Also called “Mexican Petunia”, these flowers grow everywhere around the side of our house. They seem to reseed easily. In 2006-7, the last ones disappeared in early December. The first popped up in early March. With rain they really bloom like crazy, in total shade or sun. They have a dull period in the middle of the summer, especially if they haven’t been watered. I wrote a blog post about discovering what they were: “purple showers everywhere“.
Update April 20, 2008: I highly recommend that Texas gardeners think twice before planting this. (I don’t always follow that kind of advice myself, but trust me when I say–you will be picking them out of more ordered areas of your garden for years to come.) They are pretty flowers especially in a big group, and sometimes stay evergreen year round, but they reseed and spread like mad. It is quite difficult to pull these up by their taproots without the stem breaking off (and then regrowing again), and without disturbing other plants I want to keep. I’m finding this trouble in my rose bed, as I keep pulling up little ruellia seedlings everywhere. I have heard that the dwarf Ruellia is a little more controllable.
Dave’s Garden link: Purple Ruellia, Mexican Petunia, Mexican Bluebells, etc.