Stinknet (Oncosiphon pilulifer) is a relatively new weed to North America and has been moving quickly and spreading in San Bernardino County. Gardeners may have noticed this unusual yellow-flowered plant growing in their yard this year, even though we are in a significant drought. This is a sign of how invasive this weed can be. It would be very wise to remove this plant from your yard and ensure it does not grow in your community.
Stinknet was first discovered in North America in Riverside County in the early 1980's and began to spread shortly thereafter. In the 1990's, it began to spread much more quickly and within 20 years of its discovery, it was found in multiple locations in Riverside and San Diego Counties as well as in central Arizona. Stinknet is a winter annual germinating with the fall and winter rains, blooming in the spring, and usually dying in the summer. However, stinknet seeds in the soil can germinate multiple times a year creating multiple cohorts and making it that much more difficult to remove.
Identification of stinknet:
There are four characteristics you can use to help identify stinknet. Each one by itself may or may not be helpful, and when combined they will help you correctly identify stinknet.
First, and easiest to identify, is the inflorescence is bright yellow and round, almost globe shaped. The bright flowers dry to a dark brown color and tend to hold on to their seeds for many months after the plants die into the summer.
Second stinknet, like the common name suggests, stinks. It has a strong odor which many people find unpleasant, a strong resin, turpentine, pungent pine-like odor. Similar weeds which have a round inflorescence do not have an unpleasant odor (pineapple weed has a pleasant pineapple-like odor).
Third, the leaves of stinknet are finely divided. They are doubly pinnate (bipinnate), meaning the leaves are divided at least two times into smaller divisions. While many plants have this characteristic, this can be useful if your unknown plant is not flowering, has doubly divided leaves and you can smell it.
Last, stinknet germinates in the fall and winter rains, grows as a small rosette through the winter and then starts to bolt and flower in the late winter through spring. In places that receive irrigation or in moist soils, stinknet can flower even longer from the early spring through the summer.
While size can be helpful for some plants, the size of stinknet can be highly variable. Stinknet growing in poor conditions can grow to be only 6 inches tall with a few flowers. In good condition it grows to be almost 3 feet tall with hundreds of flowers and appears almost shrub like.
Local Infestations:
San Bernardino County so far has very few large stinknet infestations, but many scattered infestations. However, this year I've noticed more and more individuals and patches of stinknet in San Bernardino County, many of which will eventually become large infestations, if left to spread.
There are very large infestations in parts of Riverside (see below) and San Diego Counties and Phoenix where stinknet has been invading the longest. In these areas, stinknet covers entire fields and roadsides, covering dozens to hundreds of acres in large patches. There have been very few places in Southern California where stinknet has not continued to spread when left unchecked.
Stinknet is a generalist and can grow in many different types of habitats including wildlands, gardens, suburban landscapes, disturbed areas such as adjacent to roadsides and parking lots, and on hiking trails. It is spreading in San Bernardino County and should thrive in the desert, inland, and at least to 4,000 ft. in elevation in the mountains, and possibly higher. If you do find this plant, report it in the iNaturalist or CalFlora apps.
Stinknet grows very well in Phoenix where winter rainfall is less than what we receive in the San Bernardino County deserts. Unfortunately, stinknet flowers every year in Southern California, even when we are in serious drought conditions, allowing it to spread farther each year.
Stinknet is one good example of why gardeners should be wary of new plants showing up in their garden. If you didn't plant it, it is likely a weed. Unfortunately, it will take at least 3 years of weeding to remove stinknet from an established site, so keep up the work and you can rid this plant from your garden and neighborhood.