Marin County Parks and Open Space Department

Invasive Plant Management

Of particular concern are invasive weeds that can rapidly dominate a landscape, displace native plants and, as a result, cause significant declines in biodiversity and habitat values. The preservation and enhancement of natural resources is inherent in our mission, and if the spread of invasive weeds is not addressed, these resources will be adversely impacted. To achieve the goal of controlling the spread of invasive weeds, we have developed and implemented a number of invasive weed management projects on particular preserves.

By visiting the projects below you can learn more about invasive weed management on Marin County Open Space District preserves. We have included information about project location, background, goals, and strategies. In each case, the strategies are based on the core principles of an Integrated Pest Management (IPM) approach.

The IPM approach is a decision-making process that uses monitoring and evaluation in determining treatment method. IPM includes a range of possible treatments, including mechanical (e.g., weed whipping), physical (e.g., hand pulling), and chemical (e.g., herbicide) tools, as well as other management practices, to control pests in a safe, cost effective, and environmentally sound manner. You can learn more about IPM principles and practices through the County of Marin’s Integrated Pest Management Program. To date, our work has targeted the worst infestations of invasive plants. We hope to broaden our efforts to control the spread, and ultimately eradicate new populations of invasive plants as they are discovered so that the adverse effects on native species and habitats are minimized

Invasive Plant Projects

Deer Island Preserve

Perennial Pepperweed

Perennial pepperweed (Lepidium latifolium) is a highly invasive weed that grows near water and tolerates relatively saline conditions. It has gained more and more attention in the Bay Area where it has become a pervasive management problem as it chokes up waterways and displaces coastal habitat. Pepperweed is a difficult weed to control and eradicate. While our largest infestation is on our Deer Island Preserve, other known populations exist at Bolinas Lagoon and Santa Venetia Marsh preserves.

Background: Perennial pepperweed (Lepidium latifolium) has expanded rapidly in Bay Area habitats that are adjacent to water, making them uninhabitable for many native plants and migratory birds. A highly invasive non-native weed, pepperweed increases erosion and soil salinity, while creating a dense mat of vegetation with a high silica content that crowds out all other vegetation. Pepperweed expands quickly by root, seed, and the spread of root fragments. Both root fragments and seeds are transported by waterways to favored habitats in marshes and other moist lands.

Much experimentation has been done regarding the effective treatment of pepperweed. Because of its ability to store energy in its roots, and reproduce from root fragments, mechanical attempts at removal often encourage its spread, while cutting it does not result in killing the plant. A number of chemical treatments have proven most effective, when timed with the budding of the flower.

Mount Burdell Preserve

Yellow Star Thistle

Yellow star thistle (Centaurea solstitialis) is a pervasive invasive weed that occurs on the grasslands of Mount Burdell and is interspersed within oak and bay woodlands. It is fairly common throughout the preserve; however the populations are often small and scattered, and as a result, our efforts have yielded significant success.

Grazing is one tool that we use to control the spread of yellow star thistle here. In addition, we have used weed whipping, herbicide, and hand pulling as control methods. The slopes of Mount Burdell support many rare plants including fragrant missionbells, Marin western flax, and the only known population, in Marin, of cupped monolopia. By preventing the spread of this invasive weed, we will preserve native plant communities and help protect dependent wildlife.

Background: Our goal is to restore an estimated 125 acres of land on the Mount Burdell Preserve that has been infested by yellow star thistle. This invasive weed spreads rapidly, displaces native plants and animals, and significantly degrades grasslands. It is harmful to wildlife, pets and horses, and can impact public use and recreation. In addition to mechanical and chemical methods, we hope to further control yellow star thistle by integrating cattle grazing with the growth cycles of native and invasive weeds. This will improve habitat and forage.


Medusa-head (Taeniatherum caput-medusae) is a winter annual grass with a two year seed bank. It has invaded many California rangelands, displacing more palatable forage and native plants. The high silica content of the plant makes it unpalatable to cattle after seed heads have developed. Established populations then create dense mats of this silica rich material, which acts as a mulch preventing other plants from emerging. Where these mats have not taken over, this weed also has an allelopathic effect on other seedlings, delaying their development, and allowing the fast growing roots of medusa-head to monopolize water resources.

