Invasive Plant Management

Oncosiphon pilulifer, commonly known as “stinknet,” has been identified as an emerging invasive weed in Western Riverside County and throughout the Southwestern United States. Deceivingly mistaken as a wildflower by many, stinknet has globe-shaped, bright yellow flowers when in bloom, between March through July, and has a strong, repugnant odor reminiscent of turpentine. When not in bloom, it takes on the appearance of low-growing leaf clumps that are “carrot like” dark green and twice dissected. In hotter months, the plants dry to a dark, almost rust brown color.

Stinknet can grow up to 4 feet tall and spreads into a dense carpet that dominates a landscape and excludes native plants. In open areas, it can cover several to hundreds of acres. Its seeds are smaller than the ball in a ball-point pen (1mm), making them difficult to see and remove from equipment, boots and clothing. The seeds disperse through a variety of mechanisms, traveling on people, equipment, vehicles, or are carried by water, wind and wildlife.

History

Stinknet is native to South Africa and was first found in California in the early 1980s. Today, it has spread to Western Australia and the southwestern United States, from Las Vegas, Nevada down to Arizona, southern California and northern Mexico.

Why is it a problem?

Stinknet has the ability to completely eliminate other native plants once established due to its natural history characteristics, including prolific seed production, multiple germination within a single season, and resistance to common treatments. It is not palatable to most native herbivores and granivores and greatly reduces the abundance of quality forage that these animals depend upon. It can increase the amount of fine fuels in invaded areas, thus increasing wildfire risk, particularly in summer months with the weed is tall and dry.

Millions of dollars are spent annually in combating non-native weeds such as stinknet, which increase costs associated with infrastructure operation, labor and maintenance costs as well as regulatory burden on development projects.

What can you do?

The monitoring and reporting of stinknet is the first step in combating this invasive weed. If you see a stinknet plant, take a photo, note the GPS coordinates and upload the information to iNaturalist,  an online database that both community and scientists can report biological data to.

Effective management of the weed involves timing treatment methods appropriately (see below for current status in Western Riverside County and recommended management actions below) and using the right herbicides that takes into account current environmental conditions and the variable blooming periods of this plant.

Avoid spreading seeds. Since the seeds can disperse through equipment, it’s important to clean vehicles, graders, mowers and tractors very thoroughly to remove the miniscule seeds. If you visit an area infested with stinknet, change your clothing and brush your shoes before visiting any other natural areas. Don’t move green waste or dirt around that has stinknet in it. When dry flowers are disturbed, they easily fall apart and its seeds are then transferred to the soil, where they can remain viable for several years, or will disperse into the wind.

Resources

For a map of stinknet reported locations on Calflora, please use this link. Create an account to report new sightings

For more information about stinknet, visit the California Invasive Plant Council: https://www.cal-ipc.org/plants/profile/oncosiphon-piluliferum-profile/

USFWS Cost of Invasives Fact Sheet

For a printable brochure on stinket, please use this link.

University of Riverside report of herbicide treatments on stinknet