Scotch broom (Cytisus scoparius) is a large, invasive shrub that has become prevalent in forest and grassland sites across much of the coastal United States. While it has a tendency to do the most damage in areas that are forested, it also is prevalent in and around roadsides and utility rights-of-way (ROW).

Dr. Timothy Harrington of the USDA Forest Service recently conducted a study testing the effects of several herbicides on the seed germination and seedling development of Scotch broom. The tests were conducted in growth chambers, where populations of 50 seeds were sown in individual, rectangular containers filled with a fixed weight of glacial outwash soil.

Two of the herbicides tested were Milestone^® and Transline^® specialty herbicide. Each herbicide was applied at rates of 0, 50 and 100 percent of the maximum broadcast label rates to the soil where the Scotch broom seeds were planted. Each application was replicated six times within the controlled growth chamber.

“The two key measures of the study were seedling emergence and mortality,” Harrington says. “Seed emergence was counted every one to three days for 90 days. Emergence was measured as a percentage of seeds sown, and mortality was measured as a percentage of final emergence.”

Positive Results

Final emergence averaged 40 percent of seeds sown, regardless of the rate of herbicide application. However, the herbicides did cause swelling of the hypocotyls and inhibited development of root hairs. Emergence of herbicide-treated seedlings was often negatively geotropic, meaning the roots grew upward, resulting in loss of moisture and mortality.

“While 40 percent might seem like a high number of seedlings to emerge after a herbicide treatment, you have to look at the condition of those seedlings after 90 days,” Harrington says. “Most were in such fragile condition, that they wouldn’t survive the season under the stress of normal field conditions.”

In the end, both Milestone and Transline provided excellent control of germinating Scotch broom seedlings with mortality rates averaging more than 80 percent at study completion when applied at the maximum broadcast labeled rates. Either treatment can serve as a highly effective tool to control the establishment of Scotch broom where a seedbank is present.

Recommendations for Selective and Residual Control

When controlling established Scotch broom, there are options that have all been used with commercial success for several years. One option is Capstone™ specialty herbicide at 8 to 9 pints per acre as a broadcast foliar treatment. Another foliar treatment option is Garlon^® 4 Ultra specialty herbicide at 2 to 4 quarts per acre, mixed with 3 to 7 ounces of Milestone or 3.3 ounces of Opensight^® specialty herbicide. Both the treatment with Capstone and the mixes of Garlon 4 Ultra and Milestone or Opensight will provide excellent control on established Scotch broom, as well as soil residual to control the newly germinating seedlings.

Although Capstone, Milestone^® and Opensight specialty herbicides are selective to many desirable plants, none of them should be used over the top of trees — only used as a directed spray under the canopy, or within the dripline, of certain trees, but not under sensitive woody species such as legume trees/shrubs. Transline also provides excellent selectivity, allowing it to be used over many conifers and hardwood species.

Additionally, Capstone, Milestone and Opensight also offer high levels of tolerance on a wide range of warm- and cool-season grasses, making them essential tools for areas where desirable vegetation needs to be maintained.

^®™Trademark of Dow AgroSciences LLC
Milestone specialty herbicide is labeled for use in forestry site preparation treatments in AL, AR, FL, GA, LA, MN, MS, NC, SC, TX and VA.
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
Consult the label before purchase or use for full details.
Always read and follow label directions.