How to Harden Off Indoor-Sown Plants

13 May 2016, written by Benedict Vanheems

It’s a proud moment: planting out all those seedlings and plants you’ve so carefully raised indoors over the past few months. It’s important to take steps to acclimatise them to their new outdoor home however, or you risk losing your plants and wasting all that hard work. This is a process known to gardeners as ‘hardening off’.

Hardening Off

Hardening off should take a minimum of a week and may take up to two. Suddenly moving plants from a stable environment to one with wide variations in temperature, light and wind can seriously weaken plants.

For most plants, start hardening off about a week before the final frost date for your area. Our Garden Planner uses data from your nearest weather station to give an indication of when it’s safe to plant outside, providing a helpful guide to work back from.

Hardening off young plants in the greenhouse

An intermediate home, such as an unheated greenhouse or cold frame, is a great tool for hardening off. Place seedlings and plants into the structure for a couple of hours on the first day, then gradually increase the length of time they are in place by two or more hours per day. After a week they can then be left there overnight, so long as there’s no danger of frost. These structures have the added advantage of windows or doors that can be opened increasingly wider with time to gradually make under cover conditions more similar to those outside.

Harden Off Plants in a Sheltered Spot

Wind speeds up evaporation, which can cause plants to wilt surprisingly quickly. Choose a sheltered position to harden off your plants, and water them before they go outside so there’s less risk of them drying out.

Pots can be clustered into crates, tubs or buckets. This not only stops them from blowing over, but acts as a windbreak around the foliage.

Sheltering seedlings in a bucket to aid hardening off

Avoid Bright Sunshine

Sudden bright sunshine on delicate plants can cause leaves to scald – the plant equivalent of sunburn – so begin hardening off on a still, cloudy day when temperatures are fairly steady. Set plants out for two hours in a shady part of the garden. The next day, leave them out for two more hours, with perhaps an hour’s direct sunshine in the morning. Gradually increase the length of outdoor time and direct sunshine over the course of one to two weeks.

Remember that areas of shade will move over the course of the day. If you can’t be at home all of the time, pay close attention to where and when the shade and sun fall. You could also use shade cloth, fleece or row covers to shield delicate seedlings from strong light – simply drape it over the top and tuck it in at the sides so it can’t blow off.

Introduce Plants to Cool Nights Gradually

In regions with cold winters, plants will need to be prepared for the cooler nights experienced earlier on in the growing season. This is especially important for tender plants such as tomatoes and capsicum, which are easily damaged by low temperatures.

If you don’t have a greenhouse or cold frame, just set your plants outside for increasingly longer periods of time. Towards the end of the hardening off period you can use fleece or row covers to protect foliage against the chill of night. Even after crops have been planted into their final positions, be on hand with crop protection to guard against any unexpected late cold snaps.

Tips for Hardening Off Success

As well as these guidelines, it’s worth considering some further points. First, avoid placing plants on the ground where they can easily be knocked over by birds or nibbled by slugs.

If you have the space, grow a few more plants than you need so you can hold some back, just in case. If you don’t need them you can always give them away.

Remember that bought-in plants may also need hardening off, particularly if they have been kept in sheltered conditions.

And finally, don’t hurry the process. Hardening off takes time but will give you stronger, more resilient plants that will ultimately be more productive.

If you’ve got a tip on hardening off plants that you’d like to share, please leave a comment below – we’d love to hear it.