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A Pretty Stinky Weed

Shaku Nair and Shujuan Li 
Department of Entomology, University of Arizona

 
Figure 1. Stinknet extending for miles on either side of SR 347 between Chandler and Maricopa, AZ in 2023. Photo: Shaku Nair.

If you drive along SR 347 in Arizona during early spring (February - March), you may witness a sea of yellow extending for miles on either side of the road (Figure 1). This spectacle may be witnessed in many other parts of the state, along the sides of roads and major highways, to wildlands and even private backyards. The pretty yellow flowers belong to the plant known as stinknet or globe chamomile, a winter annual weed in the sunflower family (Asteraceae). Native to South Africa, stinknet was first reported in the US in Riverside County, California, in 1981. It was later identified as an invasive species in Arizona during the 1990s, followed by inclusion in the state’s Noxious Weed List as a Class B noxious weed by the Arizona Department of Agriculture in 2020.

Why is stinknet such a problem?

Stinknet is a delicate-looking annual (Figure 2) which dies with the start of hot dry conditions in early summer, leaving no perennial structures. New stinknet plants grow only from seeds. The seeds do not have any special inbuilt mechanisms for dispersal (e.g., hooks, bristles or feathery appendages) that are seen in many other seeds. The dead plants stay in place for months with the seed heads intact and attached, through rain and sun until the next winter. This feature of retaining seeds on the dried plant for an extended time is likely to protect them and keeps them ready in position until an external force dislodges and breaks them apart. Ultimately, once separated from the seed head, dispersal may take place by wind, water, animals including humans, vehicles, implements, etc. The seeds then lie in the soil for favorable conditions to grow.

Figure 2. Stinknet plant with mature flowers. Photo: Andy Bridges, iNaturalist.org.

Once favorable weather conditions occur, stinknet grows vigorously. The plants thrive in a wide variety of areas, both managed and unmanaged with a preference for dry and disturbed soils. Here are some reasons that make stinknet problematic.

No natural control. Being an introduced plant, it has no natural predators. Moreover, it is not palatable to livestock or wildlife, and this allows it to grow unchecked.

Rapid spread. The small seeds spread easily via wind, water, and animals. They stay viable for an extended period of time.

Misconceptions. The delicate appearance of the leaves and bright yellow spherical blooms fascinate people who may think it is a pretty wildflower. Wildflower blooms in the low desert are known to support tourism, but they also encourage spread of the weed. Further, the name ‘globe chamomile’ leads people to believe it can be used to brew tea, similar to herbal chamomile. Some vendors in the Phoenix area farmers’ markets have been observed selling bundles of stinknet as “wild chamomile”. The effects of these concoctions are unknown.

Invasive. They compete with native wildflowers and choke them out, forming dense stands that can cover hundreds of acres. These stands smother native plant diversity (Figure 3).

Figure 3.Stinknet competing with native vegetation in Tonto National Forest, Apache Junction AZ.
Photo: Eric Hough, iNaturalist.org.

Fire risk. As an annual that dries out in early summer, it creates dense highly flammable mats which could serve as a fuel source for wildfire. It has been linked to increased wildfire intensity and spread in Arizona (Figure 4).

Figure 4. Dried stinknet is the perfect “flash fuel” and creates a severe summer fire hazard by forming dense, dried-out mats of fuel. It also acts as a "fire ladder”, bridging the gaps between native vegetation at different heights, increasing wildfire intensity and spreading, while releasing acrid, caustic smoke when burning.
Photo: Andy Bridges, iNaturalist.org.

Health hazard. The plant contains oil glands that cause contact dermatitis (rashes) in humans and pets. Its pollen and odors can cause respiratory problems and asthma flare-ups.

Life cycle

The life cycle of stinknet is similar to that of many other winter annuals growing in the low desert. The seeds start germinating from late October to November, and through the winter months if there is moisture from rain or other sources. The emerging seedlings grow into small rosettes which put out longer shoots (Figure 5), and in about two months the small yellow flower heads are formed.

Flower heads are bright yellow spheres and lack the surrounding ray florets typical of many other flowers in the sunflower family (Figure 6). Each flower in the sphere forms a seed and once all flowers in the seed head mature, the plant dries out and dies (Figure 7) around late April to May.

Figure 5. Stinknet seedlings start as a rosette and later put out side shoots. Photo: Eric Hough, iNaturalist.org.

 
Figure 6. Stinknet flower heads are bright yellow spheres consisting of multiple flowers, but lack the ray florets around the head. Photo: Lonnie Pilkington, iNaturalist.org.


Figure 7. Dried flowerheads of stinknet, showing dislodged flowers. Each flower contains a seed. 
Photo: Michael Chamberland.

