Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide


Taeniatherum caput-medusae
(medusahead wildrye)



Taeniatherum caput-medusae (medusahead wildrye)


  • Last modified
  • 20 November 2019
  • Datasheet Type(s)
  • Invasive Species
  • Pest
  • Preferred Scientific Name
  • Taeniatherum caput-medusae
  • Preferred Common Name
  • medusahead wildrye
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Plantae
  •     Phylum: Spermatophyta
  •       Subphylum: Angiospermae
  •         Class: Monocotyledonae
  • Summary of Invasiveness
  • T. caput-medusae is a self-pollinating, annual grass. It is originally from the Mediterranean region, occurring eastwards in Asia to Kyrgyzstan and northwards in Europe to Budapest in Hungary. Introduced to the...

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Taeniatherum caput-medusae (medusahead); habit, showing flower spikes.
CaptionTaeniatherum caput-medusae (medusahead); habit, showing flower spikes.
CopyrightReleased into the Public Domain by the USDA-ARS-2004 - original image by Brett Bingham
Taeniatherum caput-medusae (medusahead); habit, showing flower spikes.
HabitTaeniatherum caput-medusae (medusahead); habit, showing flower spikes.Released into the Public Domain by the USDA-ARS-2004 - original image by Brett Bingham


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Preferred Scientific Name

  • Taeniatherum caput-medusae (L.) Nevski

Preferred Common Name

  • medusahead wildrye

Other Scientific Names

  • Elymus caput-medusae L.
  • Hordelymus caput-medusae (L.) Pignattii
  • Hordeum caput-medusae (L.) Coss & Durieu
  • Taeniatherum asperum (Simonk.) Nevski

International Common Names

  • English: medusahead; medusahead-rye; Medusa's-head
  • Spanish: cabeza-de-Medusa

Local Common Names

  • Portugal: centeio-silvestre-cabeça-de-Medusa
  • Sweden: medusagräs

Summary of Invasiveness

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T. caput-medusae is a self-pollinating, annual grass. It is originally from the Mediterranean region, occurring eastwards in Asia to Kyrgyzstan and northwards in Europe to Budapest in Hungary. Introduced to the Americas in at least seven events between 1887 and 1988, it now occupies over one million hectares of rangelands in the western USA where it is considered invasive and is listed as a noxious weed in many states, and is estimated to be spreading at a rate of 12% per year. In the western USA. It is highly competitive, forming monotypic stands that not only exclude native species but transform the ecological functioning of its invaded habitat to better facilitate its own survival to the detriment of the entire invaded ecosystem.  

Taxonomic Tree

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  • Domain: Eukaryota
  •     Kingdom: Plantae
  •         Phylum: Spermatophyta
  •             Subphylum: Angiospermae
  •                 Class: Monocotyledonae
  •                     Order: Cyperales
  •                         Family: Poaceae
  •                             Genus: Taeniatherum
  •                                 Species: Taeniatherum caput-medusae

Notes on Taxonomy and Nomenclature

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After previous inclusion in Elymus, Hordeum and Hordelymus, the genus Taeniatherum was established by Nevski in 1934. Taeniatherum originally consisted of three geographically and morphologically distinct taxa, T. caput-medusa, T. asperum and T. crinitum (McKell et al., 1962a). In 1986, the genus was revised and the three taxa were reduced to subspecies: ssp asperum, spp. caput-medusae and spp. crinitum (Frederiksen, 1986). Analysis of genetic markers suggests that subspecies crinitum is genetically differentiated from the other two subspecies, and that subspecies asperum is the most variable (Peters et al., 2013). The specific name caput-medusae refers to the resemblance of the seedhead to the mythical Medusa’s head. Common names for this species typically relate to variations on the translated specific name i.e. medusahead, medusahead rye; Medusa’s head.


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T. caput-medusae is a cespitose annual grass that is primarily self-pollinated. Culm length ranges from 5 to 55 cm. It has slender, weak stems that often branch at the base. Most individuals have 2-4 leaves on the culm. The 2.5- to 7.5 cm awns are straight and compressed when green, but, upon drying, the awns twist and spread erratically in a manner reminiscent of the snake-covered head of the mythic Medusa. The three subspecies of T. caput-medusae differ most distinctly in glume length, and the spreading of the glumes in the seed stage (Frederiksen, 1986; Peters, 2013). Palea length is useful for distinguishing subspecies crinitum which has a larger seed than subspecies caput-medusae or asperum. However, palea length of subspecies caput-medusae and asperum overlapped and so this character is not a useful indicator for differentiating between these two subspecies (Peters, 2013).

