The Q&A Archives: Hardening Off

Question: I am planting Burpee flower seeds in peat pots and pony packs for replanting in an outside flower bed. How tall should the seedlings grow before the hardening off process begins? To what maximum outside temperature should they be exposed during this process?

Answer: Hardening off is the term used to describe acclimation of your seedlings to life in the great outdoors. If they went directly from a nice warm environment (your home or greenhouse) to the cold nights of spring, they'd surely go into shock. The hardening off process usually takes about a week and can begin anytime after the seedlings have developed four sets of leaves. (The plants can be moved sooner, but they will adjust better if they are more mature.) Begin the process by taking the plants outdoors, placing them in an area protected from direct sunshine, and leaving them for a few hours, then taking them back indoors. Increase the amount of time they stay outdoors for 2-3 days, then gradually move them to direct sunshine for short periods of time. After 4-5 days of increasing time outdoors and increased exposure to sunlight, they should be ready to spend the night outside. After their second night of experiencing life in the great outdoors they should be sufficiently hardened off and ready to be planted in the garden. Obviously, if the weather gets really cold and nasty, the plants should be protected with hotcaps or covers of some kind.

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