Why Is It a Noxious Weed?
This invasive shrub is a noxious weed on the west coast. It can form infestations that outcompete native and forage plants, interfere with reforestation efforts and aid the spread of wildfires into tree canopy layers. With high seed production, established infestations are difficult to eradicate.
How would I identify it?
French broom is a taprooted shrub that has fine roots associated with nitrogen-fixing bacteria.
Flowers are yellow, pea-shaped and around 0.35 to 0.47 inches (9-12 mm). Flowers are typically in head-like racemes (clusters) of 4-10 flowers at the ends of short axillary branchlets.
Leaves are alternate and compound, with three leaflets. Leaflets are oblong, variable in size with most 0.4 to 0.8 inches (10-20 mm) long. Upper and lower surface is sparsely to densely covered with appressed short, silvery hairs.
Stems upright and typically leafy and covered in silky, silvery hairs. Younger stems are green and round in cross-section and often strongly 8-10 ridged, while older stems are brown and may not have ridges.
Fruit Seed Description
The seed pods (legumes) are brown at maturity and slightly flattened, 0.4 to 1.2 inches (1-3 cm) long, and densely covered with appressed, silky, silvery to reddish gold hairs. Pods have 3-8 seeds per pod on average.
May Be Confused With
There are two other similar yellow-flowering brooms on Washington State’s noxious weed list: Scotch broom (Cytisus scoparius), a Class B noxious weed and Spanish broom (Spartium junceum), a Class A noxious weed.
Where does it grow?
French broom grows in open, disturbed sites including, roadsides and pastures. It can also be found in less disturbed to undisturbed sites such as grasslands, coastal scrub, oak woodlands, riparian areas and open forests. In Washington only one escaped population has been found on the UW campus in Seattle. Please click here to see a county level distribution map for French broom in Washington.
How Does it Reproduce?
French broom spreads by seeds and can produce over 8,000 seeds a year. French broom roots do not produce new shoots but plants can resprout when cut, frozen or burned above the crown.
How Do I Control It?
General Control Strategy
Preventing new populations of Genista monspessulana from establishing and spreading is the best way to minimize costs and impacts of this invasive shrub. It is critical to minimize soil disturbance, monitor areas being controlled and pull young plants before they produce seeds to help prevent new infestations.
Genista monspessulana can be effectively pulled with weed wrenches. Flaming--using a torch that is hot but does not have a flame--to quickly pass over plants, can be an effective method to control seedlings. Applying a layer of wood chip mulch can also decrease the germination of seedlings. These techniques can be used after adult plants have been removed.
Since French broom plants do not grow in dense shade, plant native and desirable plants in areas of French broom infestations to create canopy cover that will limit its growth and spread.
A seed beetle, Bruchidius villosus, which is used on was found to attack Genista monspessulana in the Eugene area in Oregon. Since French broom is a Class A noxious weed, this biological control would not be used in Washington since eradication is required. Using goats for grazing can be an option.
For More Information
See our Written Findings for more information about French broom (Genista monspessulana).