Plant Type

Top of page Annual
Grass / sedge
Seed propagated


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Taeniatherum occurs naturally in the Mediterranean region. It reaches eastwards in Asia to Kyrgyzstan, and northwards in Europe to Budapest in Hungary. It has been introduced in the northern and northwestern parts of Europe and in the Americas, where it acts as a weed (Frederiksen, 1986). Subspecies caput-medusae is found in the western Mediterranean and is generally restricted to Portugal, Spain, southern France, Morocco and Algeria (Young, 1992). When collected elsewhere it is probably adventitious (Frederiksen, 1986). Subspecies crinitum occurs from Greece and the Balkans into Central Asia (Frederiksen, 1986; Peters, 2013). Within its native range, T. caput-medusae spp. asperum is the most widely distributed of the subspecies, occurring across the whole geographic distribution of the species. Only subspecies asperum has been identified in North America (Peters, 2013). The introduced range of the species includes Chile and Australia as well as a number of states in the USA.

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Last updated: 10 Jan 2020
Continent/Country/Region Distribution Last Reported Origin First Reported Invasive Reference Notes


AlgeriaPresentNativeFrederiksen (1986); Missouri Botanical Garden (2015)
EgyptPresentNativeFrederiksen (1986); Missouri Botanical Garden (2015)
LibyaPresentNativeUSDA-ARS (2015)
MoroccoPresentNativeFrederiksen (1986); Missouri Botanical Garden (2015)
TunisiaPresentNativeFrederiksen (1986); Missouri Botanical Garden (2015)


AfghanistanPresentNativeFrederiksen (1986); Missouri Botanical Garden (2015)
ArmeniaPresentNativeFrederiksen (1986); Missouri Botanical Garden (2015)
AzerbaijanPresentNativeUSDA-ARS (2015)
GeorgiaPresentNativeUSDA-ARS (2015)
IndonesiaPresentCABI (Undated); Frederiksen (1986); Missouri Botanical Garden (2015)Present based on regional distribution.
IranPresentNativeFrederiksen (1986); Missouri Botanical Garden (2015)
IraqPresentNativeFrederiksen (1986); Missouri Botanical Garden (2015)
IsraelPresentNativeFrederiksen (1986); Missouri Botanical Garden (2015)
JordanPresentNativeFrederiksen (1986)
KazakhstanPresentNativeUSDA-ARS (2015)
KyrgyzstanPresentNativeFrederiksen (1986); Missouri Botanical Garden (2015)
LebanonPresentNativeFrederiksen (1986); Missouri Botanical Garden (2015)
PakistanPresentNativeUSDA-ARS (2015)
Saudi ArabiaPresentNativeFrederiksen (1986); Missouri Botanical Garden (2015)
SyriaPresentNativeFrederiksen (1986); Missouri Botanical Garden (2015)
TajikistanPresentNativeUSDA-ARS (2015)
TurkeyPresentNativeFrederiksen (1986); Missouri Botanical Garden (2015)
TurkmenistanPresentNativeFrederiksen (1986); Missouri Botanical Garden (2015)


AlbaniaPresentNativeFrederiksen (1986); Missouri Botanical Garden (2015)
BulgariaPresentNativeFrederiksen (1986); Missouri Botanical Garden (2015)
CroatiaPresentNativeUSDA-ARS (2015)
CyprusPresentNativeFrederiksen (1986); Missouri Botanical Garden (2015)
Federal Republic of YugoslaviaPresentNativeFrederiksen (1986); Missouri Botanical Garden (2015)
FrancePresentNativeFrederiksen (1986); Missouri Botanical Garden (2015)
GreecePresentNativeFrederiksen (1986); Missouri Botanical Garden (2015)
HungaryPresentNativeFrederiksen (1986); Missouri Botanical Garden (2015)
ItalyPresentNativeFrederiksen (1986); Missouri Botanical Garden (2015)
North MacedoniaPresentNativeUSDA-ARS (2015)
PortugalPresentNativeUSDA-ARS (2015)
RomaniaPresentNativeFrederiksen (1986); Missouri Botanical Garden (2015)
RussiaPresentNativeFrederiksen (1986)
SerbiaPresentNativeFrederiksen (1986); Missouri Botanical Garden (2015)
SpainPresentNativeFrederiksen (1986); Missouri Botanical Garden (2015)
UkrainePresentNativeFrederiksen (1986); Missouri Botanical Garden (2015)

