Question
Two weeks ago I planted purple wave petunias in two window boxes. They have the same soil mix and get the same amount of sun and water. I watered before I left for the beach on Sunday and when I returned on Tuesday, one was completely wilted. I watered but the leaves are still wilted. The other is unaffected. I dont see any pests. What could the problem be?

Answer
Rebecca,
Is it possible that the stem of the wilted one got bent either in planting or later? Something falling on the box, a squirrel jumping on it (unusual, I admit, but possible!) or some other way it might have been crushed?  If the main stem is bent, especially early in the season when it's tender, then the plant can't carry the water up from the roots to the top of the plant and so the plant wilts.  

Other reason a plant wilts like this is from root or stem rot - if kept too wet for long the roots or bottom part of the stem can rot and again, no water can make it to the top of the plant and it wilts.

If you still have the plant look closely at the base of the stem near the dirt line - see if it might look damaged in some way.  If the stem is collapsed and a bit darker in this area this is the sign of rot from being too wet.

The good news is that it's still early in the season and you can probably get another petunia to replace it.

I hope this helps!
C.L.