• Watch an episode of "Behind the Headlines," for a discussion of stinkwort with ecologist Claire Elliott here.
All along Interstate 280 — from Daly City to San Jose — a 2 1/2-foot-tall Christmas tree-shaped plant is buffeted by the wake of speeding cars. From July to September, it's just about the only plant of that size and shape along the roadside that is still green during the parched summer months.
The seemingly innocuous plants are less conspicuous but also present along U.S. Highway 101 from South San Francisco to south of San Jose and along Highway 92 between 101 and 280. It's as though Johnny Appleseed had strewn the seeds from a sack along the way.
But these plants are not friendly, nor do they produce benefits as delicious as apples. The only benefit of stinkwort is that it was traditionally used to treat lice in chickens in Crete, according to research by Andrea Pieroni and others in the 2006 Journal of Ethnobiology and Ethnomedicine. Commonly called stinkwort (botanical name Dittrichia graveolens) for its resinous, camphor-like smell, it's an alien poised to dominate grasslands and open spaces from the baylands to the mountains.
Stinkwort can kill grazing animals. Barbs on the fluffy-tipped seeds, which help it spread, damage the animals' digestive systems. Oils in the plant also taint the flavor of meat and milk of animals that have consumed the plants, according to various studies cited in a 2013 U.S. Department of Agriculture (USDA) "Weed Risk Assessment" report.
The plants also are known to cause allergic reactions and severe dermatitis in people who come in contact with the sticky resin, and dogs who have walked amid dense patches of stinkwort have been known to vomit, probably from ingesting or inhaling the bristles, according to some studies.
The plant's rapid spread is concerning to land managers. The USDA report states stinkwort could potentially infest 63 percent of the United States.
Santa Clara County is ground zero. The native Mediterranean plants were initially identified near Milpitas in 1984, the first known location in the country, according to multiple sources. By 2012, stinkwort had spread to 36 of California's 58 counties, researcher Rachel Brownsey reported in 2013 in the periodical California Agriculture.
The stinkwort, a member of the sunflower family, is an annual plant that grows and flowers during the hotter summer months when other plants are already dormant or have gone to seed.
It produces an estimated 71,000 seeds per plant, readily disbursed over the land and carried by the wind, water, animals, vehicles or clothing, according to the USDA report.
Its native habitats are riparian woodlands, margins of tidal marshes, vernal pools and alluvial floodplains, according to the California Agriculture report.
But stinkwort is also able to grow in many soil types and habitats, meaning that it is able to make a home in locations that are forbidding to other invasive plants, land managers said. In California, it has been found to tolerate rare serpentine and saline soils, where rare, endangered and specialized plant species reside. Locally, it has been found in or near the serpentine habitat at Edgewood Park in Redwood City and at the edge of marshlands at Palo Alto's Byxbee Park.
Stinkwort plants love disturbed soils and therefore also easily colonize construction sites, overgrazed rangelands and levees, among other areas.
At this time, stinkwort is considered a "moderate" weed by the Berkeley-based nonprofit California Invasive Plant Council, meaning it has a moderate-high impact on the environment with a high potential to spread. But stinkwort's rating could rise if left unchecked, land managers said.
• Two local plants could help keep the invasive stinkwort at bay. Find out which ones here.
A spreading weed
Stinkwort began invading Byxbee Park in 2005, said Daren Anderson, city of Palo Alto's division manager of Open Space, Parks and Golf.
"We believe it first arrived in the preserve on the tires of vehicles entering the former landfill. Staff and volunteers dedicated significant amounts of time trying to eradicate it before it spread beyond control," he wrote in an email. "We have had some success in limiting its spread, but it is challenging."
Walk up a hill just behind the Byxbee parking lot and one will encounter an entire hillside of stinkwort plants. It is less common along the marsh edges, where a few plants are scattered here and there. It also has been found at the nearby Palo Alto Airport and in other parts of the Palo Alto Baylands Preserve, according to maps by the nonprofit Calflora, which lists all plants in California and their locations.
Volunteers with the Palo Alto nonprofit Grassroots Ecology have removed stinkwort at Cooley Landing in East Palo Alto, Byrne Preserve in Los Altos Hills (where the city used an "it takes a village" concept to remove the weed) and Palo Alto's Foothills Park and Pearson-Arastradero Open Space Preserve, according to Claire Elliott, senior ecologist.
She has found stinkwort growing out of walls and between rocks off Bol Park trail in Palo Alto and in some surprising locations.
"One of the first places where it caught my eye was the roof on the green buildings at Foothill College. It was in a rain garden and had been introduced into the soil on the roof," she said.
