Increases fire frequency in deserts and coastal sage scrub and disturbed sites. The dense patches of litter can increase fuel loads beginning in the spring when plants desiccate. Increased fuel loads can persist for up to two years. Persistent dense litter from annual forbs and grassees is unusual in California, and is common in relatively few weeds in California (such as medusahead).Persistent litter can create dense fuel beds that will promote the spread of fire. Abundant along road edges and adjacent scrub vegetation, likely substantially increasing the risk of fire conveyance from roads into wildlands (Obs: J Giessow, C McDonald, M Chamberland 2020). Persistent dense litter through the fall and winter drastically increases fire risk through the peak Santa Ana winds fire season. This is seen in average to high rainfall years. (Obs: J Giessow). In coastal sage scrub the increase of stinknet fuel can alter the fire regime, which may add to the fuels created by invasive annual grasses that have already altered the fire regime in coastal sage scrub. In comparison, in desert areas where fuels are naturally sparse, stinknet can increase the fire frequency along roadsides as well as in natural areas (Chamberland 2020). This appears to be happening in and around Phoenix, Arizona, but stinknet infestations in the California Desert, as of this writing, are minimal but increasing. Lastly, stinknet seeds exposed to smoke germinate at higher rates than control seeds, suggesting it is fire adapted and may create a positive feedback cycle with frequent fires (San Diego Zoo Institute for Conservation Research, unpublished data).

Oncosiphon may likely reduce nutrient cycling. Large O. pilulifer patches produce litter that is persistent for about 2 years, while patches of annual grasses and native wildflowers have litter that generally persists for one year or less.

It is unknown whether Oncosiphon alters the water cycle, but it likely reduces soil water moisture due to the abundance of biomass produced in patches, especially in sage scrub communities. It has been shown that annual grasses can reduce soil moisture (Wood et al. 2006). If Oncosiphon acts similarly to non-native annual grasses, then it would reduce soil moisture.

Publications and the internet.

Stinknet is not palatable to livestock. The common name comes from Afrikaans, which roughly means stink only, in that the plant only stinks and it is not used as a forage by livestock or native African grazers. Stinknet has a much higher abundance on highly grazed pastures compared to lesser intensively grazed pastures, indicating it is not palatable (Rutherford and Powrie 2010). In California, very few stinknet plants shows signs of herbivore damage in the field. Very few large pollinators (bees, butterflies, beetles, flies) are observed on stinknet even when pollinators are present on nearby native wildflowers. Stinknet produces copious amounts of persistent thatch (Hedrick and McDonald 2020, Chamberland 2020). Stinknet seeds are not consumed by seed foraging rodents (such as kangaroo rats) (C Rodriguez unpub. data.)

Rutherford and Powrie 2010

Livestock grazing increases the cover and abundance of stinknet in South Africa (Rutherford and Powrie 2010). In South Africa stinknet can also be found in disturbed areas. In California, stinknet quickly invades disturbed areas, including roadsides, burned areas, mowed fields, graded soils, and plowed areas. In western Australia, stinknet is found invading small grain crops and also invades disturbed soils surrounding farms and is also invasive on livestock ranches. In central and southern Arizona, large stinknet patches can be found adjacent to roadsides, disturbed sites, ephemeral drainages, and even in cracks in the sidewalk and street. Stinknet rapidly invades areas after fire, including spreading into annual grasslands, sage scrub, and riparian habitat (Obs: J Giessow, C McDonald). As tree canopy cover increases, the stinknet population can decline, but may still remain in patches where the tree canopy is reduced.

Rutherford and Powrie 2010, Douglas and Nicholson 2019, personal observations

Stinknet can now be found in large portions of Western Riverside county, where it has created large patches over hundreds of acres. Stinknet is also found in many areas of San Diego County, where land managers are having a difficult time mapping all the new populations that have been found in the past few years. It has also increasing in San Bernardino and Los Angeles Counties. Stinknet has recently been found in the Central Valley and in Santa Barbara, Kern and Ventura Counties. Stinknet is also becoming more abundant in the California Desert especially in the Coachella Valley. Dozens of new stinknet sightings and populations have been found in the past few years in this region. It is also spreading east into central Arizona and southeastern Arizona. It can also be found in a few locations in and near Las Vegas, Nevada and in northern Mexico.