Management

Correct identification is most important in the management of stinknet.

• Look for bright yellow, globe-shaped flower heads and feathery, pungent-smelling, carrot-like leaves.
• Seedling rosettes of stinknet can potentially be confused with some native and exotic plants found in southern Arizona (e.g., pineappleweed). However, the distinctive pungent odor of stinknet may help distinguish it from other plants. Once stems initiate bolting, their identity as stinknet can be confirmed well before flower initiation.

Mechanical control is a good option for infestations in small areas.

• Stinknet has a long period of growth prior to the reproductive stage, compared to many annual weeds. This offers sufficient time for conducting mechanical control before the plants go to seed.
• Hand pulling. The entire plant should be uprooted if possible, and this is easier when growing in moist or loose soil. Wearing gloves and long sleeves is recommended, to avoid direct skin contact with the plants due to sensitive reactions in some people.
• Weed whacking or trimming. Actively growing plants can be trimmed with a weed whacker or string trimmer. The base of the trimmed plants will resprout, but this will offer some time before the plants flower again. Immature flowers on the cut plants will not mature after they are separated from the main plant, so they can be left on the ground to dry out. Stinknet does not resprout from root fragments.

• Mechanical control methods which cut or break apart the plant tissue will release the pungent chemicals that give stinknet its name. Appropriate precautions must be taken to avoid sensitive reactions.

Chemical control

• Pre-emergent herbicides remain active in the soil for several weeks to several months. Due to the staggered germination of stinknet seeds through the course of the winter, a second application may be needed.
• Pre-emergent herbicides are a great option to prevent germination of stinknet seeds. A good time to apply would be just prior to the late fall/early winter rains, prior to weed seed germination.
• Keep in mind that moisture is needed to activate the pre-emergent herbicide in the soil zone where the weed seeds germinate. So if rain is not received, irrigation or watering will have to be provided.
• For standing stinknet plants, post-emergent herbicides can be used effectively. Good coverage is essential.
• Post-emergent herbicides are most effective when applied to small and young actively growing healthy plants (past the germination stage).
• Avoid herbicide applications during very cold weather or on drought-stressed wilted plants.
• A variety of herbicides have shown effectiveness against stinknet (Umeda, 2021).
• Always follow the instructions on the herbicide label and wear personal protective equipment. Consider hiring a licensed herbicide applicator for difficult situations.

The most important goal in stinknet management is to prevent plants from going to seed.

Once plants have set seed, they are prone to dropping seeds during any physical attempt to remove the plants. Plants which have gone to seed should not be sprayed with herbicide because it is too late. The plants are dead, or will be dead soon naturally without the application of herbicides. Herbicides will not clear dried brush or kill seeds on standing plants.

Check out the University of Arizona Cooperative Extension publication on stinknet for more comprehensive information on this problematic weed: https://repository.arizona.edu/bitstream/handle/10150/671131/az1827-2023.pdf?sequence=1&isAllowed=y

What can homeowners and the general public do to manage stinknet and prevent its spread?

You can certainly make a difference! Homeowners can manage stinknet on their property following the information provided above, or for further information please refer to the resources listed below. If you are unsure what to do about stinknet in common areas or public land adjoining private property, please reach out to the following agencies/organization based on your location.

• The Arizona Department of Transportation (ADOT) has established Roadside Vegetation Management Guidelines where District Environmental Coordinators work to schedule herbicide treatments alongside mechanical cutting to prevent regrowth along roadways.
• The Desert Defenders Program is a collaborative effort bringing together citizen scientists, the Maricopa County Parks & Recreation, the Central Arizona Conservation Alliance (CAZCA), and partner organizations such as the White Tank Mountains Conservancy and McDowell Sonoran Conservancy. The program trains community members to map and remove invasive plants like stinknet. Check out this factsheet with information https://www.maricopacountyparks.net/assets/1/6/Desert_Invaders-062024.pdf
• The Southwest Vegetation Management Association (SWVMA) provides various resources and events to increase awareness about invasive vegetation and foster cooperation between vegetation managers.
• Saguaro National Park: Actively manages infestations within and near the park and encourages public reporting of sightings to their invasive species coordinator.
• Tucson Audubon Society: Manages stinknet.org, a primary hub for reporting sightings and coordinating volunteer removal efforts in Southern Arizona.
• The Sonoran Desert Cooperative Weed Management Area (SD-CWMA), a partnership through the Arizona-Sonora Desert Museum coordinates for prevention and management of invasive plant species including stinknet in Pima County, AZ. Check out their informational page https://sdcwma.org/species/stinknet.php

• Arizona Department of Agriculture, Bureau of Land Management (BLM), and Arizona State Land Department are involved in monitoring and managing the invasive species on public lands.