Bicycle and vehicle tires are probably the most common vectors. Some land managers said that could explain why they have found more isolated stands deep in open-space areas, where bicyclists might ride on trails.
"Several years ago, I worked with a group of neighbors to control the plant at Strawberry Hill (by Gunn High School), where we got it mostly under control. I fear it will be back at that location since the new VA hospital construction trucks have left it all along the new retaining wall over the hill," she said.
Although the research isn't in yet on what effects stinkwort might have on the ecosystem, Anderson and Elliott both think it will have an impact.
"I believe stinkwort, like most invasive weeds, has an impact on flora and fauna. Invasive plants can degrade and reduce wildlife habitat. The invasive weeds create monocultures and crowd out native plant species. Many wildlife species are dependent upon the native plant species," Anderson said in an email.
Midpeninsula Regional Open Space District has been working to eradicate stinkwort since 2012, said Jonathan "Coty" Sifuentes-Winter, acting senior resource specialist. The weed has been found at 15 district preserves: Bear Creek Redwoods, El Sereno, Fremont Older, La Honda Creek, Long Ridge, Los Trancos, Monte Bello, Picchetti Ranch, Pulgas Ridge, Rancho San Antonio, Ravenswood, Sierra Azul, Skyline Ridge, St. Joseph's Hill and Windy Hill.
"Some sites have extremely small infestations that were just found this year, such as Long Ridge, which has one known location on the Chestnut Trail with 11 to 50 plants," he said. Larger stands can contain hundreds of plants.
As the weed slowly makes its way into the preserves, land managers are concerned for the grasslands. Some grassland areas in the preserves have native grasses, forbs and wildflowers along with protected red-legged frogs and San Francisco garter snakes. If it gets into these areas, land managers could have difficulty figuring out how to treat it while not doing damage to the species, he said.
Driving north on Interstate 280, the rolling grassland hills below Stanford University's Dish are tawny in the morning light. But in the draws between the hills, stinkwort, now dried and brownish and in full seed, is growing up the hillsides.
At the university's Jasper Ridge Biological Preserve, sensitive habitats are also under threat by stinkwort. The plants first appeared in 2005 in an area that was mowed to create a fire break and in a fire-staging area near the boundary of Portola Valley's Westridge neighborhood. Docents and researchers recognized the plants and pulled all of them out by hand, said Nona Chiariello, a Jasper Ridge staff scientist.
But every year since, stinkwort has re-invaded. In 2007, staff found a private property near the preserve had an extensive stinkwort stand. With the landowner's cooperation, the preserve conducted its first-ever effort to manage a neighboring property, according to a 2006-07 annual report.
But since that time, Jasper Ridge staff members have seen stinkwort crop up on lands in all directions.
"Stinkwort is present along Sand Hill Road, along highway 280 and in Westridge, all of which could represent source populations for invasion into Jasper Ridge, directly or indirectly," Chiariello said in an email.
Taming a monster
Some land managers point to the highways as the most threatening source of stinkwort. At Interstate 280's interchanges, notably at Sand Hill and Page Mill roads, the plants are still lush and green from the moist conditions, even as the weeds in other locations have dropped their seeds.
The California Department of Transportation (Caltrans), which owns and maintains these highways, is responsible for controlling the weed. But Caltrans District 4 spokesman Jeff Weiss said that the problem is vexing. The department began a spraying program that was met with opposition from some environmentalists, particularly in San Mateo County, where there are many watersheds.
On the other hand, some land managers are pushing for Caltrans to do more spraying, he said. Spot spraying each plant would be a Herculean task, Weiss said.
"It's not possible to eradicate by hand (because of numbers of plants), and mowing is an ineffective way of controlling it," he said.
Weiss called stinkwort "highly virulent."
"It's so hard to control that if we can just keep it on the side of the roads, it's a victory," he said.
At Jasper Ridge, persistence has kept the monster at bay. Staff members survey for stinkwort multiple times each year, then they hand-cut and pull the weeds before the plants set flowers or seeds. Preserve staff and docents have successfully maintained near-zero stinkwort plants throughout 99 percent of the preserve.
"Our goal is to not let it get ahead of us," Chiariello said. "For 1 percent of Jasper Ridge Biological Preserve, we have tried to reduce the infestation rather than (aiming for) complete removal."
Hand weeding has also been used on other Stanford University lands, according to the office of Land Use and Environmental Planning.
