In the Phoenix area, stinknet occupies disturbed soil in residential areas, vacant lots, soil piles, along roadsides and major highways, and in cracks in pavement and sidewalks. The plant can compete with native desert vegetation and is spreading through South Mountain Park, the Phoenix Sonoran Preserve and the Tonto National Forest. Stinknet is expanding south along the I-10 corridor, becoming well established in Pinal County. Outbreaks have occurred within the Tucson metro area and in rural parts of Pima County. Observations around Sedona place it as high as 4220 ft. elevation (SEINet Portal Network, 2020).

Stinknet seeds may germinate in late October or November with the onset of winter rains. Additional germination from the soil seed bank may occur during rainfall episodes through the winter. Plants begin growth as a basal rosette and later send out shoots horizontally and vertically. After approximately two months of growth small flower heads are initiated at the tips of bolting stems. It may require another month or more before the first flowers on these heads begin to open. Seed maturation occurs as the yellow spherical flower heads dry out into tan spheres of equal size and shape. The seed heads remain intact as the plants dry out and die at the end of the growing season. Flower and seed phenology can be expected to be variable depending on temperature and rainfall at the site.

Seeds are stinknet’s method of spread. The seeds are seemingly unspecialized, miniscule tiny cones with the dried tan corolla (flower petals) remaining attached. The dried corolla may aid seed dispersal in ways which are not obvious. Wind, animals, vehicles and water all have potential to disperse the tiny seeds.

Stinknet seed heads are delicately held together. Blunt force applied to a seed head causes the sphere to break up into its component seeds. Surprisingly, seed heads may remain attached and intact for many months without dispersing seeds. Stinknet’s habit of retaining seeds on the dried plant no doubt positions seeds until a force with potential for dispersal dislodges them.

The rapid advance of stinknet on Arizona’s landscape has numerous negative implications. Stinknet is an urban weed infesting residential property and likely transported by human activity. Stinknet forms thick stands which can crowd out native winter-growing wildflowers. Stinknet can infest agricultural areas and may degrade rangeland, as the plants are reported to be unpalatable to livestock. People differ in their reaction to the odor of stinknet, which is pungent and similar to turpentine. Stinknet has been implicated in cases of respiratory distress. There are other reports of contact dermatitis from skin exposure. Stinknet dries and dies as Arizona enters early summer, which is also wildfire season. Stinknet is considered a “flash fuel” meaning it is easy to ignite from a spark or discarded cigarette. The plant has been implicated in several brush fires north of Phoenix during the summer of 2020. Stinknet’s habit of forming thick drifts bridges the gaps between plants which would otherwise impede the spread of fire through native Sonoran Desert vegetation.

Stinknet has a long period of growth prior to seed set, compared to many annual weeds. This offers an extended opportunity for mechanical control, provided the area infested is not prohibitively large. Flowering plants of stinknet cannot be easily confused with any Arizona native plants. Seedling rosettes of stinknet are less distinctive. The odor of stinknet may help distinguish it.

Stinknet may be pulled, uprooting the entire plant, when growing in moist or loose soil. It does not resprout from root fragments. Plants must bolt before they can bloom, and this provides a handhold for pulling plants at this stage. Wearing gloves and long sleeves is advisable due to sensitivity some people have experienced from contact with stinknet.

Stinknet does not seem to set seed until all flowers in the head have blossomed. It is not prone to precocious seed set. If stinknet has come into bloom, the stems may be pulled or weed-whacked and left on the ground. The flower heads will not mature into seeds after the stems are severed. The base of a weed-whacked plant will regrow however, some time is bought before it can flower again. Note, mechanical controls which break apart plant tissue will release volatile chemicals and this may irritate sensitive people.

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 an herbicide. 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. Experimental trials are underway to determine the most effective herbicides and rates of application which are maximally effective against stinknet, while also presenting minimal hazard to other vegetation and the environment.

Oncosiphon. SEINet Portal Network. http://swbiodiversity.org/seinet/taxa/index.php?taxon=16250 Accessed 14 June 2020.

Stinknet: A Weed Advancing in Southern Arizona. 2020. M. Chamberland. August 2020, AZ1827. https://extension.arizona.edu/pubs/stinknet-weed-advancing-southern-arizona