How to Harden Tomato Plants in a Cold Frame

Tomatoes grow in a range of sizes, with many different fruit varieties. All tomatoes, though, require warm temperatures, bright sun and frost-free growing seasons. They can't go outside until temperatures warm to 65 degrees. To get a jump on growing tomatoes, start the seeds indoors six to eight weeks before the last frost to lengthen the growing season. Move the seedlings outdoors when temperatures warm and ease the transition with a cold frame.

Instructions

- 1
  Place the cold frame in a partially sunny protected area of the garden. The box will protect the seedlings from dramatic temperature changes but expose them to the cooler temperature of the outdoor garden.
- 2
  Put the potted seedlings in the cold frame when temperatures reach 65 degrees. Keep the seedlings outside for two to three hours in the afternoon for the first two to three days, and move them indoors at night.
- 3
  Leave the tomatoes in the cold frame for four to five hours on the fourth and fifth days, and all day on the sixth and seventh days. Leave the seedlings overnight on the eighth and ninth days to allow them to acclimate to the outdoor environment.
- 4
  Transplant the seedlings to the outdoor garden on the 10h day to sites with full sunshine and good drainage.

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