Scotch Broom: Identification, Biology, and Integrated Management

Scotch broom (Cytisus scoparius L.) is a shrub on Montana’s noxious weed list. It can be recognized by its bright yellow flowers, clover-like leaves, and green woody stems. Scotch broom is found in disturbed sites, pastures, roadsides and open woods. Control is difficult once this species becomes established, with repeated treatments often required. Scotch broom currently has limited distribution in Montana; therefore, prevention and early detection are key to its management.

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Figure 1. Scotch broom population in Oregon. Eric Coombs, Oregon Department of Agriculture, bugwood.org.

Origin and Distribution

Scotch broom is native to central and southern Europe and North Africa. It was first reported in the United States in the 1800s in Virginia where it was introduced to provide food for sheep. In the western U.S., this species was introduced as an ornamental plant and to control soil erosion. It is considered an invasive plant in several countries including Australia, New Zealand, Canada, and the U.S. This species is found in most states in the U.S., and most reports are from the east and west coasts. Western states reporting populations of Scotch broom include California, Idaho, Montana, Nevada, Oregon, Utah, and Washington. Scotch broom is a Priority 1b noxious weed in Montana. This designation means Scotch broom is not widespread in Montana and prevention, education, and eradication are management priorities if this species is detected. It has been verified in Lincoln and Sanders counties in Montana, and reported although not verified in Missoula, Powell, and Ravalli counties. In addition to Montana, Scotch broom is listed as a noxious weed in Hawaii, Idaho, Oregon, and Washington.

Identification and Biology

Scotch broom is a deciduous shrub in the pea family (Fabaceae) that may grow up to 13 feet (4 m) tall (Figure 2). Stems are angled, green, and woody with no spines (Figure 3). Scotch broom has two different types of leaves depending where they are found on a branch. Leaves located lower on the branch are compound with three leaflets (Figure 3). The 1/5 to 1/3 inch (4-8 mm) long leaflets are typically egg-shaped and widest at the tip. Leaves on the upper branch are simple and lack a stalk or petiole. Leaf tips may be pointed or have a small spine located at the tip. Leaves may or may not be hairy.

Bright yellow flowers typically occur as groups of one to three at the leaf axils (Figure 4). Though flowers are usually yellow, they may instead have red or purple petals. Flowers are approximately 3/4 inch (2 cm) in length. Flowers of Scotch broom are typical of those in the pea family, with five petals; two wings, two keels, and a banner. Scotch broom flowers have ten stamens, with four longer than the others.

The fruit of Scotch broom is a flattened pod from 3/4 inches to 2 inches (2 to 5 cm) in length that is green when young and black when mature. The edges of the pods are fringed with hairs (Figure 5). Each pod contains between three and twelve seeds.

Figure 2. Scotch broom is a shrub in the pea family. Eric Coombs, Oregon Department of Agriculture, bugwood.org.

Figure 3. Scotch broom has green, woody stems with no spines. The leaves are compound with three leaflets (pictured), or simple (not pictured). Tom Huette, USDA Forest Service, bugwood.org.

Figure 4. Scotch broom flowers are bright yellow, and their structure is similar to other plants in the pea family. Eric Coombs, Oregon Department of Agriculture, bugwood.org.

Figure 5. Scotch broom seed pods turn black when mature, and the edges are lined with hairs. Gil Wojciech, Polish Research Institute, bugwod.org.

Not to be Confused With

Plants in the pea family with yellow flowers may be mistaken for Scotch broom including mountain goldenbanner (Thermopsis montana) and Siberian peashrub (Caragana arborescens).

Mountain goldenbanner is a Montana native forb. The most important feature of this plant that helps distinguish it from Scotch broom is this plant is a forb rather than a shrub, meaning it is not woody and typically much shorter (Figure 6).

Siberian peashrub is a shrub commonly planted in windbreaks and residential areas. Unlike Scotch broom, older stems are often brownish in color, spines occur at the bases of the leaves, and the compound leaves have more than three leaflets (Figure 7).

