Synthetic Auxin Herbicides Control Germinating Scotch Broom

Editor’s Note

The following information was summarized by Celestine Duncan from research conducted by Timothy B. Harrington and published in Weed Technology 2014 28: pp 435–442. Abstract at:

Scotch broom (Cytisus scoparius) is a large, nonnative shrub that has invaded forests and grasslands in 27 U.S. states (Figure 1). The plant was introduced as an ornamental in the 1850s and is a prolific seeder with individual shrubs producing from 100 to 14,000 seeds per year. Once the seeds are buried, their germination can be delayed for at least five years, resulting in soil seedbanks of 200 to 27,000 seeds per square meter. Without treatment, Scotch broom’s persistent seedbank ensures a continuing source of regeneration after soil disturbance. 

A variety of fire, herbicide, and mechanical treatments are effective for controlling established Scotch broom. However, observations regarding effectiveness of soil-active herbicides in controlling germinating seedlings of Scotch broom are limited.

Researchers conducted a series of studies in growth chambers beginning in 2010 to compare the effectiveness of three soil-active auxin herbicides: aminopyralid (Milestone® herbicide), clopyralid (Transline® herbicide) and aminocyclopyrachlor for controlling Scotch broom seedling germination. Herbicide application rates were 0, 50 and 100 percent of the maximum label broadcast use rate, or Milestone at 0, 3.5 and 7 fluid ounces per acre (fl oz/A) and Transline at 0, 10.5 and 21 fl oz/A.

Results of the study showed that Scotch broom seedling emergence, mortality, and biomass did not vary among herbicide treatments 90 days after application. Low herbicide application rates (50 percent of the maximum label use rate) provided 60 to 80 percent control, whereas 100 percent maximum label use rate provided 69 to 89 percent control. 

These findings have three important implications to vegetation management in the western United States. 

First, Transline is commonly used for herbaceous weed control in forestry because it is safe to apply over seedlings of Douglas-fir (Pseudotsuga menziesii var. menziesii) and other western conifer species. Findings from this research confirm field observations that operational treatments of Transline reduced regeneration of Scotch broom from existing seedbanks. 

Secondly, Milestone has been used successfully in restoration of native plant communities, because it controls many nonnative broadleaf species, yet it is tolerated by a wide variety of native grass, forb, and shrub species especially when applied at low rates. Results of this research indicate that low rates of Milestone will provide the added benefit of controlling germinating Scotch broom. 

Thirdly, depending on application rate and time since treatment, the herbicides varied in cost per unit of seedling mortality, with a general ranking of Milestone the least expensive, followed by Transline, and aminocyclopyrachlor (most expensive). Both Milestone and Transline are currently labeled for forestry and other use sites including grazed areas.

NOTE: While Milestone is not registered for use in forestry in western states, Opensight® herbicide (as Special Local Needs (SLN) label in OR, ID, and WA) and Capstone® herbicide (federal label) are registered for use on forest sites. 

Active ingredients for herbicide products mentioned in this article: Milestone (aminopyralid), Transline (clopyralid), Opensight (aminopyralid plus metsulfuron, and Capstone ( aminopyralid plus triclopyr).

®™Trademarks of Dow AgroSciences, DuPont or Pioneer and their affiliated companies or respective owners. Milestone, Capstone, and Opensight are not registered for sale or use in all states. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state. When treating areas in and around roadside or utility rights-of-way that are or will be grazed, hayed or planted to forage, important label precautions apply regarding harvesting hay from treated sites, using manure from animals grazing on treated areas or rotating the treated area to sensitive crops. See the product label for details. State restrictions on the sale and use of Transline apply. Consult the label before purchase or use for full details. Always read and follow label directions. ©2019 Corteva