North America

United StatesPresentIntroducedUSDA-NRCS (2015)
-ArizonaPresent, Few occurrencesIntroduced2000Invasive Plant Atlas of the United States (2015)
-CaliforniaPresent, WidespreadIntroduced1908InvasiveUSDA-NRCS (2015)
-ConnecticutPresentIntroducedUSDA-NRCS (2015)
-IdahoPresent, WidespreadIntroduced1994InvasiveUSDA-NRCS (2015)
-MontanaPresent, Few occurrencesIntroduced2013USDA-NRCS (2015)
-NevadaPresentIntroduced1960InvasiveUSDA-NRCS (2015)
-New YorkPresentIntroducedUSDA-NRCS (2015)
-OregonPresent, WidespreadIntroduced1887InvasiveUSDA-NRCS (2015)
-PennsylvaniaPresentIntroducedUSDA-NRCS (2015)
-UtahPresentIntroducedInvasiveUSDA-NRCS (2015)
-WashingtonPresentIntroduced1901USDA-NRCS (2015)


AustraliaPresentCABI (Undated)Present based on regional distribution.
-New South WalesPresent, Few occurrencesIntroducedCouncil of Heads of Australasian Herbaria (2015)
-South AustraliaPresent, Few occurrencesIntroduced1859Barker et al. (2005)

South America

ChilePresentIntroducedUSDA-ARS (2015)

History of Introduction and Spread

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T. caput-medusae ssp. asperum is believed to be the only subspecies of T. caput-medusae to occur in the USA (Major et al., 1960; Peters, 2013; Kyser, 2014). At least seven separate introductions of T. caput-medusae have occurred (Novak and Svorza, 2008). It was first recorded in Roseburg, Oregon, USA, in 1887. From there it appears to have spread concentrically. It was recorded in eastern Washington in 1901 and in California in 1908. It was found in Idaho by 1960 and was in Nevada in the early 1960s (Young, 1992). T. caput-medusae has continued to spread in the United States. T. caput-medusae was first collected in Utah in 1988 and in 2013 it was confirmed to be present in Montana (Novak and Svorza, 2008; USDA-NRCS, 2015).

Risk of Introduction

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T. caput-medusae is considered to be an invasive species in parts of California, Idaho, Nevada, Oregon, Utah, and Washington (Kyser et al., 2014). Although it may have arrived as a cereal grain crop contaminant, it is believed that T. caput-medusae was most likely introduced in the fur of livestock (Hilken and Miller, 1980; Kyser et al., 2014). It has a silica-barbed awn which is easily attached to fur, clothing, vehicles and machinery (Kyser et al., 2014) and is commonly spread along travel corridors (Davies et al., 2013). In addition, viable seeds have been recovered from sheep and rabbit faecal material nine days after ingestion (Sharp et al., 1957). T. caput-medusae is listed as noxious in California, Colorado, Nevada, Oregon and Utah (Archer, 2001), and is continuing to spread within the USA.


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In the western USA, T. caput-medusae generally occurs in two broad climatic regions: the Mediterranean climate of coastal California which is similar to its native range with hot, dry summers, and cool, moist winters, and the intermountain region which has warm summers and cold winters where precipitation often arrives as snow (Kyser et al., 2012; Nafus and Davies, 2014). In the Mediterranean region, T. caput-medusae occurs in annual grasslands, oak woodlands and chaparral communities (Young, 1992).  In the intermountain region, T. caput-medusae primarily occurs in the sagebrush habitat (Artemisia tridentata and Artemisia arbuscula) (Nafus and Davies, 2014).