 

     

Figure 6. Mountain goldenbanner. Matt Lavin, MSU.

 

Figure 7. Siberian peashrub. Left) Bonsak Hammeraas, NIBIO. Right) John M. Randall, TNC. bugwood.org.

Life Cycle

Scotch broom is a long-lived shrub with a lifespan of 20 years or more. Plants reach full height within 6 to 10 years. Flowering normally begins in early to late spring, depending on climate. Peak flower production typically occurs between May and June, followed by fruit set. Seed dispersal occurs from mid-July to mid-August.

Reproduction is by seed and resprouting. Seed production can begin in the second year and is extremely variable, ranging from 4,000 to 30,000 seeds per plant. Some studies indicate there may be up to 2,000 seeds per square foot of soil in dense populations. Seeds can remain viable in the soil for up to at least five years. Seeds do not need light to germinate.

SPREAD

Seeds are catapulted from mature pods when the pods dry and audibly snap apart, sending seeds 3 feet (1 m) from the parent plant. Seeds may also be spread over longer distances by animals, vehicles, and flooding. A common method of dispersal in the Pacific Northwest is through gravel and vehicles used for road construction associated with timber harvest. Horticultural plantings of Scotch broom provide a seed source for population spread.

HABITAT

Scotch broom typically inhabits disturbed habitats such as roadsides, pastures, and logged areas. It can also be found in open forests, sand dunes, rangeland, and riparian areas. It is considered invasive in coastal areas of the western United States from California to western Washington. Scotch broom can inhabit many types of soils, persisting on seasonally dry to moist sites at a range of pH, texture, and fertility. The distribution of Scotch broom in Montana may be limited by climate, particularly precipitation levels and winter temperatures. Though it is seasonally drought tolerant, Scotch broom prefers areas with annual precipitation over 20 inches (500 mm). The distribution of Scotch broom also tends to be limited to those areas that have mild winters. Seedlings and younger plants are sensitive to frost, and larger plants can experience considerable dieback during severe winters. In addition, stem photosynthesis during winter may be important for Scotch broom survival, and this process is limited by cold temperatures.

Impacts

Scotch broom can contribute to reductions in plant community diversity and can form dense, monospecific stands. There is evidence that Scotch broom may help increase abundance of other non-native plants. It is also competitive for resources like water. During dry seasons, this species has been known to reduce biomass of juvenile trees in Douglas-fir forests by as much as 96 percent.

Scotch broom is toxic to livestock due to the presence of the quinolizidine alkaloids sparteine and isosparteine, however it is rarely grazed. Livestock poisonings have been reported in Europe but are very rare in the U.S.

Management Options

PREVENTION

Identification, early detection, and control of Scotch broom is key to preventing establishment of new colonies. Preventing the spread of Scotch broom seeds by humans is critical, therefore vehicles and equipment should be regularly and thoroughly cleaned before being moved from infested to non-infested areas. Contact your local MSU Extension agent or county weed district if you think you have found Scotch broom.

MECHANICAL

Mowing or cutting Scotch broom plants at the end of summer can eventually reduce populations if done repeatedly. Cutting stems below the soil surface is an option for small infestations, but it may disturb the soil and stimulate germination of seeds from the seed bank. Mowing with a rotary mower, which involves twisting off the stems instead of cutting, has been used to control Scotch broom but does not eliminate re-sprouting and may cause damage to non-target species.

CULTURAL

The most effective way to prevent the invasion and establishment of Scotch broom is through limiting disturbance where possible and maintaining or establishing competitive plant communities. Disturbances such as overgrazing and timber harvest create gaps in vegetation and may favor the establishment of Scotch broom and other weeds. Where Scotch broom is present, retaining logging debris after timber harvest rather than piling or removing it can decrease Scotch broom establishment.

In areas of severe infestation, revegetation may be necessary to establish a plant community that meets management objectives. Herbicide application followed by seeding or planting saplings may be required to aid establishment of desirable seeded species. Due to the persistence of this species in the seed bank, this process is difficult and may require repeated treatments over multiple years.

