In this special edition of Doug’s Mountain Journal, Doug is joined by Ariel in an exploration of the saga of the thorniest weed on San Bruno Mountain.

Writing in black is by Ariel Cherbowsky Corkidi. Photos without captions also by Ariel. Other photos attributed respectively.

Writing in green is by Doug Allshouse

We are grateful to Sean Correa (Natural Resource Specialist II), Daniel Krug (County Arborist), Hannah Ormshaw (Natural Resource Manager), and Evan Cole (Natural Resource Specialist) of the San Mateo County Parks Department for providing helpful commentary and materials—as well as the terrific work they are leading to control gorse!

Some plants are appealing and others are appalling—San Bruno Mountain has both.

Appealing plants may lure the eyes—the swaying, mottled bells of checker lilies—satisfy nostrils with hints of mint and sage, or soothe fingers with velvety or smooth sensations—like stroking thimbleberry leaves or buckeye seeds.

Appalling plants foment fear for one’s flesh.

It’s hard to say which inspires more dread—gorse or poison oak.

Both have serious pain-inflicting features with their stabbing thorns or rash-inducing urushiol oil.

The sharp sting of gorse is immediate but short-lived. Meanwhile, brushing against poison oak feels fine in the moment, if noticed at all—but terrible later on once the itching and burning sensations arise and linger.

Due to all the pain such appalling plants may cause us, it feels uniquely satisfying to see the tables turned. That was the case a little more than a year ago, on February 28, 2020, when more than six acres of gorse burned on San Bruno Mountain.

The movements and colors broadcasted from the helicopter cameras were stunning—the tall wall of crimson flames belching greasy smoke into the sky as the fire crackled through dense thickets of gorse, searing their yellow-blossomed branches. The red firetrucks and firefighters in orange and yellow suits moving through the charred landscape echoed the hot palette.

Image from the San Mateo County Parks Department

Gorse is the 800-pound gorilla of the pea family that can reach 5 m in height and 3-4 m wide and grows anywhere it wants except for very wet places. Like all members of the pea family, it fixes nitrogen in the soil which increases fertility, and it produces prodigious quantities of seeds capable of remaining viable for more than 50 years in the soil. It also acidifies the soil which lays the ground bare beneath it.

Young stems are green with shoots having leaves with 3 small leaflets which modify into many branched spines 1-4 cm long. The spines are sharp, painful, and hurt after the thorn is removed. Gorse is native to the United Kingdom and Ireland and Western Europe, and has gained fame as the native substitute in the treeless fairways of Scottish links golf courses along the North Sea. In the Mediterranean climate of the Bay Area, gorse may produce flowers and seeds twice a year. It is an intricate, stiffly-branched evergreen shrub that forms impenetrable thickets. It presents an extreme fire danger, especially when it is green, because of an oily sap that quite literally explodes when ignited producing long flame-lengths that are much hotter than other shrubs, earning it the name, greasewood. Gorse is an aggressive and highly invasive shrub capable of establishing vast monocultures where nothing else exists. It demands respect and punishes those who ignore it.

There are other reasons, beyond our painful associations, to appreciate the destruction of gorse. The dense infestations of this invasive plant have consumed many acres of land on the mountain, particularly in the hills of the Saddle Area—habitat that would otherwise be wildflower meadows, freshwater wetlands, patches of riparian woodlands, or coastal scrub.

Gorse contributes absolutely nothing positive to any California ecosystem and has stolen the once-lovely grasslands of the eastern Saddle that were home to the larval host plants of the Mission Blue and Callippe Silverspot. I still remember seeing these two endangered butterflies flying about in the eastern Saddle; but no more.

A field of mature gorse is daunting, like no other weed on the mountain—impenetrable and impossible to remove at scale without a burn or a bulldozer. To see gorse scorched or scraped away and walk on a patch of earth that was previously utterly impassable is a treat—and we’ve had the opportunity to witness and do both during this past year.

Following the February burn, which moved through a patch of gorse on the west side of the connecting trail between the Saddle Loop and the Day Camp Service Road, the San Mateo County Parks Department removed an additional 7 acres on the east side of the trail in October using a Masticator (Cat 299 Skid-Steer with a mastication head)—one step to restoring habitat as well as reducing the remaining fire danger.

Video from the San Mateo County Parks Department

Image from the San Mateo County Parks Department. After Gorse Removal (Left, November 2020). Before Gorse Removal (Right, July 2020).

Follow-up work—control of re-sprouting gorse and other weeds—is being diligently carried out by the San Mateo County Parks Natural Resource Department and its contractors, as native plants begin to rise above the ashes and crushed gorse mulch.

In early February, patches of bee plant, coast red elderberry, California blackberry, and wild cucumber broke through the mulch and announced their presence. By late March, sky lupine was blooming in the flowering patches of cucumber, confirming the resilience of even our most diminutive native species.

The dichotomy was that in January huge patches of poison hemlock and gorse sprouts were appearing in sections of the masticated areas where the mulch was barely covering the ground. Poison hemlock was especially noticeable along the shoulders of the upper day camp road from the day camp to the Saddle Trail. Himalayan blackberry was also sprouting primal canes. An email with suggestions and some photos was sent to Natural Resource Specialist Sean Correa. A quick response assured that action would be taken in both areas since the burn area had turned into a thick monoculture of non-native nightshade (Solanum furcatum) the likes of which I had never experienced. Two ATVs with small spray tanks appeared at the day camp to douse the nightshade and the hemlock and gorse sprouts were dealt with by personnel with backpack spray applications.