Habitat List

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Terrestrial – ManagedCultivated / agricultural land Present, no further details Harmful (pest or invasive)
Managed grasslands (grazing systems) Present, no further details Harmful (pest or invasive)
Disturbed areas Present, no further details Harmful (pest or invasive)
Rail / roadsides Present, no further details Harmful (pest or invasive)
Terrestrial ‑ Natural / Semi-naturalNatural grasslands Present, no further details Harmful (pest or invasive)
Scrub / shrublands Present, no further details Harmful (pest or invasive)
Arid regions Present, no further details Harmful (pest or invasive)

Growth Stages

Top of page Pre-emergence, Seedling stage

Biology and Ecology

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The genome of Taeniatherum is distinct. Individuals are diploid (n = 14) (Peters, 2013). The three subspecies hybridize easily with each other in an artificial setting. However, in the greenhouse, hybrid seeds had partially degenerated endosperms which necessitated special cultivation techniques to germinate and although they grew vigorously, they did not set seeds (Frederiksen, 1986). However, there is some evidence that the subspecies asperum and caput-medusae, and asperum and crinitum, may hybridize successfully where they overlap in their geographic range (Peters, 2013). Although subspecies hybridization may occur, there is no evidence that T. caput-medusae hybridizes with other species (Kyser et al., 2014).

Reproductive Biology

T. caput-medusae is a self-pollinating, annual grass. Like all annual species, it is dependent on seed production for continued species propogation. Seeds may be ready to germinate the first season after dropping (Sharp et al., 1957; Hironaka, 1961). Frequently, however, seeds have a temperature-related afterripening dormancy which prevents germination from occurring before seeds have experienced 60-120 days of cold incubation after seed maturation (Young et al., 1968). Seventy-five percent of seeds germinated after they had reached the mid-dough stage and by the late dough stage 90% of seeds germinated (Sharp et al. 1957). Germination typically occurs with autumn precipitation and in milder climates continues through the winter and spring (Young, 1992). In colder environments, a second flush of germination occurs in the spring following snow melt (Kyser et al., 2014). Germinating seeds are extremely drought resistant. If the primary root dries out, seeds can produce multiple new adventitious roots when moisture is again available (Young 1992). Leaf development in autumn can reach several inches before cold weather stops the growth process (Young, 1992), and root growth can continue throughout winter (Hironaka, 1961). Above-ground growth resumes in spring, and flowering typically occurs sometime in early June with most seedheads maturing by July (Sharp et al., 1957) although seeds will continue to disperse from the parent plant into autumn (Davies, 2008).

Physiology and Phenology

T. caput-medusae is cespitose forming mats or dense tufts. Seed production can be phenologically plastic; an isolated plant without neighbours can produce as many seeds as 1000 more densely packed plants (Young, 1992). Under ideal conditions a stand of medusahead can produce well over 10,000 seeds·m-2 although average production is less (Clausnitzer et al., 1999).

Seeds can remain viable in the soil for at least one year, especially on sites with higher litter cover (Sharp et al., 1957). Although carryover seeds have reduced germination, enough seeds can successfully germinate to quickly replenish the seedbank. On sites where T. caput-medusae density has been partially reduced, remaining plants grow larger so that total cover and seed production is similar to pre-reduction levels (Kyser et al., 2013). Seeds from both California and France produced larger plants when grown in California soils as a result of the higher nutrient availability in Californian soils (Blank and Sforza, 2007).

T. caput-medusae has a higher silica content than other grass species (Bovey et al., 1961) which enables the formation of thick accumulations of litter to build up (Young 1992). This accumulated thatch layer creates an environment with the ideal moisture and temperature characteristics for T. caput-medusae germination, chokes out other plant species and, in semiarid Artemisia tridentata steppe, creates a continuous fuel source that promotes more frequent fire thereby providing open spaces for further T. caput-medusae invasion (Young, 1992). 

Environmental Requirements

T. caput-medusae occurs across a wide range of climatic and soil conditions. It typically occurs in areas receiving 30 – 61 cm of precipitation per year and has been found on sites with 25 – 102 cm of precipitation (Sharp et al., 1957; Major et al., 1960; Torell et al., 1961; George, 1992). T. caput-medusae is most problematic on soils with a high clay content and shrink-swell potential (Stromberg and Griffin, 1996; Sheley et al., 2008) and is capable of invading loamy soils (Sharp et al., 1957; Miller, 1996). However, it is unlikely to occur on sandy, well-drained substrates (Dahl and Tisdale, 1975) except at the higher end of its precipitation range (Kyser et al., 2014). Soils with high nutrient levels favour T. caput-medusae establishment relative to native vegetation as T. caput-medusae is better able to acquire soil resources than native vegetation especially under high nutrient conditions (Monaco et al., 2003; James, 2008a; b; Young and Mangold, 2008).


