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Hardening off tender plants

Tomato plants being hardened off.
Tomato plants being hardened off in my yard.

Here in Southeast Michigan, our last frost date is May 12th; in my family, we've always used Mother's Day as the unofficial time at which tender plants can be put out with little worry.

If you started plants indoors from seed, you'll need to spend a few days hardening them off. The reason we do this is because the shock being yanked out of a nice, warm, controlled environment and plunked into somewhat chilly soil, contending with wind and bright sunlight can all be a bit too much for tender plants. You've worked hard for the past several weeks coaxing strong little plants along; a bit more coddling is still necessary. Consider this a weaning period for your plants.

There are a couple of good ways to harden off. The first, which is a method I've used for years, is to harden off in a cold frame. Of course, this method requires having a cold frame, and having one large enough for all of your plants. The second method requires nothing but your willingness to walk plants around your yard once a day or so.

Basically, you are going to slowly but surely acclimate your plants to outdoor conditions. We're going to take five to seven days to harden your plants off. I've read advice about taking up to two weeks to do this, but the weather here in early May is often still a little iffy. I give it five to seven days, and my plants never seem to have a problem.Here is a general schedule to follow for hardening your plants off:

  • Day One: Place plants in a sheltered location (such as under an overhang or awning, or against the house) for three to four hours. Bring them back inside.
  • Day Two: Place the plants in a shelterd location for five to six hours. Bring them back inside.
  • Day Three: Place the plants in a slightly less sheltered location for four to five hours. Bring them back inside.
  • Day Four: Place the plants in an even less sheltered location, leaving them out for six to eight hours. Bring them back inside.
  • Day Five: Place the plants in an area similar to where they'll be growing (so, full sun plants in a sunny spot, without shelter) for at least eight hours. If temperatures stay above the fifties at night, you can start leaving them outside. If it drops below fifty, bring them in.
  • Days Six and Seven: Repeat Day Five. By this point your plants should be able to spend all of their time outside, and can be planted out as long as there is no frost in the forecast. If you plant, and we get a freak freeze, you can use floating row covers or even old sheets and blankets to protect your plants.

A Few Tips

Now that they're exposed to the elements, your plants will dry out much more quickly than they did indoors. Monitor soil moisture regularly and water as needed.

When you first start hardening your plants off, try to put them in an area out of prevailing winds. The wind dries plants out very quickly, and, if you're growing tall plants such as tomaotes, they may wither a bit under the stress. During the hardening off period, they'll toughen up enough that the wind won't bother them as much.

Don't place your plants in direct sunlight for the first few days. While the lights they've been growing under are bright, they are nothing compared to the brightness and heat of the sun. Plants can suffer from sunburn just like humans, so give them a bit of shelter.


  • SE Michigan Home & Living Examiner 5 years ago

    Thank you for the timely tips. My green pepper plants should be ready to start hardining off pretty soon.

    Jackie DiGiovanni

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