"The last few years, field crews have been diligently hand pulling it out of the central part of the salamander conservation area — gradually working out from the center," said Alan Launer, associate director of conversation management, in a 2017 statement provided by the university. "It is slow progress. Hand-pulling seems to be the most effective method for controlling Dittrichia graveolens in areas where herbicide use is inappropriate (most wetlands and areas where protected amphibians are present) and mowing is not allowed due to heightened fire risk."
At Midpeninsula Regional Open Space District, staff members have found stinkwort on 40 acres and removed about 3.4 acres worth of stinkwort from those sites in 2016. When plants are young in early summer they don't produce much oily resin. The district can use a low dose of herbicide to kill the plants. But later in the season the herbicides can't penetrate past the oils anymore, so staff and volunteers go into hand-weeding mode. Stinkwort plants have a taproot, but if cut an inch or so below the ground they will die. Their seed's longevity is only two to three years. Staff remains diligent by returning to previously treated sites to remove the stinkwort again and again until the seed bank in the soil is depleted.
Midpen also uses a phone app, iNaturalist, so that open-space visitors and volunteers can take a photo of a suspected infestation and add information about its location. The location is uploaded as a GPS point, allowing experts to see the observations and investigate the location.
"It's a huge help. I get alerts any time anyone sees something," Sifuentes-Winter said.
Funding for weed management throughout public lands is limited, many land managers said. Weed Management Area committees made up of county, city and nonprofit agencies were to receive state funding from the Noxious Weed Management account of the California Department of Food and Agriculture, but there hasn't been any money, said Eric Wylde, Santa Clara County deputy agricultural commissioner. The state legislature originally passed a bill creating the account in 1999. Since the program's creation, $16.1 million in private funding and resources have matched the $5.6 million state dollars. But the program was last funded in 2010, but not since the budget crisis hit, said Doug Johnson, executive director of the California Invasive Plant Council. In 2014, AB 2402 passed, which revived the program, but only after removing the $2 million funding the bill sponsors had requested.
"We've continued to advocate for renewing the program. We're hopeful that there will be a bill or budget request this spring to renew funding. It might be a standalone effort, or it might be bundled with other invasive species efforts. Addressing invasive shothole borers (an insect) is a mounting issue, for instance, that could use funding. And since that can impact urban street trees, it will have high visibility in areas with more representation," he said.
Weed management groups around the state have mostly disbanded as a result, but Santa Clara and San Mateo counties' groups still meet to share information. Sifuentes-Winter said Midpeninsula Regional Open Space has spent $15,000 per year for the past two years to control stinkwort using staff, contractors and volunteers. Those costs don't include mapping. He said staff plans to ask the district's board for more funding to fight invasive weeds, of which the district has identified 40 different species in its preserves.
Stanford spokeswoman Jean McCown said it is not possible to break out the university's cost of fighting stinkwort, as the work is just one part of open-space management.
Anderson said city of Palo Alto staff is currently working with a consultant to create a Comprehensive Conservation Plan for the Baylands Nature Preserve, which will include guidance on how to best direct the city's limited resources toward controlling invasive plant species and protecting habitat.
"Stinkwort will certainly be one of the species of concern analyzed in the plan. A similar plan will be done for Foothills Park, Pearson Arastradero Preserve and Esther Clark Park," he said.
Elliott said her group spends about $1,000 a year on hand weeding, cutting plants and mowing to control stinkwort. Most of the work is done by volunteers.
"The good news is it controls quickly. When it was on Strawberry Hill we pulled over 100 plants. In the next year, there was only a 10 percent regrowth," she said.
But even in death, the stinkwort has a few tricks up its proverbial sleeve. If the plants are flowering, even when cut and piled up, they can still produce mature seeds, she said. Elliott and the Grassroots crews must take care to cut the plants before they flower or to bag and remove flowering plants from the site. Mowing times are also critical. If mowed too early, the plants will just sprout again and make tiny flowers near to the ground that can also produce seeds. Those plants are impossible to then get out without herbicide.
Sifuentes-Winter said he has used mowing to his advantage, allowing the plants to re-sprout and then applying herbicide to the tender new shoots.
While man is battling the invasive stinkwort, Mother Nature does not appear to be sitting idly by.
Along Alpine Road in a median under Interstate 280, the advance of the stinkwort seems to stop just where a large area of the native aromatic golden aster, or telegraph weed, begins. And in a median at Sandhill Road over Interstate 280, another aromatic herb, the native hayfield tarweed, appears to be maintaining a sizable area on the edge of a stinkwort stand.
Time, and perhaps research, will tell which gets the upper hand.
"I have seen a tarweed plant and a stinkwort plant growing side by side," Elliott said. "Only one of them is going to win."