There are about 300 species of Ilex worldwide, but just one is native to Northern Ireland. It is in fact the only deciduous native evergreen tree here.
Holly, like many other evergreen trees, has leaves with a thick waxy surface, which helps it to resist water loss (through evaporation) in winter, when the tree cannot absorb any moisture from frozen soil.
The plant supports a lot of wildlife, caterpillar of one of the blue butterflies, Holly Blue (Celastrina argiolus), feeds on its flower buds.
Holly is an evergreen plant, which means it does not shed its leaves in winter.
If it does shed leaves, it is because of drought, but they would soon grow again. It has alternate, oval or elliptic leaves. They are stiff and leathery with a shiny dark green colour above and paler below.
Typically, the leaves have sharp spines up to a height of a few metres. Above this, where they are not in danger of being browsed by animals, they have smooth edges.
The white flowers appear in May and June in the axils of the leaves.
Holly has single sex flowers on each tree. This means that each tree is either a male tree or a female. To produce berries, the female flowers need pollen from a nearby male tree. Bees and other insects carry the pollen from one tree to another.
When pollination is successful, the berries ripen in early winter. They are round and bright red and have four seeds inside.
The berries may stay on the tree during the winter. They provide good food for birds, which, in return, help to disperse the seeds.
Young trees have a green stem. The green area of the stems is used for photosynthesis, (the conversion of the sun’s energy into a form that the plant can use for growth and reproduction).
When the tree is older, it has silvery or dark grey bark that is thin and smooth.
The wood is hard, white and dense, so it is good for carving. It burns brightly even when freshly felled. Even the green foliage is flammable.
Holly grows almost everywhere apart from on very wet, poorly drained sites. It grows very well in temperate, oceanic climates and is thus perfectly suited for Northern Ireland. It is very hardy, tolerates exposed sites and is well adapted to shade – it often grows as understorey in woods. It prefers neutral to acid, peaty soils.