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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