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Scotch Broom

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Oh, my aching muscles...

That Wicked Weed

By Katie Barndt and Bridget Simon, King County, WA

Scotch Broom is an upright shrub with slender green branches sprinkled with small, bright green leaves. Highlighted by profuse yellow-gold flowers, this shrub puts on a fantastic display during late spring and early summer. Drought-tolerant and thriving in poor soils, it is perfect for that sunny spot in your yard!

Tempted to add this shrub to your spring planting list? Beware: despite the tantalizing description and easy care, you will be inviting in one of the worst noxious weeds in the West.

Scotch or Scot’s broom, Cytisus scoparius, is a member of the Legume family (Fabaceae). Native to Europe and North Africa, it has become naturalized across US. First introduced on the west coast as an ornamental and used extensively as a roadside plant, Scotch broom has become a familiar sight in western Washington.

Why is it a problem?

While Scotch broom lights up the roadside in spring, it is not a welcome sight for land managers. It quickly forms very dense stands and can spread from disturbed areas into more natural vegetation. The qualities that make this plant a successful colonizer put it in direct competition with native plants, which often lose out in the battle for space and resources.

It grows rapidly and fixes nitrogen which allows it to thrive on very poor soils. It is also a prolific seed producer (up to 12,000 seeds from a mature plant) and the seeds remain viable in soil for at least five years. Its large size, the formation of copious seeds and its ability to resprout after cutting makes control of Scotch broom very labor intensive.

Scotch broom covers more than 700,000 acres in parts of California. Locally, Scotch broom is a particular problem in South Puget Sound prairies, a plant community adapted to nutrient-poor soils. Several rare plants and animals such as golden paintbrush (Castilleja levisecta) and the butterfly Mardon skipper (Polites mardon) depend on this prairie community. Scotch broom’s encroachment on these prairies is interfering with the establishment of native plants and threatens this ecosystem.

Although it is often associated with allergies, Scotch broom is not responsible. Dr. Charlie Reed, formerly professor emeritus at the Mayo Clinic who worked on allergens in the 1950s stated that the pollen of Scotch broom was not an allergen. Other sources state that the pollen of Scotch broom is too heavy and does not become airborne, unlike the alder and grass pollen that are probably the main offenders.

Scotch broom is a Class B noxious weed in Washington State. Although it is widespread in western Washington, it has not spread into eastern Washington. Prevention of seed production is required by law in counties of eastern Washington in an attempt to control its spread there. Scotch broom, unfortunately, is still available in nurseries in many states and over the web, although it is regulated by WSDA and it is illegal to buy, sell or offer Cytisus scoparius for sale in Washington. For more information on brooms visit: http://dnr.metrokc.gov/wlr/lands/weeds/index.htm. For questions contact King County Noxious Weed Control Program at 206-296-0290.

Scotch broom reseeds prolifically. If you have it on your property, cut it down before it blooms.

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