Common Name: Scotch broom
Scientific Name: Cytisus scoparius
Code: CYSC4
Group: Dicot
Family: Fabaceae
Growth Habit: Shrub
Duration: Perennial

What does it look like?

Scotch broom, a shrubby, perrinial member of the pea family, grows to about six to ten feet tall at maturity and flowers profusely with brilliant yellow “pea-like” flowers. Its sharply angled branches generally have five green ridges with hairs when young; as the branches mature the hairs fall, and the branches become tan and lose distinct ridges. The pods have hairs along the seams only. A mature Scotch broom plant can annually produce over 12,000 seeds with a survival span of five to thirty years. About 98% of the seeds are viable, contributing to the extensive spread of brooms (Bossard et al. 2000).

Scotch broom reaches reproductive maturity at two to three years, and flowers in late March to April inland and in April to June on the coast.

Where does it grow?

Scotch broom is native to Europe and North Africa and was introduced to the United States in the 1850s as an ornamental plant in the Sierra Nevada foothills and elsewhere later to prevent erosion. This invasive exotic exists along the California Coast from Monterey north to the Oregon border.

Scotch broom generally grows in disturbed areas, such as roadsides and clearcuts, and sometimes colonizes undisturbed grasslands and even open-canopy forests. It prefers open areas to shade areas and can grow in a wide array of soil moisture conditions. Brooms, a member of the pea family, fix nitrogen.

Is it in our watershed?

Scotch broom is one of the most aggressive and widespread invasive exotics growing within the Mattole River watershed. It was introduced into the Mattole as an ornamental plant, where it grew near the Redwoods Monastery in the late 1960s. Many residents valued this broom as an ornamental and often transplanted it from the Monastery area to their own yards – not aware of its tendency to spread rapidly.

Dense infestations of Scotch broom exist near the town of Petrolia, the Mattole estuary, Whitethorn, and many roadsides, clearcuts, open fields, and other ecologically disturbed sites.

What problems does it cause?

Scotch broom currently inhabits over 700,000 acres in California. It displaces native plant and forage species, and complete eradication is difficult (Bossard et al. 2000). It outcompetes other species, forming a dense monospecific stand, often shading out tree seedlings. In the Mattole, it crowds out grazing lands, decreasing the economic potential of ranchlands. It is also extremely flammable, carrying flames into forest canopies, increasing both the frequency and intensity of wildfires.

How do you get rid of it?

Manual/Mechanical Removal: It is possible and easy to pull up new shoots of young Scotch broom by hand, preferably wearing gloves. This is easiest in winter or early spring when the ground is damp and soils are pliable. Scotch broom grows quickly and the root systems soon become well-established and large, making manual removal impossible without the use of tools.

Weed wrenches are manually operated, all-steel tools designed to remove woody shrubs and plants by uprooting them with powerful gripping jaws (click here to see an example of a weed wrench). These tools have proven effective in removing Scotch broom because the wrenches remove the entire shrub including the root system, thereby controlling resprouting. This technique is labor intensive, expensive, and can disturb the seedbank, releasing seeds which can increase infestation. It is crucial to monitor projects and conduct follow-up work after any such removal project.

The use of heavy equipment has been successful in extremely large, dense infestations of Scotch broom. The use of a brush hog to clear a field is successful at removing above-ground vegetation, however, removal of root wads is important to ensure long-term suppression. A cat pulling a chain between two forks that reach underground and drag beneath the soil can remove the broom with the roots. This technique is extremely disruptive. Because it creates large-scale soil disturbances, it should only be a last-resort situation and should include proper re-vegetation.

Biological Control: In the 1960s, introduction of two USDA-approved insects as biocontrol agents, a stem miner, Leucoptera spartifoliella, and a seed beetle, Apion fusciostre, showed some success in California. Heavy grazing by goats for four to five years has proven effective in New Zealand, but goats also tend to eat the native species that begin to regenerate (Bossard et al. 2000).

Case Studies: To date, the BLM has funded many of the Scotch broom removal efforts in the Mattole, especially on the Mattole estuary floodplain as well as sites on Prosper Ridge. These projects have employed manual removal using weed wrenches and have proved successful at removing all mature Scotch broom plants. Future retreatment and monitoring projects are required to ensure long-term success.

In the spring of 2003, a Petrolia fuels reduction project cut Scotch broom with a brush hog on Cook’s pasture behind the Mattole Valley Community Center. This treatment did not kill the broom, and plants began regenerating from the roots. Crews removed the re-growth using weed wrenches. This was easier because rootwads were exposed after being mowed (Chris Larson, pers. comm.). Only about half of the Scotch broom was removed from this field due to budget constraints.

An effective removal project in the Eldorado National Forest in the Sierra Nevada foothills cut the shrubs in September and October, allowed them to dry on-site, and burned them in late May and early June. This killed the resprouts and most of the seeds within the top one-inch of the soil. Seeds below this were scarified by the heat, then germinated within two weeks and died during the summer drought. This reduced the amount of seed in the soil by 97 percent (Bossard et al. 2000).

Author:
Unity Peterson, Invasive Plant Program Coordinator, Mattole Restoration Council, Petrolia, CA.

Photographs:
Unity Peterson

References:
Bossard, CC., J.M. Randall and M. Hoshovsky. (eds.) 2000. Invasive Plants of California’s Wildlands. University of California Press, Berkeley, CA. Online version: http://groups.ucanr.org/ceppc/Invasive_Plants_of_California's_Wildlands/

Chris Larson, Executive Director, Mattole Restoration Council, Fall 2004.