Resources and further information

Arizona Department of Agriculture. Noxious Weeds. https://agriculture.az.gov/pestspest-control/agriculture-pests/noxious-weeds

National Park Service. Citizen science data inform park managers of ecological threats. https://www.nps.gov/articles/000/citizen-science-data-inform-park-managers-of-ecological-threats.htm#:~:text=In%20Phoenix%20and%20surrounding%20areas,people%20to%20keep%20up%20with.

Stinknet: A Weed Advancing in Southern Arizona. https://repository.arizona.edu/bitstream/handle/10150/671131/az1827-2023.pdf?sequence=1&isAllowed=y

Scheuring, J. and M. Chamberland. 2020. The short journey from stinknet introduction and spread to eruptive explosion in Arizona. https://aznps.com/wp-content/uploads/The-Short-Journey-from-Stinknet-Introduction-and-Spread-to-Eruptive-Explosion-in-Arizona-by-John-Scheuring1-and-Michael-Chamberland.pdf

Umeda, Kai. 2021. Demonstration of postemergence herbicides for stinknet control. https://turf.arizona.edu/20TH30%20Stinknet%20control%20Cave%20Creek%20+%20pix%20ltr.pdf

Umeda, Kai. 2021. Efficacy of postemergence herbicides for the control of stinknet in the spring. https://turf.arizona.edu/21TH03%20Stinknet%20POST%20ltr.pdf

Stinknet in Saguaro National Park. Photo: U.S. National Park Service.

Watch out for mosquitoes as the weather warms up!

Mosquitoes are the most important insect pests that affect the health and wellbeing of humans and domestic animals worldwide. In many parts of Arizona, there is no downtime for mosquitoes, and it is mosquito season year round!

Not only are mosquitoes annoying, biting pests, they can also cause a variety of health problems due to their ability to transfer (vector) viruses and other disease-causing pathogens.  An understanding about mosquito biology and behavior will help you manage mosquitoes in your environment, and related disease risks.

Humans create a lot of opportunities for mosquitoes to exploit. Many of us have mosquitoes developing in our neighborhood and even in our own backyards. Standing water left from rains or irrigation water will support increasing mosquito populations. Stagnant water in neglected swimming pools is another ideal habitat for many species, though mosquitoes cannot live in a well-maintained swimming pool. Some important vectors breed in “cryptic” breeding sites, utilizing very small amounts of water in tree-holes, artificial containers and even leaf axils.

It is equally important to avoid mosquito bites, using protective clothing or personal repellents. Personal repellents are substances applied to skin, clothing or other surfaces, or accessories used to repel or discourage biting arthropods, such as mosquitoes and ticks, from feeding on humans. They include spray, lotion, and disposable towelette products and devices that can be worn or placed in an area to repel biting pests.

Look out for more information on mosquitoes and their management in future issues this year! Meanwhile, check out a variety of resources in different formats from the University of Arizona Cooperative Extension. https://extension.arizona.edu/search?keys=mosquitoes

Is your lawn starting to green up?

Warm season turfgrasses (bermudagrass, zoysiagrass, and St. Augustinegrass) become dormant and typically lose their green color during winter. Overseeding the warm-season grass provides a green lawn through the winter until springtime, when the warm-season grasses come out of dormancy and resume their growth. Whether you overseed or not, the beginning of spring is an important time of year for your lawn and maintenance operations carried out at this time can have lasting effects on the health and functionality of your lawn through the rest of the year.

Check out our publications on turf maintenance and health: 

A Turfgrass Maintenance Guide for Residential and Commercial Lawns in the Low Elevation Arizona Desert  https://repository.arizona.edu/bitstream/handle/10150/669927/AZ1817-2020.pdf?sequence=1&isAllowed=y

Overseeding Winter Grasses into Bermudagrass Turf and Spring Transition https://extension.arizona.edu/sites/default/files/2024-08/az1683-2015.pdf

What the Heck Was This?
(From our January issue)

       

Answer: Stinknet.

Congratulations to Master Weed Detectives:

Diane Dance, Pima Co. Master Gardener
Donna Lorch, Maricopa Co. Master Gardener
Jay Subramani, U of A
Lucy Akers
Trudy Fuller

Additionally, Congratulations to Master Pest Detective Donna Lorch for correctly answering the
What the Heck question from December – Lady beetle larva. 
We apologize for skipping her name in January.

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What the Heck is This?

  
 Identify this insect.

If you know the answer, email Shaku at nairs@arizona.edu. You will not win anything if you are correct, but you will be listed as a “Master Pest Detective” in the next newsletter issue.

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