Stinknet (Oncosiphon pilulifer) is an annual weed spreading aggressively in the southwestern US. This species was first found in North America the early 1980’s in Southern California and it continues to spread aggressively in California and Arizona and a few populations have been found in Nevada. It is expected to spread beyond these areas. With the above average rainfall in Southern California in the winters of 2023 and 2024 numerous new stinknet populations have been discovered recently. Some have been adjacent to areas where stinknet was previously found and it’s also been discovered in other areas that are far from known stinknet populations. We’ve also created a UC IPM pest note for managing stinknet that can be helpful as you build your IPM program for your lands.
https://ipm.ucanr.edu/natural-area-pests/stinknet/#gsc.tab=0
While it is somewhat easy to control individual stinknet plants, it is especially challenging to control stinknet patches or populations and to eradicate new infestations. The biology of stinknet makes it difficult to control populations for many reasons.
First, stinknet has a highly variable size. Plants growing under poor conditions can be less than 6 inches tall, skinny, with just a few flowers at the end of the season. On the other hand, plants growing in optimal conditions can be 3 feet tall and wide with hundreds to even a few thousand flowers. It can be difficult to find all the stinknet plants in a population just due to the large differences in size between plants. It may also be difficult to find a few skinny stinknet plants growing among other vegetation.
Second, stinknet re-sprouts after being cut. When cut or mowed during the growing season it can grow back. Which means, if using mowing as a control technique, it often takes multiple cuttings to control stinknet and in some cases after repeated cuttings the plants will grow low to the ground and still flower. Cutting closer to the surface is more helpful, but plants can still resprout from a low cut. This resprouting can occur also after using contact herbicides, such as organic herbicides, because established plants can grow out of these burn down treatments.
Third, stinknet seedlings can grow in very dense clusters. Unlike some other weed species, stinknet seedlings can grow in dense clusters including up to 15 seedlings per square inch of soil surface. When plants are growing in such dense clusters it may be difficult to treat each individual plants. When trying to apply herbicides to each individual plant to control them all, it may take multiple treatments throughout the season to ensure applications reach each individual.
Fourth, stinknet can germinate several times each year after fall winter or spring rains. In a single population of stinknet plants there may be several cohorts including large plants nearing maturity that germinated early in the season, others that are in early flowering stage, others that are medium sized and a few that may be seedlings.
Fifth, before stinknet flowers it can be difficult to find. Early in the season stinknet is relatively small, grows low to the ground, and is often shorter than surrounding annual plants, including annual grasses. Just being one of the shorter plants makes it hard to search through taller vegetation to find. When stinknet is taller during flowering or near flowering it can be much easier to find, but then it might be too late in the season to muster effective control techniques.
And sixth, and related to the last point, stinknet can be difficult to identify. The flowers are distinct round, globe-shaped yellow balls and when blooming can be a very easy and great way to identify stinknet. If you are not very familiar with this plant or where it might be located on your lands, it is highly recommended to monitor for stinknet in the early flowering season. However, before stinknet begins to flower, it is difficult to identify. The leaves are relatively small, they are finely divided (doubly pinnate), and are covered in fine hairs. There are many plants that look similarly at these early stages. The leaves do have a strong pungent odor, even when small, so using your nose can help to identify stinknet if you have an unknown small weed with slightly hairy, finely divided leaves.
As mentioned earlier, to help control stinknet we’ve developed a stinkent pest note. This pest note details much of the current information on managing stinknet. The pest note covers how to identify stinknet, its biology and impacts in the environment, developing monitoring programs for stinknet, and covers the known successful management methods for stinknet, including nonchemical and chemical control methods and including a few options we have learned that are not effective.
We are learning more and more about this species and trying new methods, as well as fine tuning successful methods to try to control it. — By Chris McDonald, UCANR
