Reducing Milk Thistle to Zero Density

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Santa Catalina Island, located about 22 miles southwest of Los Angeles, California, is part of the Channel Island archipelago. The island supports more than 60 miles of pristine beaches and an array of plant, animal and insect species. About 90 percent (42,135 acres) of the island is protected by the Catalina Island Conservancy, a nonprofit organization established to conserve the island’s natural resources through preservation and restoration.

The native plant community is central to the ecosystem of Catalina Island. It provides habitats that offer shelter and food to the Island’s endemic animals like the Catalina Island fox (Urocyon littoralis) and Catalina quail (Callipepla californica catalinensis), and native wildlife such as bald eagles (Haliaeetus leucocephalus) among many other species. Years of importing non-native plants as livestock feed and to landscape homes has introduced more than 76 highly invasive plants to Catalina Island. 

Milk thistle (also known as Silybum marianum, milkthistle and blessed milkthistle) is an invasive plant on the island that presents a unique challenge for land managers. This winter annual can germinate throughout a seven month period and occurs in remote infestations. Tony Summers, former Invasive Plants Program Supervisor for Catalina Island Conservancy explains. “Milk thistle infests about 43 acres on the island within 257 scattered infestations. We’ve observed milk thistle germinating on the Island as early as October and as late as May, resulting in plants ranging in height from 4 inches to 8 feet tall during the late growing season.” 

The Conservancy’s milk thistle control effort historically included hand pulling, mowing, and glyphosate or Garlon®4 Ultra herbicide treatments from April through June. “We found that hand pulling disturbed the soil seed bank resulting in an increase in milk thistle seedling density, and mowing caused plants to re-sprout. Also, our spring herbicide applications were ineffective because of late-germinating plants, and large mature plants that didn’t receive a lethal dose of herbicide due to leaf overlap,” explains Summers. In 2010, a field study was initiated on the island to develop a more effective treatment strategy that would reduce milk thistle to zero density on Catalina Island. 

FIGURE 1. Percent of control of milk thistle plants within various infestation sizes following a fall application of Milestone herbicide at 6 fl oz/A to seedlings and rosettes. Green bars represent average percent control.

The new milk thistle treatment protocol included early fall applications of Milestone® herbicide at 6 fluid ounces per acre (fl oz/A). Infested sites were surveyed and Milestone herbicide was applied to newly emerged seedlings. A five-foot treatment buffer was treated around seedlings during application. Herbicide was not applied if seedlings were not visible at time of application. Late-season follow-up treatments were made in May and June on mature plants with a 5 percent solution of glyphosate or 1 percent solution of Garlon® 4 Ultra herbicide. Late spring treatments were focused on quickly killing large plants to stop flower production. Flowers that were present on mature plants at application were clipped and buried about one foot deep in soil to prevent seed dispersal and germination. 

Results of fall treatments showed that Milestone® herbicide effectively controlled milk thistle seedlings throughout the germination period. Large populations (those with greater than 1,500 plants per site) had a greater average percent reduction (97.7%) compared to smaller population possibly due to more consistent herbicide application (Figure 1). Summers explains. “About half of the milk thistle populations were completely controlled with fall applications of Milestone herbicide. On sites where we didn’t get complete control we had an average of less than 5 percent of the original milk thistle plants to treat in late spring. This saved staff time and resources in our busy spring season. The fall timing also gave us more flexibility and availability of staff resources for the control effort.” 


Results of the field work showed that fall application of Milestone® herbicide to seedlings and rosettes drastically reduced milk thistle density across the treatment area and gave some residual activity on milk thistle. “Applying Milestone early in the season, when there is minimal grass or other vegetation to obstruct the herbicide contacting the soil surface and newly germinated seedlings, was key to achieving a high level of control,” says Summers. “There were only two populations (0.8% of the original infestation) that produced seed in 2012.This puts us on a trajectory toward total eradication of milk thistle from back-country sites on Catalina Island. Although eradication island-wide will be difficult to achieve, we are confident that we can remove the plant from our back-country area. We believe that our current application strategy could be applied to other annual weeds in the Asteraceae family.” 


Treating scattered infestations of invasive plants in the backcountry of Catalina Island.

PHOTO GALLERY (click to enlarge images with captions)


  • DiTomaso JM. 2007. Weeds of California and Other Western States. University of California Agriculture and Natural Resources Publication 3488. p 391-394.

  • Hochberg ML, S Junak, R Philbrick, and S Timbrook. 1979. Botany. In Natural resources study of the Channel Islands National Monument, California, ed. DM Power, 5.1-5.91. Santa Barbara, Calif.: Santa Barbara Mus. Nat. Hist.

  • Khan MA, RE Blackshaw, KB Marwat. 2009. Biology of milk thistle (Silybum marianum) and the management options for growers in north-western Pakistan. Weed Biology and Management. 9:99-105. doi: 10.1111/j.1445-6664.2009.00326.x

  • King County Department of Natural Resources and Parks. 2009. Best Management Practices: Milk Thistle.

  • University of Maryland Medical Center. Accessed 2013. 

Herbicide products (active ingredients) mentioned in this article include Garlon 4 Ultra (triclopyr), Milestone (aminopyralid).

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Milestone is 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 Milestone apply. Consult the label before purchase or use for full details.

State restrictions on the sale and use of Garlon 4 Ultra apply. Consult the label before purchase or use for full details. Always read and follow label directions.  ©2019 Corteva

Published Sept 2013; trademark updated June 2019.