- Author: Chris McDonald
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.
- Author: Chris McDonald
Stinknet (Oncosiphon piluliferum, aka globe chamomile) is a winter annual that is spreading across Southern California and poses threats to wildlands, rangelands and agricultural areas. Stinknet was first found in western Riverside County in the early 1980's. It slowly spread to surrounding areas and by the late 1990's it was found in over a half dozen locations in Riverside and San Diego Counties. By this time, it had also spread to Phoenix. While stinknet has not been one of the fastest weeds to spread across the state (stinkwort, Dittrichia graveolens, is definitely a top contender for that spot, see here) it is now currently found in every county in Southern California (except Imperial, yet) with the largest infestations in Riverside and San Diego Counties. Stinknet continues to spread north across Los Angeles, the central valley and coast, and east across Arizona, it was also recently found in Las Vegas.
Identification
Stinknet is easiest to identify when in flower. At flowering, it grows from several inches to 3 feet tall.
Stinknet in full flower (Image credit: Chris McDonald)
A closeup of stinknet flowers (Image Credit: Chris McDonald)
Stinknet is closely related to the pineapple weeds (Matricaria spp., see here) and brass buttons (Cotula spp. see here and here), and resembles those more common weeds. The main difference is the flower of stinknet is very round, like a globe, (see above) while the flowers of pineapple weed and brass buttons tend to be slightly conical to half a sphere.
Brass buttons (Cotula coronopifilia Image credit: Carol Witham)
Pineapple weed and brass buttons also tend to grow more along the ground, and stinknet grows upright. Another diagnostic feature is that stinknet stinks. All plant parts have an unpleasant turpentine, pine or tar-like smell, and even very small plants have this smell. You can even smell the odor of a large field of stinknet when you are near a very large infestation.
Successful Control Options
Stinknet can be controlled with several herbicides that can be used in wildlands. Milestone (aminopyralid), Capstone (aminopyralid and triclopyr) and glyphosate are all highly effective at controlling stinknet, but only before the plants have flowered. Often if herbicides are applied after flowering, stinknet can finish flowering before the herbicides have killed the plant. Milestone and Capstone also provide season-long control of stinknet with suppression lasting up to and in some cases beyond 12 months. Several researchers, including myself, are working on other control measures and understanding its biology to better help managers control stinknet. Those projects should wrap up in the next year or two.
What doesn't work well at controlling stinknet?
Telar XP (chlorsulfuron) was not effective at controlling stinknet in Southern California. Transline (clopyralid) was inconsistent at controlling stinknet. Transline had shown promise of effective control in relatively dry years, but in other more wet years the treated plants were able to grow out of the treatment. Mechanical removal (mowing or string trimmers) has shown to be of limited effectiveness, mostly because the cut plants resprout and flower closer to the ground. Multiple cuttings close to the ground alleviate this problem and can provide good control, but a simple single cutting is not very effective. Stinkent can grow in dense patches so hand pulling will only work on a very small scale, and multiple sessions are needed. Stinknet is not palatable to livestock, so grazing will not be an effective management strategy and can make the problem worse.
Threats to California Agriculture
In Western Australia, stinknet is a problem weed in small grain crops. In both South Africa (the home range of stinknet) and Austrailia stinknet is a problem weed in rangelands too. This is because stinknet is unpalatable to many livestock. While stinknet is not currently known to be toxic, there are reports it can taint the meat and milk from those animals. If the patterns in Australia and South Africa hold in the US (and so far in a few observed cases it appears to be similar) and if stinknet continues to spread into rangelands and agricultural areas in California, then it will cause problems.
EDRR
Right now, the best way to keep stinknet from spreading locally in California is to identify early infestations and rapidly respond to those initial infestations. This strategy is called early detection and rapid response (EDRR). Fortunately, the largest stinknet populations are found in only a few locations in Riverside, San Bernardino and San Diego Counties. However, numerous small populations occur across Southern California. I have also noticed that very few stinknet patches decline in size, once stinknet colonizes a site it tends to expand, or even hold its ground during a drought. The areas with the earliest known stinknet populations, in both California and Arizona, now have large, very high-density patches. Those patches can be over a dozen and up to hundreds of acres in extent. Stinknet appears to be a strong competitor especially in disturbed areas, but also in undisturbed wildlands too, and can quickly become the dominant plant. It does this by creating numerous small patches and as those patches grow, they form large blankets carpeting acres or long strips along roads and trails. Stopping those small stinknet patches can prevent them from turning into large infestations.
More information on stinknet will be available as our research progresses and we learn more about its biology, spread and controlling it.
- Author: Cheryl A. Wilen
I recently attended a Santa Ana River Orange County Weed Management Area (SAROCWMA) meeting and there was an opportunity for participants to update the group about new invasive plants as well as give an update on management of these and others. During the discussion, Ron Vanderhoff from the Orange County Native Plant Society (OC-CNPS), reported new findings of a plant I'd never heard of. In fact, when the group was talking about it, I wasn't sure if I heard the name right.
The plant is called stinknet (Oncosiphon piluliferum), which to me sounds like a game played by 10 year olds. However, the California Invasive Plant Council considers it an emerging invasive weed (http://www.occnps.org/PDF/HYS-Oncosiphon-piluliferum.pdf). On the OC-CNPS weedmapper, it was first reported in Orange County in 2003, but was in San Diego County as early as 1998 and Riverside County in 1981.
According to the USDA-NRCS plants database, stinknet has a limited distribution in the U.S., being found only in Orange, Riverside, and San Diego Counties in California and Maricopa, Pinal, and Yavapai Counties in Arizona. However, Cal-Flora has unconfirmed sightings of it in San Bernardino, Imperial, and Kern Counties.
Stinknet or globe chamomile is a relatively small annual plant that could easily be confused with the turf and landscape weed pineappleweed until you smell it. Pineappleweed flowers have a pleasant sweet smell while, as you may guess from the name, stinknet has the opposite odor. It is most noticeable when flowering (March to July in Southern California) so now is a good time to spot it.
Although it is not listed as a noxious weed, land managers should still be on the lookout for it especially along the coast and inland. It can be particularly damaging in coastal sage scrub because of its tendency to fill in open spaces, and it can reduce growth of other native annuals and impact animals that depend on the openings in these areas.