Plant introductions to the British Isles can be divided into those introduced before 1500, the ‘archaeophytes’, numbering c. 150 species and considered in Chapter 35, and those introduced later, the ‘neophytes’. The neophytes comprise a very large number in total, but with about 230 species having spread far from their original introduction site. Most neophytes have spread in the twentieth century. A remarkable number spread mainly or entirely by vegetative means. Neophytes occur throughout the British Isles, but are most common in disturbed sites and especially in south-east England. Certain neophytes have a particularly interesting historical distribution or exhibit taxonomic and ecological problems that have generated much research. Freshwater habitats contain several fast spreading and abundant neophytes that spread vegetatively.

Most neophytes are attractive insect-pollinated plants introduced initially for their ornamental qualities and, though they can become abundant in places, do not pose ecological problems. Examples are the Ivy-leaved Toadflax, some bellfowers and the Buddleia. They are broadly welcomed. A few have invaded sensitive habitats, but widespread ‘problem’ plants are very few in number: Japanese Knotweed, ‘Wild’ Rhododendron, New Zealand Pigmyweed, Himalayan Balsam and in a few places Hottentot Fig; maybe one or two others locally.

The widespread planting of trees, both native and non-native, and the spread of ‘wild flower mixes’ for roadside and meadow plantings has led to some mixing of native and non-native genetic material. Most of these have become inextricably mixed. […]