When first moving into an area, T. caput-medusae tends to first replace more desirable annual grasses and forbs (Sharp et al., 1957). In California, T. caput-medusae can invade grasslands, oak savannah, oak woodland, and chaparral communities (Young, 1992). In Oregon and Idaho, T. caput-medusae often replaces Bromus tectorum (Hironaka, 1961; McKell et al., 1962b; Young and Evans, 1970; Dahl and Tisdale, 1975). In the Great Basin, established perennial bunchgrasses are the most important functional group to prevent successful establishment of T. caput-medusae (Davies, 2008; Sheley and James, 2010). T. caput-medusae responds well to disturbance and is quick to establish and spread on sites following disturbance such as wildfire (D'Antonio and Vitousek, 1992; Maret and Wilson, 2005).

In the Pacific Northwest, T. caput-medusae may be associated with forbs such as Centaurea solstitialis, Achillea millefolium, and Balsamorhiza sagittata (Bell et al., 1992; Sheley et al., 1993; Roche et al., 1994; Archer, 2001). T. caput-medusae can also replace the understory of Quercus garryana /Toxicodendron diversilobum and Pinus ponderosa or Juniperus occidentalis forests in northern California and western Oregon (Young and Evans, 1970; Smith, 1985).



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BS - Steppe climate Preferred > 430mm and < 860mm annual precipitation
BW - Desert climate Tolerated < 430mm annual precipitation
Cs - Warm temperate climate with dry summer Preferred Warm average temp. > 10°C, Cold average temp. > 0°C, dry summers


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ParameterLower limitUpper limitDescription
Mean annual rainfall2501270mm; lower/upper limits

Rainfall Regime

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Soil Tolerances

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Soil drainage

  • free
  • impeded

Soil reaction

  • acid
  • neutral

Soil texture

  • heavy
  • medium

Natural enemies

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Natural enemyTypeLife stagesSpecificityReferencesBiological control inBiological control on
Fusarium arthrosporioides Pathogen Leaves not specific
Pseudomonas fluorescens Pathogen Whole plant not specific
Ustilago phrygica Pathogen Seeds not specific

Means of Movement and Dispersal

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Natural Dispersal

Seeds generally disperse relatively short distances in the absence of large animal, human or vehicle dispersal. Seventy-five percent of seeds land 0.5 m from the invasion front and most of the remaining seeds disperse no further than 2 m from the invasion front (Davies, 2008).

Accidental Introduction

Seeds have long awns covered in small silica barbs which facilitate adhesion to animals, clothing and vehicles (Nafus and Davies, 2014). Small mammals may move seeds into new areas and plant them in caches (Miller, 1996). Roadways are a primary vector of T. caput-medusae spread, both because vehicles are an important vector of spread and because roadside areas are conducive to T. caput-medusae establishment (Davies et al., 2013).  

Pathway Causes

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CauseNotesLong DistanceLocalReferences
Animal productionAccidental Yes Yes Kyser et al., 2014
Hitchhiker Yes Yes Kyser et al., 2014

Pathway Vectors

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VectorNotesLong DistanceLocalReferences
Clothing, footwear and possessionsSeeds get trapped in clothing/footwear Yes Kyser et al., 2014
Land vehicles Yes Yes Nafus and Davies, 2014
Livestock Yes Yes Nafus and Davies, 2014

Impact Summary

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Economic/livelihood Negative
Environment (generally) Negative

Economic Impact

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T. caput-medusae spp. asperum is of little value to livestock production or wildlife. Infestations can displace native vegetation leading to lower productivity and increased management costs (Kyser et al., 2014). Species with spiny awns, such as T. caput-medusae, can cause eye and mouth injuries to livestock and wildlife species (Currie et al., 1987). Dense infestations can reduce livestock grazing capacity on rangelands by 50-80% and severely degrade wildlife habitat which can have significant economic costs for ranchers and rural communities (Hironaka, 1961; Goebel et al., 1988; Kyser et al., 2014). Invasions of annual grasses such as T. caput-medusae, are associated with increased wildfire frequency especially in areas with higher human traffic and can necessitate increased wildfire suppression. Wildfire suppression in the USA cost over a billion US Dollars annually in four out of seven years (1999-2006) (Gebert et al., 2008). 

