Hollies, the ilex genus, include over 500 species and are native to every continent except Australia, according to Christopher Bailes in his book, "Hollies for Gardeners." Depending on the species, a holly may be a low-growing bush or a tall tree.
All hollies are woody plants and most holly species are shrubs, rather than trees. All ilex species are dioecious, meaning fruit production requires two plants, one with male flowers and one with female flowers. Some holly species are evergreen and some are deciduous, however.
The red-fruited holly (Ilex opaca) used in Christmas decorations is an evergreen tree that can grow to over 30 metres (100 feet) tall. In contrast, other native hollies are bushes, not trees. Inkberry hollies (Ilex glabra) are black-fruited evergreens that grow no more than 1.8 metres (6 feet) tall. Winterberries (Ilex verticillata) are red-fruited, deciduous shrubs that grow to no more than 3 metres (10 feet) tall.
If you are choosing a holly for your garden, size matters. For evergreen hollies with red fruits to grow as bushes rather than trees, go for the blue holly hybrids (Ilex x meserveae), which are hardy in the UK. Blue hollies grow from 1.5 to 4.5 metres (5 to 15 feet) tall.