It's too early to plant! wait for mid-May or Memorial Day. Have you done any pinching to produce stockier plants? if you haven't do it now. You can be "brutal" with pinching, too... the plant won't really care. Pinching forces the plant to put out more roots and develope multiple stems, this is good because the plant doesn't have a chance to get tall and "weedy". If you are new to plants this is a scarey thing to do! I almost fainted the first time... I pinched my Cascade petunias to just about soil level... thought they'd die... but NO! they produced 3 stems and were absolutely gorgeous, full, and lush all season long.

If they really begin to over grow their places you can always "pot them on"... but try pinching first... all the greenhouses are doing the very same thing. And I always take the plants out of the containers... they never break down efficiently and the plant invariably remains potbound and fails to grow vigorously throughout the season.

Can you transplant them into a larger container and save the origianl containers for another time?