Environmental Impact

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Impact on Habitats

T. caput-medusae spp. asperum is estimated to have invaded at least one million hectares in the western USA and is believed to be spreading at a rate of 12% per year (Rice, 2005). It is considered to be an ecosystem transformer species (Richardson et al., 2000) which means that it transforms ecosystem functions to favour its own survival and propagation at the expense of more desirable species (Kyser et al., 2014). T. caput-medusae produces a heavy thatch layer that favors T. caput-medusae establishment and germination and decreases germination and establishment of desirable rangeland species (Evans and Young, 1970; Davies and Svejcar, 2008). The thatch layer also leads to increased fire frequency which is detrimental to the less fire-adapted native vegetation species and is beneficial to further T. caput-medusae dominance. This leads to a grass-fire cycle that promotes T. caput-medusae dominance to the detriment of native vegetation (D'Antonio and Vitousek, 1992).

Impact on Biodiversity

T. caput-medusae is associated with decreases in native vegetation and biodiversity (Young, 1992; Davies and Svejcar, 2008; Davies, 2011). Heavily invaded plant communities produce only 13% of the native plant biomass of uninvaded communities (Davies and Svejcar, 2008). Potentially threatened species such as sagegrouse (Centrocercus urophasianus) are threatened by loss of habitat (Connelly et al., 2000) caused, in part, by T. caput-medusae invasion (Davies and Svejcar, 2008). T. caput-medusae seedlings are highly competitive against native bunchgrass species, especially at the seedling state, in part, because T. caput-medusae seedlings are able to more rapidly uptake soil nutrients and have higher growth rates than native species, even in low nutrient conditions (Monaco et al., 2003; James, 2008b; a; Young and Mangold, 2008; Mangla et al., 2011).

Risk and Impact Factors

Top of page Invasiveness
  • Proved invasive outside its native range
  • Has a broad native range
  • Highly adaptable to different environments
  • Tolerates, or benefits from, cultivation, browsing pressure, mutilation, fire etc
  • Pioneering in disturbed areas
  • Has propagules that can remain viable for more than one year
Impact outcomes
  • Ecosystem change/ habitat alteration
  • Modification of fire regime
  • Modification of successional patterns
  • Monoculture formation
  • Negatively impacts agriculture
  • Negatively impacts animal health
  • Negatively impacts livelihoods
  • Reduced native biodiversity
  • Threat to/ loss of endangered species
  • Threat to/ loss of native species
Impact mechanisms
  • Competition - monopolizing resources

Similarities to Other Species/Conditions

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Taeniatherum, of which the three subspecies of T. caput-medusae are the only members, is a distinct genus which most closely resembles species of Elymus, Hordelymus and Hordeum. It differs from Elymus spp. by being an annual and by having a one-flowered spikelet with connate, subulate glumes. Taeniatherum differs from Hordelymus spp. as it is an annual with a sessile spikelet with connate glumes and flattened lemma awns. Taeniatherum differs from Hordeum spp. by having a tough rachis, a rigid spike with a terminal spikelet and the sessile spikelet occurring in pairs (Frederiksen, 1986). The two most similar species to T. caput-medusae in the intermountain western USA are Hordeum jubatum and Elymus elymoides, which both have long awned seeds that arise from the seedhead in a manner somewhat similar in appearance to that of T. caput-medusae. However, unlike H. jubatum and E. elymoides, T. caput-medusae is an annual species with shallow roots and seedheads that do not disarticulate at maturity. 

Prevention and Control

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Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.


Prevention is the single best use of resources and the most effective management strategy for T. caput-medusae (Cal-IPC, 2012; Nafus and Davies, 2014). Effective prevention programs consist of two main strategies: 1) prevent T. caput-medusae from reaching a site; and 2) promote biotic resistance of the desired plant community so that T. caput-medusae seeds that reach the site are less likely to successfully establish (Nafus and Davies, 2014).

