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

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

Pampas Grass and Jubata Grass

The Ring Mountain Preserve is home to the greatest concentration of rare plants throughout all of our preserves, including the endemic Tiburon Mariposa Lily (Calochortus tiburonensis) which occurs no place else on earth. Yet many invasive weeds threaten the serpentine outcrops and grasslands that host these native plants. Some of the biggest concerns on the preserve are pampas grass (Cortaderia spp.), Jubata grass (Cortaderia jubata), mayten (Maytenus boaria), tower of jewels (Echium spp.), French broom (Genista monspessulana), and fennel (Foeniculum vulgare).

Background: Pampas grass (Cortaderia) is a genus of large, dense grass with species native to regions of the Andes Mountains in South America. Their size makes them both conspicuous, and able to crowd out much other native vegetation. Many populations in the Bay Area have formed dense monocultures of this large grass where little else can grow. The Ring Mountain Preserve is home to many rare, native plants, including the endemic Tiburon Mariposa Lily, and hosts one of the only intact native grasslands in the county. Years ago, towering Jubata grass could be found all over the preserve, particularly filling up the wet drainages.

The Jubata grass removal project represents one of our most dramatic successes with invasive weed control. We have removed thousands of weeds that once covered more than 20 acres of the preserve, and we have virtually eliminated mature plants that were once common on the preserve. In 2005, we began treatment by mechanically digging out the large weeds on the preserve. The largest area proved very challenging to treat due to the size and density of the weeds. At the same time, we began using seasonal staff to locate and remove isolated patches of Jubata grass. The following year, we manually removed some of the dense populations along the drainages. By 2007, many of the large root balls left behind were resprouting, so brush cutters and herbicide was used to treat those weeds. Since then, we have manually removed the remaining weeds before they mature to reproductive age.

Every year, we must work to address the occurrence of new sprouts that continue to appear. This includes many isolated patches, dense populations in drainages, and one monoculture of almost two acres on the south face of the preserve. In addition, more than 150 flowering plants remain in the areas directly adjacent to the preserve. As one might expect from the fluffy plumes, the seed of Jubata grass is designed to disperse by wind, with each plume producing up to 100,000 seeds in one season.

In support of this effort, we have initiated monitoring protocols for a number of rare plants, including detailed surveys of the Tiburon paintbrush (Castilleja affinis ssp. neglecta) and the Tiburon mariposa lily (Calochortus tiburonensis). These surveys are being conducted at least once every three years to monitor long term trends in plant populations. And lastly, we have been working to educate the many private landowners that abut the preserve about this invasive plant, that may occur on their land, and thus pose a threat to the preserve.

Mayten Tree

Mayten (Maytenus boaria) is a small tree that spreads by shoots and forms small colonies. Though not commonly found as a wildland weed, this growth pattern has earned it a reputation for being difficult to control in certain landscapes. The population of mayten on the Ring Mountain Preserve is the only known population in Marin County. It stems from a larger population that occurs down slope and outside of the preserve boundary. We are working to control the spread of this invasive tree onto Ring Mountain Preserve, where many rare native plants are found.

Background: Most of our large invasive plant management projects are focused on well known problem species that are common in Marin County. In this case, we are dealing with the first population of Mayten identified in Marin. We have used small scale, mechanical control methods over the past few years, while at the same time attempting to find out more about the characteristics of this invasive plant. Though the trees are small with narrow trunks, pulling them up by the root was ineffective, because the individual trees are connected below the surface, and the brittle wood simply snapped under the pressure of the tool. We also attempted to cut all of the stems in one area, but in the following year we found that they had resprouted. Following these efforts, we used herbicide to treat the stumps of the cut trees to kill the underground growth.


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.