Hardening Off — Don’t Forget!

This is a reminder I’ve made over the last couple of years around this time.

If you are new to gardening, and have started plants indoors under lights, don’t forget to harden them off before moving them outside or they will probably die!

Hardening off simply means to introduce them slowly to the outdoor weather, usually over the course of about a week. Otherwise the shock of moving from a sheltered room and grow lights to outdoors will just be too much for the plants.

I harden off my plants according to the following schedule of outdoor time:

Day 1: 15 minutes

Day 2: 30 minutes

Day 3: 1 hour

Day 4: 2 hours

Day 5: 4 hours

Day 6: 8 hours

Day 7: Plants are ready to be left outdoors

These times are approximate! Don’t drive yourself crazy over them.

While hardening the plants, you should also introduce them slowly to full sun.

Many people have other ways or systems of hardening plants. For example some people make use of a coldframe or greenhouse and slowly let in more outside air. Other people have different times for different days. Some people make use of plastic milk jugs. As long as it takes at least a week, and you are introducing your plants gradually to the outdoors, it should work fine.

A lot of people ask me questions like what about moving plants from a polytunnel or some other in between situation? I don’t really know. I suggest, if you have a plant you can afford to lose if necessay, set it out for an hour while carefully watching it for signs of stress. Look at the plant again the following day. If there are no signs of stress, than it is probably okay to just set the plants outdoors. To be certain, it’s still better to harden them first.

Hardening off plants is a risky time! Many gardeners lose plants while trying to harden them. Watch your plants closely, and if any start showing signs of stress, move them back inside for a few days and try hardening them again from the beginning.

If you move your plants back indoors after hardening, you will need to reharden them!

9 Replies to “Hardening Off — Don’t Forget!”

  1. I may have to do stuff with my soil but at least I don’t have to raise my seedlings inside! It is great to read about how people have to adapt to their own conditions.

  2. I am glad you posted this, I did not realize that the first two days were such short periods of time. For some reason I thought the first day would be at least 1 or 2 hours. Hopefully my trees will be in good shape this spring thanks to your hardening off instructions.

  3. Thanks for the info.

    P.S. Thanks also for the seeds. I’ve been meaning to write you and tell you that but I keep forgetting (my nuerons are fried by life).

  4. Hardening off is a big chore for me…the flats are carried indoors and outdoors to the unheated “greenhouse” until they can survive the nights out there. I keep a High Low thermometer in the greenhouse for a week or so before I move them out there, so I can see the conditions…and still I loose my fair share occasionally….

  5. Hi Cyndy,

    I find hardening off a chore too. Lots of carrying trays of plants back and forth, usually bending over. It’s good we only have to do it once a year!

  6. I hopped on here to see if I could harden off my plants in an unheated greenhouse (my husband is still assembling). Squirrels have tended to get too inquisitive when my flats are out on the deck. However, I was surprised by your ‘hardening off’ schedule. I guess, in my case, ignorance has been bliss because for the last 4 years, all I’ve done is set them out on the deck for the entire day, bringing them in at night. I only did this for 3 nights then left them out for good, of course checking the low temperature for each day. (zone 5) They are not in full sun there, more like filtered early morning and late afternoon sun, so maybe that’s why I was successful. Just before our frost date, I sell them (heirlooms).

  7. Hi Betsy,

    Thanks for the comment. Yes, different people have different methods, and it depends on the plant you are hardening too. While the schedule I give here works fine, but that doesn’t mean there aren’t other ways to do it too.

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