Weed of the Week: Scotch Broom

The 19th century brought about more than the popularity of brick ovens among bakers, consumption of imported whiskey among gold miners, and trading of ornamentals among gardeners. Trading among settlers also drove the invasion of Scotch broom (Cytisus scoparius) into North America, which, after more than 150 years since it was introduced, is still rapidly invading coastal regions of British Columbia.

Originally planted along highways to prevent soil erosion, Scotch broom has spread far beyond the bounds of cultivation and is now considered an invasive plant. Invasive plants grow rapidly and spread quickly, causing damage to the environment, economy and our health. They are also the second greatest threat to biodiversity after habitat loss, according to the International Union for Conservation of Nature (IUCN).

Many invasive species, including Scotch broom, contain toxins that can sicken livestock and people when ingested. People tend to rush for the medicine cabinet to combat the uncomfortable allergy symptoms of wheezing and sneezing with the spring arrival of Scotch broom’s yellow blooms.

More than an allergic irritant, Scotch broom is a highly competitive evergreen shrub that has photosynthetic stems, enabling year-round growth and allowing it to form impenetrable thickets, invade rangelands, and replace desirable plants. For instance, Scotch broom is a serious competitor to conifer seedlings; Douglas fir (Pseudotsuga menziesii) plantation failures in Oregon and Washington have been credited to such infestations.

Scotch broom was sold as an ornamental in California in the 1860s and by 1900 it became invasive on Vancouver Island. Scotch broom is also common west of the Coast-Cascade Mountains in southwest BC, and has been reported on the Queen Charlotte Islands as well as in parts of the Kootenays and North Okanagan–Shuswap.

Scotch broom is not only an escaped ornamental; it was also used in fresh cut bundles to package cases of imported whiskey for gold camps across California and northward into BC. Packaging was readily discarded, scattering hardy seedpods and likely creating new infestations along these corridors. Additionally, a hand-held whisk broom known as “bisom” made these plants essential to bakers for cleaning the cooking surface of brick ovens.

Once introduced, Scotch broom sprouts through seed dispersal into high-density infestations that are highly flammable and can increase wildfire fuel loads, resulting in escalated wildfire intensity. Dense patches, first purposely established as a soil binder, now obstruct site lines on roads and require increased maintenance expenses for removal. Thickets may also be impacting endangered Garry oak ecosystems in southwestern BC.

Due to its affinity for light-dominated, disturbed areas, any disturbance activity, such as road or home construction near infested areas, can enhance spread.

This plant can also spread through seed transport by vehicles and machinery, and several species of ants are attracted to the seed appendages and disperse seeds while foraging.

Scotch broom is identifiable by bright yellow, pea-like flowers and woody stems. Shrubs grow 1-3 metres in height and have a lifespan of 15-20 years. Flat, hairy seedpods are initially green, turning brown or black with maturity.

Mature plants can produce up to 3500 pods, each containing 5-12 seeds. As seedpods dry they split and spiral, expelling the contained seeds near the parent plant. Seedpods are hardy, remaining viable in water, soil and gravel for more than 30 years!

Help your community protect local resources by managing invasive plants like Scotch broom. Hand-pulling, cutting or mowing is effective against Scotch broom with prompt revegetation of competitive, native shrubbery such as snowberry, salmonberry, thimbleberry, and Oregon grape. Herbicides are available, though currently there are no approved biocontrol methods in BC. Fire has also been used to eliminate large impenetrable thickets and prepare areas for easier follow up treatments.

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