Answer: If I were you, I'd pot the seedlings up in larger containers and place them in a sunny spot, once you've finished hardening them off. You might place them where they'll get only morning sunshine for a few weeks to help them get acclimated. Remember that potted plants will need more frequent watering than garden-grown plants. But by putting them in containers, you'll be able to give them a little extra TLC until they get sturdier.
Butterfly weed can be quite slow-growing, as you've seen. For best germination, chill the seed for several weeks before sowing, and/or chill the seeds once they've been potted up. Even then, they can take 30-90 days to germinate. That's why it's recommended that you start seeds about 10 weeks before your last frost date to give them time to grow; even then, they may still be quite small.
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