Background: Grazing has been an important management tool throughout the history of the Mount Burdell Preserve. However, previous grazing patterns likely encouraged the establishment of medusa-head, and the cattle were likely the means by which the weed was introduced in the first place, as medusa-head is a common rangeland weed in California. In the past, grazing was focused on one of three pastures at a time, which left two of the three pastures un-grazed during the period when medusa-head was going to seed. The main area of medusa-head is found in these two pastures.

Beginning in 2010, the grazing practices on Mount Burdell were adjusted to apply grazing in a more even fashion across the landscape. We also began to consider soil moisture and growing season. Research has shown that the optimum grazing time for control is a very narrow window. We monitor plots within areas of varying grazing intensity to compare changes in medusa-head density.

Old St. Hilary's Preserve

French Broom and Jubata Grass

Since 1997, we have been working to remove the highly invasive French broom (Genista monspessulana), while also restoring native habitats and plants such as purple needle grass (Nasella pulchra), on the Old St. Hilary's Preserve. In fact, many of the same volunteers who are active in this work today were pivotal many years ago in assuring the protection of the lands that now make up the preserve. In conjunction with our Environmental Stewardship Program, a local community group known as The Broom Busters turns out monthly to hand pull French broom. The group clocks hundreds of volunteer hours each year. In addition, we have worked with adjacent landowners to address and limit new infestations. For example, in 2009 we established a partnership with the Marinero Owner's Association to tackle the manual removal of French broom and invasive Jubata grass (Cortaderia jubata) along the northern boundary of the preserve. Plans for follow up treatments are in place for future years. This is an important example of how collaborative partnerships with neighbors can help us achieve the goal of controlling invasive plant species on our preserves.

Background: The history of the Old St. Hilary's Preserve is tied to an active local group called the Last Chance Committee, which was involved in the acquisition of the land years ago when it was at risk of development. In late 1996, shortly after the acquisition and dedication of the preserve, the Last Chance Committee worked with us to form a volunteer stewardship group called The Broom Busters. This group focused its efforts on protecting native habitat from the invasion of non-native plants. Though Old St. Hilary’s is one of our smallest preserves, it has one of the highest concentrations of rare plants, which are found primarily on its serpentine soils. Its small size and the fact that it is surrounded by development, make the preserve very susceptible to invasion by non-native weeds that can threaten sensitive habitat. For example, French broom (Marin County's most pervasive and threatening invasive plant), was introduced to Old St. Hilary's decades ago and spread rapidly. The first step in addressing the spread of French broom was to eliminate the large seed source by removing all of the mature plants. Subsequent steps included the treatment of stumps to prohibit new growth and help the establishment of native plants in these areas. The Broom Busters have organized volunteer habitat protection and restoration workdays on the first Saturday of every month since 1996.

Ring Mountain Open Space Preserve

Ring Mountain Invasive Weed Removal Project May 28, 2015

The Ring Mountain Open Space Preserve is home to some of the most intact native serpentine grassland habitat in California. This habitat includes a number of state and federally-protected rare and endangered plants, one of which – the Tiburon mariposa lily – is found nowhere else on earth. Because of its ecological significance, Ring Mountain must be preserved and restored.

Nonnative, invasive weeds, especially aggressive perennial grasses, are the biggest threat to Ring Mountain’s habitats. The more these invaders colonize Ring Mountain, the less room there is for Ring Mountain’s native plants to thrive and multiply. The acreage of nonnative, invasive weeds would be increasing were it not for the Marin County Parks’ efforts to limit and reduce their spread.

As the primary steward of Ring Mountain, Marin County Parks manages vegetation and combats these destructive nonnative plants pursuant to a multi-year plan approved by The Nature Conservancy, the previous owner of the preserve. As part of this program, Marin County Parks has mapped Ring Mountain’s plant communities, both native and nonnative.

