There are about 400 species in the genus Ilex but few are as distinctive and immediately recognisable as holly. Holly is the only broadleaved, evergreen tree native to the British Isles. It has a vast native Eurasian distribution from Norway to Algeria, and from the Atlantic coast to China. In its native range holly is threatened by browsing animals; it is excellent forage. However, in parts of North America, where it has been introduced, it may become invasive.
As a small, slow-growing tree it is often found as part of the woodland understorey; the thick, shiny, dark green leaves are photosynthetically highly efficient at low light levels. Leaves on the lower parts of wild trees are usually very prickly those on the upper parts often lack prickles. This is thought to be an adaptation for protection against browsing animals. Until the eighteenth century, holly was an important winter fodder for cattle and sheep. Holly is also one of the densest native timbers and has been used by engravers for woodcuts, as a rival to box wood, and for the manufacture of fine musical instruments, hinges and tool handles.
Holly is dioecious; populations have separate male and female plants in broadly equal proportions but genders cannot be distinguished until they flower. Female trees take about 20 years to start producing white, insect-pollinate flowers in early summer. By November the well-known scarlet berries, so strongly associated with Christmas, will have developed; there are good and bad fruiting years. The fruits contain four seeds, which are mostly dispersed by birds. Heavily browsed or pruned trees will not flower.
Many holly cultivars have been selected by gardeners over hundreds of years. One of the oldest is a particularly well armed cultivar, 'Ferox', which has small, wrinkled leaves with short prickles covering the upper leaf surfaces. The Bobarts, father and son, appear to have been growing this cultivar in the Oxford Botanic Garden during the late seventeenth century. Gardeners wanting berries must plant female cultivars, although their names may be confusing, for example, 'Golden King' is female, whilst 'Golden Queen' is male. Berries are not the only reason to grow holly cultivars, there is a tremendous array of leaf form, shape, colour and texture, as well as habit, fruit colour and shape.
Holly fruits contain caffeine but it the leaves of another holly, Ilex paraguariensis, which is the source of the South American drink mate.
Hemery G and Simblet S (2014) The new sylva. A discourse of forest & orchard trees for the twenty-first century. Bloomsbury.
Peterken GF and Lloyd PS 1967. Ilex aquifolium L. Journal of Ecology 55: 841-858.