A couple common questions we get each spring are, "What's that yellow plant we see along so many highways, and why don't we do something about it?"
The plant with the bright yellow flowers and distinctive smell is Scotch Broom, or Cytisus scoparious, an invasive species visible along many roadways this time of year. There's a long-standing rumor that our agency planted the invasive Scotch Broom along our roadways many years ago, but that is false. Prior to the 1970s we did plant some "cousins" of the Scotch Broom – called Moonlight Broom and Highway Broom – but those are ornamental varieties and do not produce viable seeds, and thus are not invasive.
|Scotch Broom broke out on the cut-slope coming up from the Nisqually Delta on I-5 near Olympia|
shortly after freeway construction in the mid-1960s.
How did Scotch Broom get here?
Scotch broom may be pretty, but as an invasive species it acts like an invader, crowding out native plants that are better suited to supporting native wildlife and pollinators. It was first brought here by settlers in the 1800s, who carried with them seeds for plants they enjoyed back in jolly old England or elsewhere, without realizing the threat to native species. It was further spread through clear-cuts by the booming logging industry during the 20th century, and also carried by ground transportation throughout western Washington.
Since we now know the plants are invasive and bad for the environment, why are they still there?
- First, like most invasive species, Scotch Broom is hardy and hard to remove completely. It thrives in disturbed soil – such as along highway projects – and each mature plant produces hundreds of seeds each summer. Mowing slows but doesn't kill the plants, and their seeds can lay dormant for 60 to 70 years and still germinate in the right conditions.
- Second, they're also very wide spread in places. The northwest coastal areas of Washington, Oregon and British Columbia are pretty much a perfect breeding ground for Scotch Broom once it's introduced. So there are a lot of plants to contend with. It's not just roads, either. Some of the most widespread and oldest infestations are in the low-lying forest logging areas throughout the region.
- And third, while we do work to eradicate Scotch Broom in some areas – including around sensitive agriculture fields throughout eastern Washington – limited state funding means we have a containment rather than removal approach in many areas.
Unfortunately, given funding constraints and the challenges of Scotch Broom itself, there is no way we will ever be able to fully control this plant on all our highways – and restoring an infested area takes decades of work and follow through. So until there is more money and time to devote to this issue, we are focused on protecting currently uninfested areas and preventing further spread. Our strategy is to prioritize control of plants that show up in otherwise uninfested areas as well as some other specially designated restoration areas.
Here are the locations we actively manage Scotch Broom:
- In counties and areas with specific Scotch Broom concerns and requirements, including all counties in eastern Washington, Skagit County because of agricultural concerns, Pacific County, which is almost entirely Scotch Broom-free, and Clallam and Jefferson counties in gravel pits. In these locations, we are working with the county noxious weed control boards on long-term plans for management and control.
- In existing naturally pristine sites such as national parks and national recreation areas, and highways on the west slopes of the Cascades approaching the passes.
- Where individual plants or just small isolated patches exist in otherwise uninfested areas. This prevents the plant from producing seeds and spreading into new areas.
- In select sites where highway construction work has added new roadside plantings, or where we are using a limited amount of maintenance funding to restore the roadside to a native, pollinator-friendly condition.
- In strategic locations where the Department of Agriculture has approved releasing several types of seed-eating insects. When established, these bugs can reduce seed production in a patch of Scotch Broom by up to 90%.
Our plans and approach are detailed for each section of the state as one part of our Integrated Roadside Vegetation Management Plans. These area-specific plans explain all of the vegetation management related work we do throughout the state, including where and how we control Scotch Broom.
Scotch Broom on the roadsides also got a bit of a "break" in 2015, when we implemented a "reduced mowing policy" to save money and promote better pollinator habitats. Since then, designated roadside areas have been growing Scotch Broom and other assortments of non-regulated weed species like crazy! Eventually, we'd like to further restore those unmowed areas with more native plants, but that would require additional funding for roadside restoration. That means, for now at least, the Scotch Broom is here to stay.
You can help
Residents in eastern Washington may not even be familiar with Scotch Broom, but it will grow just fine on the "dry side" of the state if you give it a chance. It's just highly regulated and controlled in eastern Washington due to the potential damage it could do to agricultural crops. If you see a Scotch Broom plant east of the Cascades, be sure and report it to the local county noxious weed control board, and pull it out of the ground if you can!
Throughout the month of May, the Washington Invasive Species Council has been conducting the Great Scotch Broom Census of 2020 across the state, and they have received overwhelming response, including many reports of roadside sightings. There's still a couple of days to take part.
You also can learn more about this plant and other fascinating (some scary!) non-native threatening species that threaten our state's environment and economy by visiting the WISC website. They also have online tools – including an app that lists where invasive species are known to be present and ways to report any you spot – as well as a wealth of information on all types of invasive species and their impacts.