List of trees of Great Britain and Ireland

Many additional species have been imported by humans; the total list of all introduced trees numbers several thousand.

A far smaller number of these have become widely naturalised, spreading by their own accord without recourse to further human assistance.

A tree can be defined as a large, perennial, woody plant with secondary branches supported by a primary stem (compare with shrub).

There is no set definition regarding minimum size, though most authors cite a tree species as being one which regularly reaches 6 m (20 ft) tall with a single stem.

[1] Species like Blackthorn (Prunus spinosa) and Purple willow (Salix purpurea), which may reach 6 m but not on a single stem, are not treated as trees.