On This Page
Intro
Stinknet (Oncosiphon pilulifer) is an herbaceous plant native to South Africa that was found in Southern California in the early 1980s. Since its first recorded observation in Perris, CA in 1981, this species has continued to spread across the southwest US and Mexico.
Identification and Biology
When blooming, this species is generally 2 to 3 feet tall. However, individuals can be smaller than 6 inches tall when growing under dry conditions, and larger than 3 feet and bushy when growing in optimal conditions. Seedlings are very small, less than ΒΌ inch, and the plant remains small throughout the beginning of the growing season. The leaves are doubly divided (bipinnate) and are generally 2 inches long and about 1 1/2 inches wide.
Stinknet inflorescences contain hundreds of extremely small seeds per seed head that easily disperse. Most seeds from a stinknet plant fall near the parent plant, forming large dense patches of stinknet that expand in size over time, however some seeds disperse long distances. The seeds can be transported via clothes, boots, animal fur (including wildlife, pets, and livestock), as well as on vehicles and heavy equipment such as road maintenance equipment. Seeds or entire plants may also be blown in strong winds and float downhill when it rains. New infestations have been found from several miles to hundreds of miles from known sources. The inflorescences remain attached after the plants senesce, and when they are disturbed, the seeds easily anchor to clothes, footwear and animal fur or feathers.
The inflorescences grow at the tips of the branches, and are nearly spherical, or form a globe shape (one alternative common name is globe chamomile) and are 1/4 inch in diameter. The plants, especially the leaves and stems, have a strong oily, pine-like or turpentine odor. The smell can be noticed near large masses of the plant in the spring, and many find the smell of the plants and large patches unpleasant. Stinknet seeds mature several weeks after flowering and are very tiny, less than 1/16 inch long (less than 1 mm).
Stinknet is a winter annual, germinating with fall or winter rains and flowering during the late winter throughout the spring. In some wetter locations, this species can flower into the early summer. Germination begins several days after the first large rainfall of the fall or winter. Stinknet can produce several cohorts each year. Flowering lasts several weeks, and cohorts often bloom in synchrony. Senesced plants can persist on the landscape for over a year.
Impact
Stinknet has been spreading rapidly across the southwestern US. Currently it is established across Southern California, Arizona, and northern Mexico. In 2018, stinknet was reported growing in Southern Nevada. In California, stinknet creates dense stands. These dense stands reduce native plant diversity, as stinknet dominates hundreds of acres and alters plant community composition. In Southern California, stinknet invasion is threatening critical habitat for several endangered and sensitive species, including the Stephens kangaroo rat (Dipodomys stephensi).
Stinknet is not palatable to livestock and thus reduces the amount of available forage for livestock when it displaces annual grasses and more palatable forbs. In South Africa, the cover of stinknet was 10 times higher on intensely grazed pastures compared to pastures that were grazed at low intensities. This indicates livestock do not readily graze this species, and this has been anecdotally observed in Southern California. Stinknet is likely not palatable to wildlife as well. Very few stinknet plants in wildlands are found with grazing damage and this contributes to the expansion of large patches where grazing or browsing occurs on more palatable species.
It is unknown how long stinknet seeds can survive in the soil in the US. In Australia, stinknet seeds can live up to 5 years. However, the majority of buried stinknet seeds are likely to naturally die before then. Stinknet can produce several cohorts of plants each growing season. In years with adequate rainfall, there can be at least 4 cohorts of stinknet plants in an infested site, with each new cohort growing after each large storm. This creates problems managing stinknet since once large plants are removed, small plants growing under the taller stinknet plants can survive and thrive. It often takes several treatments to remove all the stinknet plants from an infested area during a growing season.
Management
Monitoring
Stinknet is best monitored when it is flowering from late winter through early summer, although it is easiest to monitor for new stinknet populations during the spring. Spring is when plants are in full flower and can be most easily detected from a distance by their bright yellow inflorescence and few other plants have similar stature and globe-shaped flowers at that time. However, seeds mature within a few weeks of flowering, so plants should be removed as soon as possible and bagged when in flower.
Occasionally, when resources or labor are limited and immediate removal will be impossible, it is also efficient to map an entire population in the spring when it is easy to detect, and then remove that population the following fall or winter when a new crop of stinknet plants germinate.
Monitoring stinknet earlier in the growing season can be difficult since individuals are harder to detect as seedlings. Stinknet seedlings appear about 1 week after the first significant storm of the fall or winter, however it is generally best to wait a few weeks for the plants to grow large enough to easily identify so they will not be confused with a few other similar looking weeds. Other weeds, especially those with yellow and round flower heads (discoid flowers), such as pineapple weed (Matricaria discoidea) could also be confused with stinknet at a distance. Vegetative stinknet plants can be identified by the highly dissected shape of the leaves and the pungent smell of the plant when crushed.
Biological control
There are no biological control agents for stinknet, nor does it appear that herbivores eat stinknet in the US in significant abundance. It appears unpalatable to many insect species and herbivores.
Cultural Control
Livestock grazing is not an acceptable means of reducing stinknet. Livestock do not readily graze stinknet and are very likely to spread the small seeds when livestock move to a new pasture.
Mulching may control stinknet since its seeds are small and unlikely to germinate under or through a thick layer of mulch.
