugs can drop down from. Use some organic slug pellets such as SlugClear Ultra 3 around the pots to help keep them safe.
5. Bring them in at night! This is essential. Try not to forget and leave them outside until morning.
6. Repeat this for between 7-10 days and then you can start to leave them outside at night for a few more days.
7. After this, your little plants will have toughened up and be ready to be planted outside in their permanent positions, ready to cope with whatever nature throws at them!

Cold frames

These are perfect if you have the space for hardening off plants. They can be bought ready made or can be an easy DIY project. The important part is that they have a slanted glass lid that can be opened during the day and then closed at night. They are usually made from wood and only about 50cm – 1 metre high with gravel put in the bottom to help deter slugs and snails. Pop them in a shady part of the garden and they can be used all year round not just for hardening off but for over wintering, growing cuttings, and hardy veg.