How to Harden Off Plants

Preparing Seedlings for Transplant by Hardening Them Off

April 12, 2018

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Find out how to move your indoor-sown young plants to the garden outdoors—without stalling their growth!

How to Successfully Harden Off Indoor-Sown Plants

‘Hardening off’ is the process of gradually acclimatizing indoor-sown plants to outdoor conditions.

For most plants, begin hardening off a week before the final frost date for your area. Our Garden Planner uses data from your closest weather station to recommend when it’s safe to plant out, providing a helpful guide to work back from.

Choose a sheltered spot to harden off your plants. An unheated greenhouse or cold frame is a great tool for this, or you can cluster pots into buckets, crates or boxes to keep the wind off. Don’t place pots directly on the ground where they can easily be knocked over by birds or attacked by slugs.

Begin hardening off on a still, cloudy day when temperatures are fairly steady. Water plants before they go outside. Place them into your sheltered spot for just two hours on the first day. 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 one to two weeks. You can then leave them out overnight if there’s no danger of frost.

In cold winter regions, plants – particularly tender plants such as tomatoes and peppers – will need to be prepared for the cooler nights early in the growing season. Towards the end of the hardening off period, cover your crops with fleece or row covers to protect them overnight. Once crops have been planted into their final positions, be alert for unexpected cold snaps and cover tender crops if necessary.

It’s a good idea to grow a few more plants than you need so you can hold some back just in case. Bought-in plants may also need hardening off, particularly if they have been kept in sheltered conditions.

Read our full page about transplanting seedlings.

Also: Be sure to check out the Almanac Garden Planner! We’re offering a free 7-day trial to create your best garden.


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Hardening off young plants

I have a garden cart that I use for hardening off young plants. I put them all in the cart, bring them outside and each day, increase the length of time. This way I don't have to move each plant.


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