BIOLOGICAL

Two insect species, a twig-boring moth (Leucoptera spartifoliella) and a seed weevil (Exapion fuscirostre), were released in the 1970s and 1980s as biocontrol agents for Scotch broom in California. The effectiveness of these insects has been limited. A third insect species, a broom seed beetle (Bruchidius villosus) is currently used as a biocontrol agent in Oregon. These insects are not known to occur in Montana.

Grazing by goats during active growth of Scotch broom has been used successfully in New Zealand and requires several seasons for effective control. Goats are non-selective grazers, so they may also impact desired plants. Grazing by llamas has been successful in California. No toxicity has been reported in goats or llamas. Sheep would not eat Scotch broom during a field trial in British Columbia.

PRESCRIBED BURNING

Prescribed burning can be effective and requires repeated burns to control this species. Burning can be used prior to herbicide application to improve control of Scotch broom. Burning is most effective when done in mid-summer as new plants are sprouting. A high-intensity burn followed by a low-intensity burn two to three years later can provide long-term control. Trials conducted on French broom (Genista monspessulana) in Australia and New Zealand indicated that burning reduced existing plant populations, and increased seed germination from the seed bank. Repeated burning can deplete the seed bank over time by killing the newly establishing plants.

CHEMICAL

There are a variety of herbicides labelled for Scotch broom control in Montana. Glyphosate (e.g., Roundup®) can be used as a foliar herbicide to control established plants during active growth and after all leaves have expanded. This herbicide should be applied with a non-ionic surfactant (based on label recommendations) at a rate of 1.25 to 1.5 percent solution. Triclopyr + 2,4-D (Crossbow®) can be used as either a broadcast application (1.5 gal/acre) or high- volume foliar application (1 to 1.5 percent mixture) during active plant growth. Triclopyr (e.g., Garlon® 3A or 4) or aminopyralid + triclopyr (e.g., Capstone® or Milestone VM Plus®) can also be used for control of Scotch broom in non-crop areas like rangelands, pastures, and rights-of-way. These herbicides can be used as a foliar treatment when plants are growing rapidly or as a cut-stump or basal bark application any time the ground is not frozen. Always read and follow herbicide product labels.

Integrated Weed Management

• Learn to identify Scotch broom so it can be detected and controlled before it can spread.
• Clean vehicles and equipment to prevent transport of seeds between sites. Avoid transporting gravel from areas where Scotch broom is prevalent.
• Limit disturbance and minimize bare ground to prevent weed encroachment.
• Monitor vegetation, especially after a disturbance, and control weeds as soon as possible. Continue monitoring following control treatments.
• Small to moderate infestations may be controlled by cutting in combination with herbicides.
• Severe infestations may require repeated treatment with herbicides. Revegetation may be necessary in cases of severe infestation.
• Contact your local MSU Extension office or county weed district if you suspect you have found Scotch broom.

Glossary

Angled – square-stemmed; not round.

Banner - largest petal of a pea flower.

Deciduous - losing its leaves at the end of the growing season.

Keel - two lower fused petals of a pea flower.

Simple - single, not divided into parts.

Wing - the two lateral separate petals of some pea flowers.

Additional Resources

DiTomaso, J.M. G.B. Kyser et al. 2013. Weed Control in Natural Areas in the Western United States. Weed research and Information Center, University of California. 544 pp

Lesica, P. 2012. Manual of Montana Vascular Plants. Botanical Research Institute of Texas. 779 pp.

Peterson, D.J. and R. Prasad. 1998. The biology of Canadian weeds. 109. Cytisus scoparius (L.) Link. Canadian Journal of Plant Science. 78: 497-504.

Acknowledgements

This publication is based on information from MSU Extension publication number EB0202 by Melissa Graves, Jane Mangold, and Jim Jacobs. The authors would like to thank Shawna Bautista (U.S. Forest Service), Dr. Tim Harrington (U.S. Forest Service, retired), Jasmine Reimer (Montana Department of Agriculture) for reviewing this publication.