While there are large patches in California and Arizona, there are areas adjacent to infestations that are not heavily invaded and only have a few isolated plants. There are many isolated plants that have dispersed a long distance that have the potential to increase in size and become large isolated patches. This may change as more plants reproduce in small isolated patches and form large patches.

Personal observations, conversations with land managers, CalFlora, iNaturalist.

San Diego County data can be seen here:
https://sdmmpgis.maps.arcgis.com/apps/View/index.html?appid=be81637fe366455c85c767bcfafc307e

Stinknet seeds easily disperse on clothes, footwear, pets, tools and vehicles. The seeds are tiny (less than 1 mm long) and also disperse as a contaminant of agricultural and construction products and materials. The seeds are also tightly packed into the inflorescence, where they form a globe shape and the infloresence does not easily break apart. It is likely that these inflorescence balls roll on the ground, float downstream or downhill or break off from the plant. They are also carried in brisk winds. The inflorescenses are also picked up and carried in ‘dust devils.’ Once the inflorescence is forcefully hit, it begins to break apart and easily releases its seeds. Once the seeds are released from the inflorescence, the seeds can easily be carried on clothes and footwear because they are tiny. The seeds also appear to float and may be dispersed downhill with rainwater or downstream in a current. Stinknet seeds exposed to water release a viscous, sticky substance that could help them adhere to boots, tires, and animal fur or feathers. (San Diego Zoo Institute for Conservation Research, unpublished data). Many new stinknet patches are found near sites that are disturbed and also have human activity (such as adjacent to parking lots, hiking trails, highway rest stops, gravel stockpiles on roadsides, recently graded roadsides, etc.).

Parsons, W.T and Cuthbertson, E.G. (1992) Noxious weeds of Australia, Calomba Daisy pp.296-298. Inkata Press as cited in Douglas and Nichols 2019.
Hedrick and McDonald 2020.

Stinknet is still spreading in California. It is likely expanding its range in South Africa as it is spreading into disturbed areas (such as farms) there. It is also expanding its range in Western Australia. It is currently unknown how far across the Southwestern US it will expand. Since stinknet is a winter annual it should occupy areas with adequate winter and spring moisture.

Currently stinknet has been found adjacent to the coast, in interior ecosystems, such as chaparral, California sage scrub and annual grasslands, it has also been found in the Mojave and Sonoran Deserts of California as well as in Arizona and Nevada. As of this writing one population has been found in Santa Clara county near the bay.

It has found above 4000 ft in elevation in Arizona, however it is highly likely it will spread higher than found in its current distribution given time.

Personal observations, CalFlora, iNaturalist, SEINet.

Stinknet was first discovered in Riverside County in 1981. Within 10 years it had spread beyond a single introduction location and into several patches on adjacent lands. In the 1990's it had spread to San Diego County as well as Maricopa County, Arizona. In all three of these locations where stinknet populations have been spreading for 20 years or more, stinknet is locally widespread and locally abundant forming very large infestations. At these oldest invaded locations stinknet can dominate the herbaceous flora, forming large dense patches while only a few other herbaceous plants are present.

Stinknet is still spreading in California. It is currently unknown how far across the Southwestern US and what variety of habitats is will invade. Since stinknet is a winter annual it should occupy areas with adequate winter and spring moisture.

Currently stinknet has been found adjacent to the coast, in interior ecosystems, such as chaparral, California sage scrub and annual grasslands, it has also been found in the Mojave and Sonoran Deserts of California as well as in Arizona, Nevada and northern Mexico. As of this writing one population has been found in a disturbed site in Santa Clara county near the bay.

After a fire, stinknet has been found to invade a variety of habitats including annual grasslands, sage scrub, chaparral and woodlands. As taller vegetation recovers such as when riparian tree canopy re-grows stinknet abundances may decline in the shade (Obs: J Giessow).

It has found above 4000 ft in elevation in Arizona, in woodland or forested communities, however it is highly likely it will spread higher than found in its current distribution given time.

CalFlora, SEINet, personal observations, Chamberland 2020.