University of Maryland Extension

Hardening Off Vegetable Seedlings

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Hardening is the process of exposing transplants (seedlings) gradually to outdoor conditions. It enables your transplants to withstand the changes in environmental conditions they will face when planted outside in the garden. It encourages a change from soft, succulent growth to a firmer, harder growth.

  • Begin hardening transplants 1-2 weeks prior to setting out plants in your garden.
  • The easiest way to harden transplants is to place them outside in a shaded, protected spot on warm days, bringing them in at night. Each day, increase the amount of sunlight the transplants receive.
  • Don’t put tender seedlings outdoors on windy days or when temperatures are below 45° F. Even cold-hardy plants will be hurt if exposed to freezing temperatures before they are hardened.
  • Reduce the frequency of watering to slow plant growth, but don’t allow plants to wilt.
  • A cold frame provides an excellent environment for hardening off transplants.
  • After proper hardening, tomato plants can usually tolerate light and unexpected frosts with minimum damage.
  • The hardening process is intended to slow plant growth. If carried to the extreme of actually stopping plant growth, significant damage can be done to certain crops. For example, cauliflower will produce thumb-sized heads and fail to develop further. Cucumbers and melons will stop growing if hardened too severely. They may be left outside overnight if the temperature will not fall below 50° F.

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