Distribution

More info for the terms: cover, shrub

The following information on the general distribution of Scotch and Portuguese broom is based primarily on literature reviews ([17,34,88,97,148]), unless otherwise cited.

Scotch broom is native to northern Africa and parts of Europe, ranging from northern Africa north to Sweden and the British Isles and east to Ukraine. Throughout its native range it is invasive in neglected areas and encroaches onto poorer pastures in Spain and Portugal. It is also abundant on the hillsides around Rome. Eastward in Europe it is not found beyond Poland and Hungary. In the British Isles, it is a weed of forests and hill grazing land.

Scotch broom has been introduced to many parts of the world as an ornamental (e.g. Canada, Chile, India, Iran, Australia, New Zealand, South Africa, and the United States). It is a major weed problem in Australia and New Zealand. In North America, Scotch broom was introduced to Virginia in the early 1800s for use as fodder for domestic sheep. It was considered invasive in this area by 1860 [70]. Scotch broom was introduced to California as an ornamental in the 1850s, was widely used for roadside erosion control in the early 1900s, and was recognized as a problem in California in the 1930s [74]. It was introduced to Vancouver Island in 1850 as an ornamental; and from 3 surviving plants, it spread over most of Vancouver Island over the next century and a half [100]. Deliberate plantings of Scotch broom by the British Columbia Ministry of Highways have accelerated the spread of Scotch broom during the past 50 years.

The current North American distribution of Scotch broom is along the Atlantic coast from Nova Scotia to Massachusetts, Delaware, Virginia, and Georgia; along the Pacific coast from British Columbia to central California; and inland to Idaho, Montana, and Utah. It also has scattered occurrences in several inland states in the eastern United States, and occurs in Hawaii. The worst infestations of Scotch broom occur from British Columbia to central California, from the coast to the inland valleys: primarily west of the Cascade Range in Washington, Oregon and west of the Sierra Nevada in California [26]. In California, Scotch broom occurs on more than 700,000 acres (283,000 ha) in central to northwest coastal and Sierra Nevada foothill regions. Occurrences are also reported further south in the interior valleys of Los Angeles, San Bernardino, Riverside, and Orange counties, and Scotch broom is beginning to invade chaparral and lower montane habitats in the San Bernardino Mountains [122]. Roughly 10% of Vancouver Island is infested with Scotch broom and common gorse (Ulex europaeus). Isolated patches of Scotch broom have also been reported along Kootenai Lake and Castlegar in interior British Columbia [103].

Portuguese broom is native to the Iberian Peninsula. It is the least common of the broom species in North America, occurring in California and Oregon. In California, it occurs in the San Francisco Bay area, southern Coast, and Peninsular ranges, where it is locally abundant [54]. It occupies about 65 acres (26 ha) in the Marin Headlands, Marin County, where it was introduced in the 1960s for landscaping and slope stabilization. It now forms a dense cover, with 1 mature shrub per m². It is found occasionally in other parts of the Bay area, and has been reported in Mendocino and San Diego counties [4]. Plants database provides state distribution maps of Scotch broom and its infrataxa and of Portuguese broom.

Scotch broom and Portuguese broom are 2 of 4 nonnative invasive broom species that occur in North America. Spanish broom (Spartium junceum) and French broom (Genista monspessulana) occur in similar habitats and have some similar morphological and ecological characteristics. Common gorse is another leguminous shrub that occurs in similar habitats, but is morphologically distinct from the brooms.

The following lists include vegetation types in which Scotch broom and Portuguese broom are known or thought to be potentially invasive, based on reported occurrence and biological tolerances to site conditions. These mostly apply to Scotch broom; as described above, Portuguese broom has a very limited North American distribution. Precise distribution information is limited, especially in eastern North America; therefore, these lists may not be exhaustive.