Brooms are a group of evergreen, semi-evergreen, and deciduous shrubs in the subfamily Faboideae of the legume family Fabaceae, mainly in the three genera Chamaecytisus, Cytisus and Genista, but also in five other small genera (see box, right). All genera in this group are from the tribe Genisteae (syn. Cytiseae). These genera are all closely related and share similar characteristics of dense, slender green stems and very small leaves, adaptations to dry growing conditions. Most of the species have yellow flowers, but a few have white, orange, red, pink or purple flowers.
All the brooms and their relatives (including Laburnum and Ulex) are natives of Europe, north Africa and southwest Asia, with the greatest diversity in the Mediterranean region. Many brooms (though not all) are fire-climax species, adapted to regular stand-replacing fires which kill the above-ground parts of the plants, but create conditions for regrowth from the roots and also for germination of stored seeds in the soil.
The largest species of broom is Mount Etna broom (Genista aetnensis), which can make a small tree to 10 m tall; by contrast, some other species, e.g. dyer's broom Genista tinctoria, are low sub-shrubs, barely woody at all.
Species of broom popular in horticulture are purple broom (Chamaecytisus purpureus; purple flowers), Atlas broom (or Moroccan broom) (Argyrocytisus battandieri, syn. Cytisus battandieri, with silvery foliage), dwarf broom (Cytisus procumbens), Provence broom (Cytisus purgans) and Spanish broom (Spartium junceum).
Many of the most popular brooms in gardens are hybrids, notably Kew broom (Cytisus × kewensis, hybrid between C. ardoinii and C. multiflorus) and Warminster broom (Cytisus × praecox, hybrid between C. purgans and C. multiflorus).
Genista tinctoria (dyer's broom, also known as dyer's greenweed or dyer's greenwood), provides a useful yellow dye and was grown commercially for this purpose in parts of Britain into the early 19th century. Woollen cloth, mordanted with alum, was dyed yellow with dyer's greenweed, then dipped into a vat of blue dye (woad or, later, indigo) to produce the once-famous "Kendal Green" (largely superseded by the brighter "Saxon Green" in the 1770s). Kendal green is a local common name for the plant.
The flower buds and flowers of Cytisus scoparius have been used as a salad ingredient, raw or pickled, and were a popular ingredient for salmagundi or "grand sallet" during the 17th and 18th century.
A traditional rhyme from Sussex says: "Sweep the house with blossed broom in May/sweep the head of the household away." Despite this, it was also common to include a decorated bundle of broom at weddings. Ashes of broom were used to treat dropsy, while its strong smell was said to be able to tame wild horses and dogs.