Can you pick wild flowers from the roadside?

Sure you can. Is it legal? Yes, mostly. You aren't allowed to trespass, and you aren't allowed to damage another's property -- not even the State's property, which would include roadsides. There's an Excellent article in "Related links" below about picking Texas' state flower, the bluebonnet, from roadsides. The article also mentions other flowers in other areas, and gives links to special plants that actually are protected.