Broom species have been identified as the second most problematic weed by Weed Management Area managers. They block light and use up water, resulting in many native species becoming locally extinct. It reduces forage and creates stands which are inaccessible and unpalatable to wildlife. Brooms can produce up to 12,000 seeds per plant – making it difficult to control once established. They form dense stands that cover 100% and eliminate native habitats. Brooms can invade even intact native ecosystems – and regrow after fire and grazing are used to control them.

Getting a handle on broom - Scotch, French, Spanish and Portugese Brooms in California

Cytisus scoparius reproduces primarily via seed. Its seed capsules have a capacity for ballistic dispersal, most notably when dry. Once ejected, dispersal of the seeds can be further aided by wind. Moving water is also a possible method of seed dispersal. Some vegetative reproduction can occur in the form of resprouting.

Mechanical Removal
Mowing or cutting Scotch broom plants at the end of a dry season has been shown to significantly reduce re-sprouting and if done repeatedly can eventually reduce populations. Cutting the stems below the soil surface may disturb the soil and stimulate germination of seeds from the seed bank. Bush hog removal, which involves twisting the stems off instead of cutting, has been used to control Scotch broom, but does not eliminate re-sprouting and may cause damage to non-target species.

Prescribed Burning
Prescribed burning can be used successfully, but requires multiple burns to effectively control this species. This technique can also be used prior to herbicide application to increase control of Scotch broom. Burning is more effective when done in mid-summer as new plants are sprouting. A high-intensity burn combined with a low-intensity burn 2-3 years later can result in long-term control. Burn trials conducted on French broom (Genista monspessulana) in Australia and New Zealand indicated that burning reduced the existing plant populations, but increased seed germination from the seed bank. Higher intensity burning is required to prevent seed germination, but this technique damages seeds of desirable species in the seed bank. This can result in increases in weed species.

Grazing Management
Livestock losses after grazing Scotch broom have been reported and therefore using grazing management to control populations should be approached with caution. During field grazing trials in British Columbia, sheep would not eat Scotch broom. However, La Manchia goats grazed Scotch broom on small field plots on Vancouver Island. Grazing by goats during active growth of Scotch broom has been used successfully in New Zealand, but requires several seasons for effective control. Native plant species are also at risk with this method since goats are non-selective grazers. Grazing by llamas has been used with some success at a few sites in California. No toxicity has been reported in goats or llamas.

Biological Control
Two insect species, a twig-boring moth (Leucoptera spartifoliela) and a seed weevil (Apion fuscirostre) were released in the 1970’s and 1980’s as biocontrol agents for Scotch broom in California. The effectiveness of these insects for controlling this species has been very limited however. The twig-boring moth is subject to insect parasitism. The seed weevil larvae consume seeds and may help reduce plant stands, but they have no impact on adult plants. These insects are not known to occur in Montana. Several new biocontrol insects are being tested for use in New Zealand and Australia, but are not currently available in the United States.

Integrated Pest Management

1. Prevention is key. Maintain soils and desirable plant species at optimum quality levels. In the event Scotch broom plants are found, immediate treatment is necessary to prevent spread.
2. Monitor and clean vehicles and equipment to prevent transport of seeds between sites.
3. Disturbance will facilitate weed establishment. Routinely monitor vegetation, especially after a disturbance, and remove weeds as soon as possible. Continue monitoring following removal.
4. Small to moderate infestations may be controlled by cutting in combination with chemical treatment.
5. Severe infestations may require repeated chemical treatment. Re-seeding of infested areas may be necessary in cases of severe.