If it is the regular wave petunia, it grows flat and will cover the soil in not too long. Easy Wave, a little more mounding. I buy them and had one mostly eaten by the slugs and it came back. They are tough. You can give it some shade by putting a board over it held up by plastic pots. That would block the noon sun and it could have morning and afternoon sun. To begin with it will take a bit more frequent watering because it's roots are in the top part of the soil in a tight little square. Once it can get a root to the bottom of the container it will be resilient. It is stressed and it needs babying for a short while.

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.

Gee, My Petunias were/are awesome. Petunias are really cold hardy as seedlings, so I planted them out really early...they were very tiny, like 1 true leaf stage. Hey! I had a LOT of stuff to plant out and I had to start early! The hardest part was planting them one-by-one and pulling them apart, and thenplanting them a foot or more apart. Right now we kick them off the grass where they have overstepped the boundaries, but I expect in the next couple of weeks I'll have to trim them back with clippers or scissors. I hate the stickiness of them this time of year. The Zinnias are a little different story -- I didn't think the wintersowing idea was good for me, so I just did a few Profusions. The reason why I didn't want to wintersow them is because the powdery mildew they get is particularly awful here. BUT if you don't start them until mid-late May, they somehow avoid it. I don't know why, but it's true. I have only seen one reference to it, and that was in the Burpee catalog, and just recently. http://www.burpee.com/product/annual+flowers/zinnias/zinnia+giant-flowered+series++-+mixed+colors+-+1+pkt.+%28375+seeds%29.do?search=basic&keyword=zinnia&sortby=newArrivals&page=2 I did have several no shows in my WS jugs -- maybe 10 or 12%, but upon further examination, they were all seeds that had an iffy provenance. (defined as Proof of authenticity or of past ownership. Used of art works and antiques. ) Gotta love the swaps and round robins! (And I do love them still. Everything that is really, really cool I have growing was from a trade or swap -- maybe not so much a round robin, tho. Lime

I believe it is a type of mite. See the link for pictures/more info. I had a significant amount of this on a large clump of "Purple Showers" Mexican petunia last year. I did nothing to combat it (i.e. no "treatment"). This year there is only one small area on a single stem. Despite the fact that Maxican petunia is considered common, I also like this plant. As you said, it blooms all the time and both the flowers and the dark green stems are attractive and add a pleasing contrast to the garden. http://dougsbugs-n-shrubs.blogspot.com/2009/10/white-stuff-on-mexican-petunia.html