How to Harden off Cool Weather Vegetables

Cool weather crops, including lettuce, broccoli and spinach, can survive light frost and prefer the cooler temperatures of spring or fall for best production. Starting the seeds indoors allows you to get started on the growing season earlier, but the plants require hardening off to accustom them to outdoor conditions before you can transplant. Moving from the warmer, climate-controlled conditions indoors to the cold and sometimes freezing outdoor conditions can otherwise kill the young seedlings.

Cold frame or cloche

Instructions

- 1
  Set the seedlings outside in a protected area, such as on a patio, on a frost-free day. Choose a location protected from direct sunlight and high winds.
- 2
  Leave the plants outdoors in the protected area for two to four hours the first day, then bring them inside. Place them back outdoors each day as long as temperatures are above freezing. Gradually increase the plants' exposure to sunlight and increase the amount of time spent outdoors over a five- to seven-day period, advises Norma Rossel, quality assurance manager for Johnny's Selected Seeds.
- 3
  Set the plants, still in their pots, in the garden bed after finishing the first week of hardening off. Place them inside a cold frame, if available.
- 4
  Leave the cold frame open during the day. Close the frame at night or if frost threatens. If you don't have a cold frame, cover the plants with a cloche at night or during frost.
- 5
  Transplant the plants into the garden soil after five to seen days of hardening off in the cold frame or cloche. Cover the plants with a cloche during frosty weather during the first three to five days after transplanting when the roots are still establishing in the garden bed.

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