Ask a Master Gardener: Scotch broom continues to invade rural landscape

Franz Eugen Kšhler, Kšhler's Medizinal-Pflanzen

One can’t help but notice the highly visible flowers of Scotch broom this time of year. Much of the rural landscape throughout our coastal area, including forests, pastures, and rights of way, have been transformed into a carpet of gold. Scotch broom was introduced as a garden ornamental in California in the 1850s and later used to prevent erosion and stabilize banks and sand dunes. Since then, the plant has invaded much of the Pacific Northwest. While not as abundant, it is also present in most counties east of the Cascades in Oregon and Washington. 

Scotch broom spreads slowly, but persistently. It is a copious seed producer; a single plant can produce more than 12,000 seeds a year. Seedpods split suddenly at maturity and eject the seeds. The seeds have hard coats enabling them to survive in the environment for up to 80 years. The seeds are transported from place to place in mud stuck to vehicles, equipment, shoes and the feet of animals. In addition, ants aggressively collect the seed and assist in its dispersal. Birds also assist with spread, but how well the seeds survive digestion varies with the species of the bird. Scotch broom can grow from crowns even after aboveground growth is removed by clipping, frost or fire

Scotch broom is evergreen, and plants can live up to 20 years. The species is host to nitrogen-fixing bacteria, which contribute to its ability to establish in disturbed, nutrient-poor soils. Plants have no inherent dormancy; growth is limited only by cold temperatures or drought, giving it a real advantage over native plants.

A woody, leguminous shrub, Scotch broom establishes quickly in disturbed areas, often outcompeting native plants to form dense stands. Once established, Scotch broom can substantially reduce livestock forage and shade out conifer seedlings, making vegetation management and reforestation expensive. Scotch broom’s economic impact can be significant; the state of Oregon loses more than $40 million annually in lost timber revenue and in expenses incurred in controlling this invasive plant.

Scotch broom is not readily grazed by animals, which is fortunate as it contains quinolizidine alkaloids that, if eaten, can cause muscle degeneration and birth defects in livestock.

Why isn’t Scotch broom being controlled?

According to Nancy Ness, coordinator of the Grays Harbor County Noxious Weed Control Board, each county’s weed board has the option each year of designating target weeds to be controlled. Unfortunately, when weeds like Scotch broom get listed, they are usually placed in the “C” category, which means that the infestation is already so prolific that enforced control is no longer feasible. “The very best we can do is to educate the public on options for control. We are particularly concerned that people become aware of Scotch broom’s potential as a fire hazard,” added Ness. 

Domestic goats are reported to browse Scotch broom without apparent ill effects. Given time, goats will probably control a patch of Scotch broom if the plants are not too tall. Plants can also be controlled by grubbing out the crowns. After removing existing large plants, repeated cultivation will destroy seedlings of this weed. Cutting or mowing the plant down to ground level immediately after flowering has proven to be an effective means of control. 

Selected herbicides currently recommended by WSU weed scientists for the control of Scotch broom include triclopyr (Crossbow)* and glyphosate (Roundup)*. Both of these materials can be found in most retail garden stores and provide effective control when used according to labeled directions.

Although many people complain about having “hayfever” symptoms of watery eyes and sneezing when Scotch broom is in bloom, more than likely, the broom is not to blame. Scotch broom is insect pollinated. Being heavy and sticky, the pollen does not become airborne; thus the potential for an allergenic reaction is minimal except perhaps when an individual actually handles a plant in bloom. Most allergenic reactions that result in “hayfever” symptoms are the result of wind-borne pollens such as grass pollen, which is distributed in large amounts at about the same time that Scotch broom is in bloom.

Some folks who complain about being allergic to Scotch broom are in reality allergic to the fragrance, resulting in a hyper-responsive airway which may require medical treatment. 

*Trade names are mentioned for educational purposes only. No product endorsement is intended or implied.

More from this section

Recommended for you

(0) comments

Welcome to the discussion.

Keep it Clean. Please avoid obscene, vulgar, lewd, racist or sexually-oriented language.
Don't Threaten. Threats of harming another person will not be tolerated.
Be Truthful. Don't knowingly lie about anyone or anything.
Be Nice. No racism, sexism or any sort of -ism that is degrading to another person.
Be Proactive. Use the 'Report' link on each comment to let us know of abusive posts.
Share with Us. We'd love to hear eyewitness accounts, the history behind an article.