Plant Assessment Form

Oncosiphon pilulifer

Synonyms: Matricaria globifera, Pentzia globifera, Oncosiphon piluliferum

Common Names: stinknet; globe chamomile

Evaluated on: 5 Jan 2021

List committee review date: 05/02/2021

Re-evaluation date:


Chris McDonald
University of California, Dept. of Agriculture & Natural Resources

List committee members

Jutta Burger
Carlos de la Rosa
Jason Giessow
Marla Knight
Lynn Sweet

General Comments

No general comments for this species

Table 2. Criteria, Section, and Overall Scores

Overall Score? High
Alert Status? No Alert
Documentation? 3 out of 5
Score Documentation
1.1 ?Impact on abiotic ecosystem processes A. Severe Reviewed Scientific Publication
Four-part score AABD Total Score
1.2 ?Impact on plant community A. Severe Observational
1.3 ?Impact on higher trophic levels B. Moderate Other Published Material
1.4 ?Impact on genetic integrity D. None Other Published Material
2.1 ?Role of anthropogenic and natural disturbance in establishment A. Severe Reviewed Scientific Publication
Total Points
19 Total Score A
2.2 ?Local rate of spread with no management A. Increases rapidly Observational
2.3 ?Recent trend in total area infested within state A. Increasing rapidly Observational
2.4 ?Innate reproductive potential
(see Worksheet A)
A. High Other Published Material
2.5 ?Potential for human-caused dispersal A. High Other Published Material
2.6 ? Potential for natural long-distance dispersal B. Occasional Observational
2.7 ?Other regions invaded B. Invades 1 or 2 ecological types Observational
3.1 ?Ecological amplitude/Range
(see Worksheet C)
B. Moderate Other Published Material
Total Score C
3.2 ?Distribution/Peak frequency
(see Worksheet C)
D. Very low Other Published Material

Table 3. Documentation

Scores are explained in the "Criteria for Categorizing Invasive Non-Native Plants that Threaten Wildlands".

Section 1: Impact
Question 1.1 Impact on abiotic ecosystem processes? A Reviewed Scientific Publication
Identify ecosystem processes impacted:

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.

Sources of information:

Publications and the internet.

Question 1.2 Impact on plant community composition,
structure, and interactions?
A Observational
Identify type of impact or alteration:

Stinknet patches form nearly monospecific stands. In years with adequate rainfall, stinknet seedlings can grow at very high densities, at over one plant per square centimeter, or 10,000 per square meter, and continue to grow in dense stands throughout the beginning of the growing season. Stinknet plants produce relatively large amounts of persistent litter, compared to annual grasses or native wildflowers, and stinknet litter can persist for 2 years. This in turn prevents and diminishes the growth of native wildflowers likely creating a positive feedback cycle. Stinknet patches also appear to reduce the vigor and success of native perennial seedlings, some of which do not survive competition with stinknet.

Sources of information:

Field observations, personal contacts with scientists and land managers.

Question 1.3 Impact on higher trophic levels? B Other Published Material
Identify type of impact or alteration:

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.)

Sources of information:

Rutherford and Powrie 2010

Question 1.4 Impact on genetic integrity? D Other Published Material

There are no native plants in California that are in the same genus as stinknet. The genus is native to Africa. There are also no weeds in California in the same genus. It is unknown if it causes any genetic impacts.

Sources of information:

Jepson eFlora, CalFlora.

Section 2: Invasiveness
Question 2.1 Role of anthropogenic and natural disturbance
in establishment?
A Observational
Describe role of disturbance:

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.

Sources of information:

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

Question 2.2 Local rate of spread with no management? A Observational
Describe rate of spread:

In a series of years with average or above average rainfall in Southern California, stinknet patches can easily double in size in a few years. In metropolitan Phoenix Arizona, stinknet patches dominate large portions of roadsides, where few stinknet plants existed ten to twenty years ago. Small patches easily spread and increase in size in just a few years also creating an abundance of small patches adjacent to main infestations. Stinknet was first found in California in 1981 in Riverside County and in less than 40 years it has spread over 500 miles as measured from east to west.

Sources of information:

Personal observations, conversations with land managers. CalFlora, iNaturalist for distribution information

Question 2.3 Recent trend in total area infested within state? A Observational
Describe trend:

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.

Sources of information:

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

San Diego County data can be seen here:

Question 2.4 Innate reproductive potential? A Other Published Material
Describe key reproductive characteristics:

Stinknet is a winter annual. It generally produces one large synchronized blooming event in the late winter through spring, and sometimes a few smaller blooms will occur later depending on rainfall patterns. However, patches with additional soil moisture can continue to reproduce for several months after peak blooming. Plants produce numerous flowers and can produce 3,000-5,000 seeds per plant with large plants producing many more seeds (Douglas and Nicholson 2019). Seeds can survive in the soil for 5 years in Western Australia (Dodd and Lloyd 1988).

Sources of information:

Douglas and Nicholson 2019, Dodd and Lloyd 1988

Question 2.5 Potential for human-caused dispersal? A Other Published Material
Identify dispersal mechanisms:

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.).