At Ring Mountain, Marin County Parks uses a combination of methods to control invasive weeds including mechanical (hand pulling and removal by machinery), seed head removal, steaming, tarping, and herbicides. Because of the extent of the invasive plant population at Ring Mountain, the effectiveness of one single method is extremely limited. Relying on any one of these methods alone would not result in effective control of the nonnative, invasive weeds, and the long-fought battle to save Ring Mountain’s rare and endangered plants would be lost.

Based on our long experience at Ring Mountain, the limited use of herbicides may be the only effective method of controlling these tenacious invasive weeds. The herbicide treatments used at Ring Mountain do not involve indiscriminate, broadcast spraying over large areas. When herbicides such as glyphosate are used by Marin County Parks, they are used in very small quantities (measured in ounces) and diluted to the maximum amount (the concentration used is 98% water or other inactive ingredients) while still remaining effective. The herbicides are applied to individual plants, one by one and with a wand, by trained and state-certified pest control applicators. Herbicides are applied only at a specific time of year when they are most effective. Treatments outside of this “window of opportunity” are either not effective or less effective, giving the Ring Mountain’s nonnative, invasive weeds the opportunity to increase their acreage at the expense of native plant populations. Because of Marin County Parks’ intimate knowledge of Ring Mountain’s unique plant communities, our actions are targeted at protecting them.

Concerned Marin citizens have questioned the potential risks posed by the use of herbicides, particularly glyphosate. We want you to know that we hear you and we share your concerns.

The World Health Organization recently determined that glyphosate should be classified as a “probable carcinogen to humans.” The full paper for the aforementioned determination has not yet been released. Marin County has been a leader in limiting its herbicide use, and we continue to closely track the scientific literature on this issue. The preponderance of scientific study to date as well as the Environmental Protection Agency and many other foreign regulatory agencies, including the European Union, continue to conclude that glyphosate does not pose any unacceptable risk to human health. However, we continue to evaluate new research in our decision-making process as it becomes available.

The threat that invasive species pose to Marin County’s biodiversity is significant. We use the least toxic, most effective method possible on a case-by-case basis. In the case of Ring Mountain, after other efforts to control invasive weeds have had limited success, we propose to use very limited (measured in ounces) amounts of herbicide in low concentrations (98% water and other inactive ingredients) applied by hand to protect the rare and unique species found there. We believe that this type of treatment will make it easier to hand-pull the invasive weeds found there in the future. After 20 years of stewarding Ring Mountain, we truly believe that an integrated approach, including the limited use of herbicide, is the best way to protect Ring Mountain’s unique biodiversity.

Terra Linda/Sleepy Hollow Preserve

Barbed Goatgrass

For more ten years, we have focused on the removal of barbed goatgrass (Aegilops triuncialis) at the Terra Linda/Sleepy Hollow Preserve. Barbed goatgrass is recognized by the California Department of Food and Agriculture (CDFA) as a high priority weed to be targeted for control and eradication. We have made this effort a top priority due to the fact that barbed goatgrass has been reported to exist in only three areas in Marin County, and one of those is on this preserve. As a result, we have made this weed a top priority, and over time have employed a variety of methods to treat it depending on the location of the population, soil type, slope, and season. To maximize our effectiveness in controlling its spread, we have implemented a variety of methods throughout the early spring and early summer months. Those include treatment with weed whipping, hand pulling, controlled burning, and grass-specific and general purpose herbicides. As with all of our projects, we will continue to use an adaptive management approach in addressing the control of this highly invasive weed.

Background: According to the CDFA, barbed goatgrass is a B-rated noxious weed in California. As such, CDFA mandates its control primarily because the seed heads contain sharp barbs that pose a threat to cattle. Two of the three known populations of barbed goatgrass in Marin County occur on our lands, thus compounding the urgency to control this invasive weed.

We were made aware of infestations on the Terra Linda/Sleepy Hollow Preserve in 2003. After a couple years of investigation, it became clear that there were many sub-populations spread throughout this preserve, covering over 60 acres. The infestations initially identified were on serpentine soil, which supports a limited number of native species specifically adapted to its unique chemical composition. The ability of barbed goatgrass to colonize serpentine soil, and thus threaten many rare and endemic plants that specialize to this difficult environment, is one reason why barbed goatgrass is distinguished from other invasive weeds.