The fire and mechanical removal of gorse in 2020, affecting about 13 acres of gorse, augments a reduction of nearly 290 acres of gorse, in varying densities, on the mountain from 1982 to 2007 (2007 San Bruno Mountain Habitat Management Plan).

Map from the San Mateo County Parks Department

The recent work borders and expands on an area that had undergone gorse-removal during a project from 2004 to 2008 overseen by the San Mateo County Parks and implemented by May & Associates.

This was during a time when the County did not employ a biologist. Sam Herzberg, Senior Parks Planner, asked Jake Sigg and me to read several proposals and recommend a contractor. The May & Associates proposal was recommended because it also dealt with hand removal of outlying populations of gorse on the very eastern edge of the Saddle. The bulk of the removal was done by the caterpillar in the photo using a sadistic rotating drum with large teeth rotating at many RPMs on the front of the cat. In all, slightly over 40 acres were removed.

Jake and I feared that the gorse seed bank would erupt the following winter with the rains. Interestingly that did not happen. Velvet grass moved in and it secretes a chemical that repels sow bugs that would have otherwise devoured the mulch that covered the seeds. Now there are islands of pearly everlasting scattered about over a dozen years later.

As we excitedly applaud the continued reductions of gorse—both intentional, or spontaneous like the wildfire—it’s not a plant we can expect to forget.

After all, gorse will continue to be a thorn in the mountain’s side—it isn’t likely that it will be completely eradicated, especially since it has spread to steep topography, intermixed with other vegetation, where removal would be more challenging, like the slopes above Carter Street.

Furthermore, this plant has become rooted in the stories of the mountain and “piqued” the interest of many.

This specimen from a 1993 collection on San Bruno Mountain provides an example of a botanical record of gorse. Collection data from the participants of the Consortium of California Herbaria (ucjeps.berkeley.edu/consortium/).

The earliest botanical record of this species from San Bruno Mountain was collected for the California Academy of Sciences sixty-five years ago, on March 30th, 1956. The collection was made on the northern foothills of the mountain, south of Geneva Avenue and Brookdale Street, by a nineteen-year-old undergraduate studying at the University of California, Berkeley.

The student was Peter Hamilton Raven, who has come to be one of the most respected plant scientists on the planet—Director of the Missouri Botanical Garden for forty years and well-known author of countless articles, books, and textbooks, and a wider range of environmental topics.

Just like the prominence of Peter H. Raven grew over time, so did the fame of the prickly weed he collected and pressed.

Gorse became one of many polarizing figures in the debates over the Habitat Conservation Plan (HCP) on San Bruno Mountain in the early 1980s, an unprecedented agreement that allowed for limited construction of endangered species habitat in return for the creation of a management plan that would be funded through fees assessed on the residential and commercial developments.

All agreed that gorse was trouble for the grassland habitat of the mountain’s endangered mission blue and callippe silverspot butterflies.

However, the developers and those in favor of the HCP compromise elevated gorse as one of the greatest risks. They argued that the fees collected from development projects would fund control efforts that would limit the spread of gorse and convert existing gorse-infested areas back into suitable grassland habitat.

San Francisco Examiner. January 18, 1987

Those skeptical about the promise of the HCP countered that the ability to convert dense stands of gorse back to acceptable grassland habitat was unproven, arguing that the quality of existing grassland habitat in the proposed development areas was irreplaceable. Furthermore, they believed that gorse would not continue spreading to other parts of the mountain and that the disturbance from construction activities would create fertile ground for the spread of existing weeds or the introduction of new ones. In their view, the acreage to be chewed up by construction, not gorse, was the biggest threat.

Looking back, nearly 40 years later, it seems the initial publicity surrounding gorse may have drawn too much attention—land managers on the mountain and recent studies show that forces other than gorse collectively pose a greater threat to the quality and quantity of grassland habitat on the mountain, and thus the survival of the mission blue and callippe silverspot butterflies.

These include non-native annual grasses, other weeds like oxalis and fennel, and native scrub encroachment following the periodic cessation of indigenous prescribed burning practices and historic grazing. All are symptoms of the mix of disturbances of the nineteenth and twentieth centuries—the dispossession of native people, ranching, farming, mining, road-building, urbanization, exotic horticulture, soil chemistry altered by deposition of local car emissions, and more.

The typical land management solution in the past was to focus on the macro-weeds because, after all, they were large and caused the most problems, or so we thought. It wasn’t until the 21st Century that the focus began to turn to the micro-weeds. There were so many more species of them and their effects took more time to manifest destructions to the ecosystems; often to the point of no return

Still, in terms of plants, none might be as effective a mascot as gorse to evoke the fearful consequences of invasive plant introductions.

The haunting thorns and thickets of gorse tell a more convincing tale of the pain people can let loose on sensitive landscapes and delicate butterflies, than do the bright flowers of oxalis, the fragrant stands of fennel, or the green-to-gold fields of invasive wild oat or quaking grass.

Its removal, too, is more dramatic and raw than most other ongoing weed control efforts on the mountain—and the heavy-duty efforts required to remove gorse communicate the costly and challenging nature of intensive weed management.

The attention-grabbing nature of gorse makes it a fitting ghost story (gorse-story, perhaps) in the mountain’s folklore. It is riveting to get drawn into the drama of gorse—just don’t lose sight of other, seemingly less intimidating, “mountain monsters.”