The Waves probably could fill the 25-inch barrel planter just fine. In the garden they are good spreaders. Incidentally, with a little tender loving care, you can grow cuttings from petunias.

Put it in the barrel planter. They will probably fill it in just fine. First tip it out of the pot and loosen the roots. They are probably root bound and will not do well unless you do. I imagine they are growing round and round in their pot and they will continue to do that even in a bigger pot. If they do they can't spread out and get the nutrients they need so untangle them up and try to spread them out. Don't worry if you break a few. Remember that you are doing them a favor. They may droop a bit after transplanted but will soon pick up especially if you do this on a cooler overcast day

I'll go one step further and encourage you to cut into the root ball ball with a sharp knife, making several shallow vertical, shallow slashes. Yiu might also carve off the bottom couple of inches of roots completely. Pulling roots in an attempt to untangle them results in tearing and shredding, unless they aren't grown together. New roots will recover and regenerate very slowly.

You already know that cutting the top back encourages new growth! The exact same principles work for the roots. You would be amazed at how rapidly a flush of new roots begins to energe from each severed root. Root pruning has been a technique used to rejuvenate root systems of container grown plants for thousands (literally) of years.

It's important that the planting medium you select be coarse textured and fast draining. The planter must have drainage holes. With a little care, your petunias will do just fine.

Hi, I'm new to this forum , what is wrong with my petunia !?

Here is a link that might be useful: http://s1279.photobucket.com/user/seantan181/media/image_zps75759b60.jpg.html