Biological Flora of Britain and Ireland: <i>Cytisus scoparius</i> *

Abstract This account presents information on all aspects of the biology of Cytisus scoparius (L.) Link (Broom or Scotch Broom) ( Sarothamnus scoparius (L.) W.D.J. Koch) that are relevant to understanding its ecological characteristics and behaviour. The main topics are presented within the standard framework of the Biological Flora of the British Isles : distribution, habitat, communities, responses to biotic factors, responses to the environment, structure and physiology, phenology, floral and seed characters, herbivores and disease, history, and conservation. Broom is shade‐intolerant and drought‐tolerant, aided by c . 40% of net carbon fixation coming from stem photosynthesis and an ability to shed leaves in inclement periods. It is at its best in lowland, open, disturbed habitats but also open woodland; widespread across Britain, Ireland and much of Europe. It has been widely introduced to all continents (except Antarctica) where due to its prolific seeding and rapid spread it is often an invasive shrub. Broom thrives in an oceanic climate on a range of moist soils but is capable of invading arid and poor soils, helped by its ability to acquire 50% of its nitrogen from bacteria in root nodules. Broom is an obligate seeder, pollinated by a limited range of insects, primarily bees large enough to trip the explosive pollination mechanism. Seeds are dispersed explosively from the fruit and secondarily moved by ants attracted to the elaiosome. The water impermeable seed coat results in high dormancy, and so broom forms a significant seed bank. In Britain and Ireland, populations are showing little decline and broom is not considered to be at threat of extinction. In introduced areas, broom can rapidly spread despite browsing by native species and livestock. Biological control using a range of insects introduced from native areas, particularly seed eaters, together with mechanical and possibly chemical treatment, is showing some promise in its control.