Bougainvillea” doubles as a botanical name (genus name) in plant taxonomy and as a common name. When using it as a common name, I do not capitalize.
Botanically, the plants are classified as woody vines, although they often behave more like shrubs. In fact, they can easily be trained to grow as shrubs (see below). The foliage of these tropical flowers is usually evergreen in warm climates, but their leaves will sometimes drop if watering has been insufficient.