Cytisus scoparius is one of the west coast of North America’s most notorious invasive species. Commonly known as Scot’s broom, its yellow flowers will flood disturbed landscapes of the Lower Mainland and Southern Vancouver Island each spring.
My first experience with Scot’s broom came two springs ago in the local Iona Beach Regional Park, where its yellow flowers radiated beautifully throughout the area. I assumed it was a native species, and was impressed by the amount of colour it brought to the landscape; even the birds were loving it. When the blooming flowers faded away as spring passed, I definitely felt as though the park lost some of its liveliness. I wonder if I’d have had all the same sentiments had I known about its extreme invasiveness.
Scot’s broom is a 1 to 3m (3.3–9.8 ft.) tall deciduous shrub with broom-like upright stems (the specific epithet, scoparius, means “broom-like”). Its trifoliate leaves are small and alternately-arranged along the branches. The yellow flowers are sometimes tinged with purple or red markings. Each plant produces up to 3500 flattened black pods, with each of these containing 5-12 readily-dispersed seeds. In addition, Scot’s broom can be self-seeding. Plants strictly occupy dry sunny sites at low elevations, often after some sort of soil disturbance.
Native to the Mediterranean regions of Europe, Cytisus scoparius has been introduced to many parts of the world, frequently becoming ecologically destructive to the native ecosystems. It was intentionally introduced to British Columbia in 1850 when it was planted on a farm on Vancouver Island. With a fierce ability to outcompete other plants on open sites, it quickly replaces forage plants on rangeland. As it establishes and grows rapidly, it can also take over areas after disturbances such as clearcut logging– in those situations, preventing regrowth of conifer seedlings. Eventually, Scot’s broom becomes a tall thicket that is practically impossible to pass through. This further prevents the (re-)establishment of native plants, as well as limiting animal movement and increasing the fuel load for wildfires.
Eradication is a long and laborious process. Once Scot’s broom lays its foot down on a site, only forceful removal will free up the land as it nearly halts natural successional processes. Scot’s broom also fixes nitrogen through a symbiotic relationship with a bacterium. Objectively speaking, this is often considered a positive ecosystem service for the soil and other organisms. One could argue the existence of a silver lining for its nitrogen-fixing properties and abundant, attractive bee-pollinated flowers, but the damage to native biotic communities far outweighs these services. In British Columbia, Scot’s broom has proved to be a particular nuisance in endangered Garry oak ecosystems.
Wikipedia’s page on Scot’s broom makes note of its medicinal uses, presence in old royal symbols, and appearances in folklore and myth.