Answer: It depends on the hardiness of the plants, but I would wait a little while, then gradually harden the new plants off by bringing them outdoors gradually. Place them in a sheltered place for an hour or two the first day, gradually increasing their daily time outdoors over a week or two until they are out full-time. Keep a close eye on them while they are hardening off--small pots dry out quickly, and tender plants can get sunburned!
I suggest you plan your hardening off so that you can plant them just a week or two before the last frost date. You don't want to take any chances with your new plants, and expose them to a very hard frost. In the meantime, you'll need to keep them under lights, keep them well-watered, and perhaps turn a fan on in the room to increase air circulation to discourage disease. Also, keep a close eye out for insect pests.
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