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How to Harden Off Plants

Acclimating and Preparing Seedlings for Transplant by Hardening Them Off

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Young, pampered seedlings that were grown either indoors or in a greenhouse will need a period to adjust and acclimate to outdoor conditions, prior to planting in the garden. This transition period is called "hardening off". Hardening off gradually exposes the tender plants to wind, sun and rain and toughens them up by thickening the cuticle on the leaves so that the leaves lose less water. This helps prevent transplant shock; seedlings that languish, become stunted or die from sudden changes in temperature. Hardening off times depend on the type of plants you are growing and the temperature and temperature fluctuations. So be flexible when hardening off your seedlings and be prepared to whisk them indoors if there's a late freeze and snow.

There are 3 approaches to hardening off:

  1. Gradually Longer periods of Time Outdoors
  2. Place in a Cold Frame
  3. Withholding Water


Gradually Longer periods of Time Outdoors

  1. Begin 7 - 10 days before your transplant date.

  2. Place plants in a sheltered, shady spot outdoors. Under a tree or even on your back porch is fine. Leave them for 3-4 hours and gradually increase the time spent outside by 1-2 hours per day.

  3. Bring plants back indoors each night.

  4. After 2-3 days, move the plants from their shady spot into morning sun, return them to the shade in the afternoon.

  5. After 7 days, the plants should be able to handle sun all day and stay out at night, if temperatures stay around 50 degrees F. Keep an eye out that the soil doesn't dry and bake the plants, if the weather is warm.

  6. After 7 -10 days your plants are ready to transplant. Try to do so on a cloudy day and be sure to water well after planting.


Place in a Cold Frame

  1. Begin 7 - 10 days before your transplant date.

  2. Indoor grown plants can be moved and left in a cold frame, for hardening off.

  3. Turn off heating cables and/or open the cold frame cover for gradually longer periods of time. Start with 3-4 hours and gradually increase the exposure time by 1-2 hours per day.

  4. Close the cover and resume heating at night, if temperatures dip below about 40 degrees F.

  5. Plants should be ready to transplant in 7 - 10 days. Try to do so on a cloudy day and be sure to water well after planting.


Withholding Water

Allowing seedlings to wilt has the same effect as gradually exposing them to the elements.
  1. Starting about 2 weeks before your transplant date, don't water your seedlings until they start to wilt.

  2. At this point, water normally, then wait for them to wilt again.

  3. After 2 weeks of this process, seedlings should be ready to transplant. Try to do so on a cloudy day and be sure to water well after planting.


Tips

  • The moving in and out process can be made easier by putting your plants on a wagon or wheelbarrow and simply wheeling them into the garage for the night.
  • Protect your young seedlings from animals and snails or slugs.

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