Common Name: Scotch broom; broomtop; common broom; European broom; Irish broom
Scientific Name: Cytisus scoparius (L.)
Subkingdom: Tracheobionta (vascular plants)
Superdivision: Spermatophyta (seed plants)
Division: Magnoliophyta (flowering plants)
Class: Magnoliopsida (dicotyledons)
Family: Fabaceae alt. Leguminosae. Also placed in Papilionaceae.
Identification: Scotch broom is a perennial that grows in thickets, with individual plants becoming as high as 10-12 feet. The branches are green, slender and angled so sharply that they often appear tangled; until flowering occurs, some of them may remain naked. The tips of the branches often die back later in the season. Leaves are alternate, palmately compound, with the entire leaf measuring one-half to one inch across; the leaflet is egg-shaped, attached to the petiole at the narrower end. The leaves are dark green above and paler and fuzzy below. Buds are very small, but the bright-yellow flowers that appear in May to June are about an inch long, resembling pea flowers.
The fruit of Scotch broom look like shiny beans, about one-eighth inch long; several of them are held in one to one-and-a-half inch flat pods that turn brown or black with fuzzy edges in late summer. The bark is greenish brown, with light strips developing as the shrub ages; when young the bark is smooth, but it develops shallow fissures as the shrub matures.
Original Distribution: Scotch broom is native to all of Europe, including the British Isles, and to the islands of the Azores, the Canaries, and Madeira.
Current Distribution: In North America, Scotch broom is found on the east coast from Nova Scotia south to Georgia; in the west, it is found in California and throughout the Pacific Northwest of both the United States and British Columbia, primarily west of the Cascade Mountains but more recently taking hold on the eastern slopes as well. It is also found in Chile, India, Iran, Australia, New Zealand, and South Africa.
Site and Date of Introduction: In North America, Scotch broom made its first appearance on the east coast (date unknown). It was being sold as an ornamental in California in the 1860s and by 1900 had become naturalized on Vancouver Island, where it was introduced by Captain Walter Grant in 1850.
Mode(s) of Introduction: Scotch broom entered California, and probably other areas of North America as well, by being sold as an ornamental.
Reason(s) Why it has Become Established: Scotch broom began to spread rapidly in North America soon after it was introduced because it was frequently planted in gardens as an ornamental; in California, it was for a time planted along highway cuts and fills as a soil binder. The species is well suited to invasive dispersion in many environments. It tolerates a wide range of soils and and can grow for most of the year given adequate precipitation and reasonably mild temperatures. It produces long-lasting seeds that can be spread widely in a variety of ways: carried for long distances along roads by vehicles, or in gravel hauled from river bottoms; transported by birds or other animals; or carried on the wind after the pods have exploded. It has even been known to be dispersed by ants. Where it has been introduced, Scotch broom invades pastures and cultivated fields, dry scrubland and wasteland, native grasslands, and along roadsides, dry riverbeds, and other waterways. It does not do well in forested areas, but invades rapidly after trees have been logged or cleared by burning. It can reproduce vegetatively or by seed sprouts back after cutting.
Scotch broom is a prolific and tenacious species: a single bush can produce up to 60 seed pods, with each pod containing five to eight seeds. Because the seeds have hard coats, they can survive being carried in river gravels, and seeds can last in the soil for up to 60 years. It is also relatively fast growing: within the first year, plants can grow over a meter tall. It is well adapted to early succession on degraded land because of photosynthetic tissue dispersed throughout the crown in long twigs and leaves.
Ecological Role: In its native habitats, Scotch broom is subject to predation by a variety of insects. This is not the case in North America, although insects have been introduced in attempts to control its growth biologically.
Benefit(s): Because broom is leafless from late summer to early spring, it allows light to reach later-arriving successional plants; this may make it preferable in disturbed areas to other early successional plants, such as gorse. Also, it produces a sparse, easily decomposed litter, unlike the acidic litter of gorse, and fixes nitrogen in the soil that can be used by other plants after the broom has died. (While it is alive, it uses all the nitrogen that it fixes itself.)
As shown by its widespread use as an ornamental, Scotch broom is aesthetically pleasing in residential areas, especially in early spring when its flowers add splashes of bright yellow to landscapes that are otherwise without a lot of color. This is also true of bushes growing in clumps in the wild, though in this case the disadvantages probably outweigh the aesthetic benefit.
Threat(s): Because of its spreading habit, scotch broom threatens grasslands and hillsides throughout California. The Nature Conservancy quotes Mobley (1954) as follows: "It is very aggressive....growing so dense that it is often impenetrable. It prevents reforestation, creates a high fire hazard, renders rangeland worthless....Even wildlife suffers...as the growth becomes too dense for quail to thrive and there is no forage left for deer." In Oregon, broom is reported to interfere with the growth of young seedlings on forestry plantations, since it enters the area after it has been logged and grows much faster than the new conifer plantings.
Control Level Diagnosis: Medium priority. Scotch broom is still continuing to spread more than 150 years after first being introduced to North America, as evidenced by reports of its range expansion to the east of the Cascade Mountains. In non-residential areas it has become a pernicious pest, particularly in degraded areas where it is likely to take over and produce shade before native forbs, grasses, and trees have a chance to sprout and gain a foothold. Because of its efficient dispersion methods, however, controlling Scotch broom will remain very difficult as long as gardeners and nurserymen continue to view it as an ornamental and to plant it widely.Wildlife and forestry managers, among others, are therefore likely to face an ongoing need to seek out new growths and destroy them before they grow tall enough to shade out more desirable vegetation or disperse seeds into other areas.
Control Method: Control methods are numerous, with the optimal choice depending on the site and and on the resources available. Following are some methods that have been used with at least a modicum of success.