Stinknet appears to grow poorly under the deep shade of larger plants like large shrubs with a closed canopy. Under incomplete shade, stinknet can grow well. Growing larger competitive plants can reduce the abundance of stinknet plants but will not eliminate it. In some areas stinknet can outcompete annual grasses in wildlands after several seasons of growing together. Stinknet appears to reduce the vigor of other annual plants. Large patches of stinknet have been found in areas where invasive winter annual grasses previously dominated.
Stinknet can become abundant on disturbed soils and roadsides. It appears to thrive in recently burned areas, likely due to the long-lived seed bank. Prescribed fires that occur after plants have flowered do not provide good control since many of the tightly packed inflorescences are not burned. Reducing disturbances, soil disturbances, and planting competitive plants in disturbed areas will help to slow the spread of stinknet, but not eliminate it.
New infestations can be prevented by thoroughly cleaning equipment and having workers or visitors clean their equipment, clothes, and shoes after leaving an infested area, as well as before entering a clean, uninfected area. New populations can be controlled using early detection and rapid response techniques, such as monitoring for new populations and removing them once discovered.
Mechanical Control
Stinknet produces many seedlings starting in the fall or winter after significant precipitation or near irrigated areas and continues germinating until the spring. These seedlings can be easily managed with a very shallow cultivation, raking, or harrowing from the soil surface without deeply disturbing the soil. Cultivation can reduce native seedlings and when repeated over multiple years can disturb the soil and encourage more stinknet. When the plants are larger, they are more difficult to control mechanically. Stinknet patches can be dense. It can take multiple treatments to hoe or scuffle hoe all the seedlings in a patch. Later in the season when the plants are taller and begin budding, the plants can be mowed or cut with a string trimmer. Often multiple treatments will be needed when mowing or cutting the plants as they often re-grow after being cut or smaller cohorts will grow through the skeletons of the cut plants. However, mowing or string trimming after the plants have flowered can spread seeds around and is generally not a viable management strategy.
Wear pants, long sleeved shirts, eye protection, and gloves when working with this species since some people develop contact dermatitis after contact. Wear gloves to pull small stinknet infestations, or individual plants.
Chemical Control
Several herbicide options, including both postemergence and preemergence herbicides, provide control of stinknet. Often, chemical control options are needed to treat large stinknet patches or populations when other methods, including mechanical control, are difficult to implement as stand-alone methods. Long-term success when managing stinknet, especially with large patches that often require chemical control, will be more successful if restoration of native plants follows successful management techniques.
Postemergence Control
For postemergence control of stinknet, the timing of application is very important. While several herbicides provide excellent postemergence control, few provide the same level of control when the plants are flowering or in the late bud stage. Applications of herbicides are most effective when applied to plants in the rosette or bolting stages, and before flowering. While some land managers have had success controlling stinknet at the flowering stage, control at this stage can be inconsistent to minimal.
The non-selective herbicide glyphosate can provide excellent in-season control of stinknet. The selective herbicides aminopyralid and aminopyralid/triclopyr also provide excellent in-season control of stinknet, and many grasses will tolerate an application of these herbicides. While clopyralid has been highly effective in some years, in other years, particularly those with above average rainfall for southern California, control has been lacking. Clopyralid is not recommended for postemergence control but does show promise as a preemergence option.
Preemergence Control
Several herbicides provide excellent control of stinknet as a preemergence application. These herbicides can be applied in the fall before the first winter rains. They need rain or irrigation after application to effectively incorporate them into the soil to prevent seed establishment. Preemergence herbicides are often applied as a tank mix partner with postemergence herbicides. A tank mix of postemergence and preemergence herbicides can be applied any time after seedlings have emerged from the soil, and before flowering. This will continue to control stinknet that has already germinated as well as any seedlings that emerge throughout the growing season.
The selective herbicide aminopyralid can provide season-long control of stinknet, as well as excellent postemergence control. Indaziflam as well as isoxaben or isoxaben + dithiopyr provide excellent preemergence control only. Clopyralid, while it provides inconsistent postemergence control, provides great preemergence control of stinknet. These herbicides may provide preemergence control of stinknet the next growing season after application. Preemergence applications of aminopyralid and clopyralid are acceptable if the label does not prohibit that application type.
In areas where protecting native perennials is warranted, the preemergence herbicides indaziflam or isoxaben + dithiopyr can be used to control stinknet. These herbicides will generally not harm, or may produce minimal effects, on established native perennial plants, while control of stinknet populations will be high. However, these herbicides control most annuals, including native annuals, as well as any perennial seedlings that germinate that year.
References
Chamberland M. 2020. Stinknet: A Weed Advancing in Southern Arizona. Extension Publication Az1827
https://extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1827-2020.pdf
(Accessed October 23, 2023)
Douglas A, Nicholson D. 2019. Biology and Management of matricaria (Oncosiphon piluliferum). Grains Research and Development Corporation Australia. https://grdc.com.au/resources-and-publications/grdc-update-papers/tab-content/grdc-update-papers/2019/02/biology-and-management-of-matricaria-oncosiphon-piluliferum
(Accessed October 23, 2023)
Hedrick PW, McDonald CJ. 2020. Stinknet, A New Invasive, Non-native Plant in the Southwestern US. Desert Plants 36(1) 5-16. https://sdcwma.org/docs/stinknet_desert_plants.pdf
(Accessed October 23, 2023)
Rutherford MC, Powrie LW. 2010. Severely degraded rangeland: Implications for plant diversity from a case study in Succulent Karoo, South Africa. Journal of Arid Environments, 74(6): 692-701.
