By Michael Viramontes | Contributing Columnist
In the wild open spaces of Southern California, one small plant is creating a big stink for land managers. Stinknet, or Oncosiphon piluliferum, is an invasive plant posing a serious threat to our region’s native habitats. Also known as globe chamomile, most would consider it too sweet a name for this highly invasive plant native to South Africa.
It was first noted in Riverside County in 1981 and has since spread throughout the whole Southern California region. However, we’re not the only state dealing with it. Arizona first noted stinknet in the early 1990s and southern Nevada in the late 2010s.
The small herbaceous annual plant ranges in height, from just a few inches to three feet, depending on conditions like water availability, soil type and competition. First popping up in early winter, its frilly leaves and stems are a fluorescent green and produce a slightly sticky resin. When you smell a single plant, it may be reminiscent of a pine-scented cleanser. A whole field of stinknet on a hot summer day can bring an unpleasant odor more deserving of its name.
The weed has spherical bright yellow flower heads typically emerging in March to April. As the plants mature in April to May the leaves and stems shift to a rusty red then brown, while the flowers fade to a dull yellow then brown. Each spherical flowerhead can produce hundreds of seeds, which may remain viable for up to five years. Stinknet’s rate of seed production and length of seed survival, allow it to skip germination in dry years and have a population boom in wet years. The California Invasive Plant Council has assigned it an invasiveness rating of “high,” the top category.
Typically, the plants will colonize bare ground before eventually blanketing an area and smothering any comparably sized plants. In natural areas it can outcompete native species for water, nutrients, and sunlight. For this reason it seems to grow particularly well in more sparsely vegetated habitats of the inland southwest where it has room to grow unopposed.
Examples of how the species can dominate natural areas can be seen around Lake Perris, as well as in Phoenix, where stinknet is creating a fire risk. In summer and fall, the spent plants are dry, dense, and resinous making them highly flammable.
Still, even in the Inland Empire’s wildland-urban interface, stinknet seems to be spreading fast. Populations pop up following any ground disturbances like grading or continued use of dirt roads, road shoulders, or trails. The website iNaturalist shows the first observation along the Santa Ana River in 2018 just upstream of the Market Street bridge in Colton. Now there are over 60 observations of the species along the river between Market Street and the 71 Freeway. Its seeds, the size of dust particles, are dispersed by driving, biking, or walking through patches of the mature plant.
To help control stinknet at home, we should familiarize ourselves with it and pull it out where we can. According to the University of California’s Integrated Pest Management Program the plants seeds mature within a few weeks of flowering. Dr. Chris McDonald, natural resource adviser for UC Cooperative Extension has studied control of the species. “For all invasive plants, the best advice is to bag them and put them in the trash. We don’t want the seeds to spread,” McDonald said, further recommending to double bag highly invasive plants, like stinknet, to avoid seed spreading if a bag tears.
Our stewardship team at Rivers & Lands Conservancy battles a never-ending war against invasive species. You can join us in the fight by volunteering at one of our habitat restoration events or in your own backyards. Together we can give our native plants and the wildlife that depends on them a fighting chance.
Rivers & Lands Conservancy connects our community to natural, wild, and open spaces of Southern California through land conservation, stewardship, and education.
Michael Viramontes is the stewardship manager of Rivers & Lands Conservancy, has a bachelor of science in environmental studies and natural resource management from San Jose State University.
