More to explore:
A wagon wheel herb garden... - cool idea, but I wouldn't be able to spend the years after watching that old wheel rot away, in the dirt... maybe use something else in the shape of a wagon wheel so it comes across as the same idea for the herb garden... and do something a little less "lets watch this cool old wagon wheel relic of the past rot away" with the wagon wheel, like furniture, a chandelier, wall hanging, table, I don't know sand and clear coat and use as a towel rack...
Old wagon wheel herb garden, with the spokes separating plant groups.
Old wagon wheel. Herb Garden Idea!
Real cute outdoor herb garden idea, guess it could be indoor too if you have a mini wagon wheel