Rhododendrons and Azaleas
Azaleas and Rhododendrons have been interbred since the first plants were scooped out of the dirt, and for a while these "crossbreeds" were even referred to by some ghastly name — Azaleodendrons or the like.
The old rule of thumb used to be that Azaleas are deciduous and Rhododendrons evergreen, but if that was ever true, it isn't anymore.
The differences between the two are slight, so slight in fact that botanists do not break them into the two groups that gardeners have come to know. Rhododendrons have 10 stamens per flower, while Azaleas have only five. While both can be deciduous, that trait is typically seen more in Azaleas than Rhododendrons. The shape of the leaves is often another indicator, with larger, paddle-shaped leaves typical of Rhododendrons, Azaleas often have smaller, elliptical leaves and trumpet, or tubular-shaped flowers at the ends of the shoots.