Your petunias look great. You can grow them in your flats until you're ready to plant them into the garden or containers. In a few weeks, if you feel they are getting too big for the cell packs,you could consider potting them into a larger pot size, such as a 3 or 4 inch nursery pot. But that's a lot of pots and a lot of work. (The ones you buy at garden center usually come in the little packs.) Once these plants really start to take off, you might need to pinch them back if you continue growing them in the small cell packs.

Your plants look great!

I would leave them in the cells until ready to move outside (harden off first of course...) but if you think about how large the plants are when you buy them at a greenhouse - yours have a long way to go (in size) before they need to be moved. ALSO - potting up, will slow the growth as the roots try to catch up again, another reason to leave them.

Just be sure to check them daily - as they will dry out more quickly as the roots fill up the cells.

Great job!!

Missti

At zone 8, you should take them outside today!

Definitely keep them in their cell trays until they get sturdier. And as the other posters said, do keep them watered as the little containers dry out faster.

Congratulations!