What's that PNW Weed: Invasive Scotch Broom
- Katie Burnham
- Jul 21, 2024
- 4 min read
Updated: Jul 24, 2024
Discovering Weeds in the Pacific Northwest
Cytisus scoparius, more commonly known as Scotch broom or Scot's broom, is an invasive shrub from Europe in the Fabaceae family. Listed as an invasive plant species in Washington state in 1988, Scotch broom is commonly found growing along roadsides, pastures, grassy areas, and places with newly disturbed soil.
General Description of Scotch Broom
Scotch broom is a perennial shrub with many branches and can grow 3-10 feet tall. Similar to other Fabaceae plants in the Pea family, Scotch broom has pea-like flowers with 5 bright yellow petals about 3/4 inch long. Flowers typically bloom from March to June. If pollinated, the flower forms a hairy seed pod (pea pods), which changes from bright green to black with maturity.
Scotch broom stems are somewhat woody (not as woody as a tree trunk, but sturdy), remain dark green year-long, and have five ridges with hairs when young. Older wood is more glabrous (smooth and free from hair).
Scotch broom is a deciduous invasive plant, which loses its leaves in the early fall. This plant has two different types of leaves: simple or compound 3-parted. Simple leaves are found at the upper part of the plant, while 3-parted leaves are on the lower part of the plant. Leaves are obovate (egg-shaped), where the widest part of the leaf is above the middle, with a pointed tip, and smooth outer margin.
Generally, the best way to identify Scotch broom is from a distance during the springtime. The bright yellow flowers and green woody stems are such distinguishing characteristics that it's difficult to misidentify from afar. However, up close and during months while not in flower, you may mistake Scotch broom as another Fabaceae (pea family) member.
Scotch broom prefers growing in full sun and can tolerate dry, sandy soils with a pH of 4.5-7.5. It does not grow well in dense forests, but will rapidly outcompete native species after disturbances such as logging.
Why Scotch Broom is an Invasive Weed
Scotch broom is not from America--or even the North American continent! It's originally from Europe and was purposefully brought to America as an ornamental shrub, prized for its bright yellow spring blooms and as a soil stabilizer on hillsides. Unfortunately, the ideal growing conditions in Washington have led to its naturalization and the displacement of native PNW plants. These are the tell-tale characteristics of an invasive species. If left to grow in a pasture or grassland, Scotch broom will quickly outcompete ground cover, wildflowers, and grasses--turning areas of pasture into a brushy, undesirable monoculture. Monocultures replace native species, reducing food and habitat for PNW native fauna.
Moreover, Scotch broom seeds can remain viable in the soil for 30+ years (holy cow that's a long time to sit around and wait!). Some tests have shown seeds to remain viable for up to 80 years! Simply put, seeds can remain dormant in undisturbed soil for 30+ years; once the soil is disturbed, those dormant seeds may sprout given favorable water and light conditions.
Each Scotch broom plant can produce between 4,000-30,000 seeds a year. And seeds are dispersed mechanically (pods pop open and launch seeds up to 20 feet), by ants, animals, or through soil transportation (like mud on the bottom of your shoe). Simply put, it only takes one Scotch broom plant to turn a beautiful, open pasture into a densely-populated Scotch broom forest.
How to Manage Scotch Broom
Managing Scotch broom can be an arduous and long undertaking. If you have only a few plants growing in your yard, the best control method is mechanical removal. Dig up the roots and completely remove all parts of the plant. The best time of year to do this is before flowers turn to seed pods (disturbing seed pods may scatter seeds, exacerbating the problem). Removed plants should be bagged and thrown in the garbage or burned. Do not add Scotch broom (or other invasive plants) to compost, as seeds may remain viable.
Another form of mechanical management of Scotch broom is to continuously mow short plants. Shorter plants are less woody and can be cut with a lawn mower or powerful weed whacker. If plants are mowed down continuously, the plant may die eventually. Mowing will also reduce height and ability to flower and reproduce seeds. If plants are too large for mowing, use loppers to cut the plant flush with the soil surface. Unfortunately, if the plant doesn't die from the mowing/cutting, it may become dense and displace grass or other shrubs.
Another form of management is to reduce soil disturbance. If you live in an area with Scotch broom, you may want to think twice before tilling or disrupting soil. As seeds can remain dormant and viable for up to 80 years, the best method of reducing germination is to limit soil disturbance. No disturbance means no seed germination. In circumstances when soil disruption is unavoidable, it's important to regularly inspect for Scotch broom seedlings and promptly mechanically remove.
Others have found success in managing Scotch broom with herbicides, but I don't like using chemicals unless the situation is completely dire and all other methods have been attempted without success. Remember, herbicides aren't plant-specific. So if you use an herbicide to kill a Scotch broom, it'll also kill every other nearby plant. Additionally, chemical herbicides remain in the soil for years (if not decades) after application, preventing future plants from growing.
Recap
Cytisus scoparius (Scotch broom) is an invasive, woody shrub originally from Europe that aggressively outcompetes native Pacific Northwest flora. Scotch broom is in the Fabaceae family and has bright yellow pea-like flowers that mature into hairy, black pea pods. One plant can produce thousands of seeds a year, which can remain dormant and viable in the soil for up to 80 years. The best mitigation methods are to mechanically remove plants and reduce soil disturbance.
Sources
Research for this article utilized the following resources:
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