Sources of information:

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.

Question 2.6 Potential for natural long-distance dispersal? B Observational
Identify dispersal mechanisms:

Seeds are tiny so they could stick to birds and mammals which may migrate long distances. A large number of cases of a single or a few plants have been found far (hundreds of yards) from potential human sources (roads, hiking trails) or in areas where human traffic is less frequent (non-public access nature preserves). Birds are not regularly seen foraging on seed heads. Human assisted dispersal is arguably the more likely long-distance dispersal mechanism, as many new long-distance populations are near heavily trafficked sites, roadsides, parking lots, etc.

Some locations do appear to be farther from human activity. Some new stinknet individuals are found far from roads, trails or where people frequent, so long-distance (miles) dispersal by animals cannot be ruled out. Several patches of stinknet have been found in areas where winds are concentrated or have significant force during large regional wind events, such as during Santa Ana winds. The inflorescences are also picked up and carried in ‘dust devils.’ It is very likely that during strong winds, either in large regional wind events like Santa Ana winds or in 'dust devils', stinknet inflorescences and seeds are picked up and carried long distances. Stinknet seeds appear to float and can be carried downstream. Stinknet seeds are also very likely carried downstream during flooding events, and flash flood events after desert monsoon storms.

Sources of information:

Hedrick and McDonald 2020

Question 2.7 Other regions invaded? B Observational
Identify other regions:

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.

Sources of information:

Personal observations, CalFlora, iNaturalist, SEINet.

Section 3: Distribution
Question 3.1 Ecological amplitude/Range? B Other Published Material

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.

Sources of information:

CalFlora, SEINet, personal observations, Chamberland 2020.

Question 3.2 Distribution/Peak frequency? D Other Published Material
Describe distribution:

Since its discovery in 1981, stinknet has since spread to 8 counties in California. As of the time of writing it can be found in 7 counties in Southern California and has also been found in Santa Clara County near the San Francisco Bay. It has also been found in at least 5 counties in Arizona and two sites in Nevada, as well as in northern Mexico. New locations are often found each year during years with average or above average precipitation.

Sources of information:

CalFlora, SEINet, personal observations

Worksheet A - Innate reproductive potential

Reaches reproductive maturity in 2 years or less Yes
Dense infestations produce >1,000 viable seed per square meter Yes
Populations of this species produce seeds every year. Yes
Seed production sustained over 3 or more months within a population annually Yes
Seeds remain viable in soil for three or more years Yes
Viable seed produced with both self-pollination and cross-pollination Unknown
Has quickly spreading vegetative structures (rhizomes, roots, etc.) that may root at nodes No
Fragments easily and fragments can become established elsewhere No
Resprouts readily when cut, grazed, or burned Yes
Total points: 8
Total unknowns: 1
Total score: A?

Related traits:

Worksheet B - Arizona Ecological Types is not included here

Worksheet C - California Ecological Types

(sensu Holland 1986)
Major Ecological Types Minor Ecological Types Code?
Marine Systemsmarine systemsU, Unknown
Freshwater and Estuarine lakes, ponds, reservoirsU, Unknown
Aquatic Systemsrivers, streams, canalsD, < 5%
estuariesU, Unknown
DunescoastalD, < 5%
desertD, < 5%
interiorD, < 5%
Scrub and Chaparralcoastal bluff scrubD, < 5%
coastal scrubD, < 5%
Sonoran desert scrubD, < 5%
Mojavean desert scrub (incl. Joshua tree woodland)D, < 5%
Great Basin scrubU, Unknown
chenopod scrubU, Unknown
montane dwarf scrubU, Unknown
Upper Sonoran subshrub scrubU, Unknown
chaparralD, < 5%
Grasslands, Vernal Pools, Meadows, and other Herb Communitiescoastal prairieU, Unknown
valley and foothill grasslandD, < 5%
Great Basin grasslandU, Unknown
vernal poolD, < 5%
meadow and seepU, Unknown
alkali playaU, Unknown
pebble plainU, Unknown
Bog and Marshbog and fenU, Unknown
marsh and swampU, Unknown
Riparian and Bottomland habitatriparian forestD, < 5%
riparian woodlandU, Unknown
riparian scrub (incl.desert washes)D, < 5%
Woodlandcismontane woodlandU, Unknown
piñon and juniper woodlandU, Unknown
Sonoran thorn woodlandU, Unknown
Forestbroadleaved upland forestU, Unknown
North Coast coniferous forestU, Unknown
closed cone coniferous forestU, Unknown
lower montane coniferous forestU, Unknown
upper montane coniferous forestU, Unknown
subalpine coniferous forestU, Unknown
Alpine Habitatsalpine boulder and rock fieldU, Unknown
alpine dwarf scrubU, Unknown
Amplitude (breadth): A
Distribution (highest score): D

Infested Jepson Regions

Click here for a map of Jepson regions

  • CA Floristic Province
  • Great Valley
  • Southwest
  • Desert Province
  • Mojave Desert
  • Sonoran Desert