Along the banks of highways all over the Pacific Northwest, you’ll see these bright yellow flowers. But don’t be fooled by their pretty color: They grow on a plant called Scotch broom, Cytisus scoparius, a member of the pea family, which is not a decorative plant but an invasive, noxious weed.
Scotch broom is native to Britain and central Europe. It was introduced in North America in the 1860s as a garden ornamental and was planted along roadsides and open banks to prevent soil erosion. But because Scotch broom can tolerate a wide range of soil and moisture conditions, it quickly became invasive. Invasive species create monocultures, dense areas of growth that displace native and beneficial plants and cause loss of grassland, such as pastures, and open forest habitat. These monocultures impede movement of wildlife and increase both the frequency and intensity of fires.
Scotch broom is a fast-growing deciduous shrub from five to 10 feet tall. Each shrub may live as long as 30 years. An excerpt from the book Weed Control in Natural Areas in the Western United States lists the weed’s range as:
The entire Atlantic and Pacific coasts from Alaska to British Columbia to California, and from Nova Scotia through Georgia.
Also Idaho, Montana and Utah, as well as one Hawaiian island.
According to Jim Pojar and Andy MacKinnon in Plants of the Pacific Northwest Coast, revised edition:
‘Broom’ is derived from the Anglo-Saxon brom meaning ‘foliage.’ The word was applied to shrubs that were used for making ‘besoms,’ which are bunches of twigs us as brooms.
Peak flowering time for Scotch broom is from March or April until June, but some blooms may appear sporadically during the year. The plants often drop their leaves during dry summer months and may be leafless for most of the year. Seeds are produced in seedpods at the end of the summer. When mature, the pods split open and eject seeds up to 20 feet. According to the Washington State Noxious Weed Control Board, each plant can produce thousands of seeds each year, and the seeds can survive in soil for more than 30 years, with some estimates as high as 80 years. This enormous production and long life of its seeds is another reason why Scotch broom is so invasive.
Washington State University Extension warns that, if eaten, all parts of the plants are toxic to livestock, horses, and humans.
The yellow flowers may look pretty, but they represent vegetation devastation. A 2011 article in The Olympian, the newspaper in Washington’s capital of Olympia, reports that Scotch broom causes around $100 million in agricultural and forestry losses each year in Oregon and Washington.