The Q&A Archives: Tips on Hardening Off Seedlings

Question: I am perplexed by the use of and term of "hardening off". All references to it are different. One book says to take plants outside a couple hours every day for 3-4 days. Another says for 2 weeks, etc. Another says to take germinated seedlings directly to a cold frame. Then start the hardening off process. Which is correct? I have three flats of seedlings in the basement (according to a couple sources, you want to drop the temp. after germination to get hardier plants) that are roughly 1/4" to 2"tall. I started them in March. I have since made a cold frame to take them outdoors. Do I first take them into the cold frame a couple of hours everyday then bring them back indoors? Or do I just put them in the coldframe and open the lid progressively over time? Will the 40 degree temps. we are still experiencing here at night harm the seedlings? (HA,HHA,HP,HHP) Is it too late in the season to start seeds in a cold frame directly outdoors? If I purchase bareroot plants at the store (that havebeen at store temp. for who knows how long, do I plant them directly outdoors? And what about the plants I purchase from a greenhouse. They have been inside a heated greenhouse growing, do I need to harden these off? If not, why? I really hope you can help me out with this, I am a beginning gardener and have found this hardening off process to be quite perplexing. Trudi Ratican Grovertown, IN

Answer: There is no hard-and-fast rule to hardening off. The amount of hardening off you need to do depends on several factors, including the type of plant, the temperature at which it was grown, the outdoor temperature, etc. You need to harden plants off to get them used to the cooler outdoor temperatures, wind, and strong sunlight. Tender indoor plants will get a sunburn if left outside for too long too soon. They need to build up protection against the strong rays. Remember that if it is cool outdoors, your plants will grow more slowly than they do indoors, so keep heat-loving plant like tomatoes and peppers indoors under lights until the temperature warms up reliably. I usually put my seedlings out for a few hours in a sheltered spot (out of the strong wind) for a few days. If they start to look a little ragged I'll reduce the time; if they look great I'll lengthen it. If the temperature is predicted to drop into the 40's I'll bring the warm weather crops indoors again until it warms up. Certain crops like broccoli or leeks, can withstand cooler temperatures. Just keep an eye on the seedlings, and observe whether they look stressed--wilted, brown leaf margins, etc.--and nurture those a bit more. They're living creatures! I would begin to place the seedlings outdoors in your cold frame, for a few hours the first few days, then lengthening the time they stay out. I would harden off greenhouse-grown plants in the same way. Bare-root plants should be ready to plant and not need hardening off, since they are in a dormant state. And you can certainly start more seeds now. Just remember, most seeds germinate more reliably in warm, rather than very cool, temperatures.

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