Preventing introduction of seed: Vehicles and equipment, especially agricultural, construction and fire-fighting equipment, should be cleaned prior to moving from infested areas. Field workers should remove seeds from their clothing prior to leaving infested areas. Seeds are most commonly found in socks, trouser cuffs, shoelaces and shoe eyelets (Kyser et al., 2014). Transporting livestock from infested sites to uninvaded sites should be minimized, especially during the summer months when T. caput-medusae seeds are actively dispersing (Davies, 2008). When livestock transport is necessary, livestock should be contained for a few days to shed seeds and containment areas should be monitored periodically to prevent infestation (Kyser et al., 2014).

Maintain or increase biotic resistance: Proper grazing management is important for maintaining community biotic resistance. Heavy spring grazing in low-elevation rangeland can reduce seed production of more desirable annual grasses and should, therefore, be avoided just before or while more desirable species are flowering (Kyser et al., 2014). Heavy spring grazing in high-elevation rangelands can reduce vigour of perennial bunchgrasses. 

Early warning systems

In management of invasive weed species, early detection is critical to minimizing the development of new infestations and to reduce costs of future eradication and treatment (Sheley et al., 2003; Davies and Johnson, 2011).  A monitoring system to detect new infestations should be concentrated in locations at highest risk of invasion especially along roads and animal trails, facilities, animal staging sites, recently disturbed areas and uninvaded areas near existing infestations (Davies et al., 2013; Kyser et al., 2014).

Rapid response

When new infestations are identified, pulling, hoeing or chemical spot-treating should be done as soon as possible. If seeds may be viable, plants should be bagged, removed from the site and disposed of properly. The site should be GPS marked so that continued monitoring can be performed (Kyser et al., 2014).

Public awareness

It is important to educate ranchers, management agency personnel, recreational users (e.g.: hunters, hikers, campers, etc) and work crews (e.g.: fire crews, construction workers) on how to identify T. caput-medusae, the potential environmental and economic ramifications of invasion, how to prevent seed spread and how to report newly detected infestations (Kyser et al., 2014). Unfortunately, people may be less responsive to education unless they have had direct negative interactions with invasive species. Johnson et al. (2011) found that ranchers were less likely to take preventative action or attend educational opportunities to increase knowledge of invasive species if they did not already have direct experience with invasive species.

Cultural control

Although T. caput-meusae recovers well after fire (D'Antonio and Vitousek, 1992), under certain conditions, prescribed burning can be an effective tool for T. caput-medusae control. In low-elevation annual grasslands, later T. caput-medusae seed production relative to more desirable annual grasses allows for selective burning to reduce T. caput-medusae and favour more desirable species (Meyer and Schiffman, 1999; Kyser et al., 2008). Fuel loading, preburn species composition, fire characteristics, and weather conditions during and after the burn can all affect burn effectiveness (Harrison et al., 2003; Kyser et al., 2008). A slow burn that maximizes heat intensity performed during the late dough stage before seedheads mature can provide a temporary reduction in viable T. caput-medusae seeds (Sharp et al., 1957; McKell et al., 1962b; Pollak and Kan, 1998; Sheley et al., 2007). Burning after desirable annual grass seed drop and before T. caput-medusae seed drop is necessary as seed mortality is greater when burned prior to seed drop because seeds on the soil surface are less likely to receive sufficient heat to exceed the mortality threshold than are seeds still in the inflorescence (Sweet et al., 2008; Kyser et al., 2014).

In low elevation, warm winter areas, high annual grass fuel loads are more conducive to the high intensity burning necessary to achieve successful T. caput-medusae control than in semiarid, cold-winter areas of the intermountain region. In the intermountain region, burns must often be conducted later in the season when perennial bunchgrasses are dry enough to carry fire. Late season burning and insufficient combustible biomass may make it difficult to achieve satisfactory control in these areas (Kyser et al., 2008). In these ecosystems, burning T. caput-medusae, especially after seed maturation, generally only serves to increase T. caput-medusae dominance unless further treatments are applied (Young et al., 1972; Maret and Wilson, 2005; Davies, 2010; Davies and Sheley, 2011). Because burning reduces the thatch layer, any burn, even accidental burns, should be utilized as an opportunity to apply further control methods such as grazing, revegetation and preemergence herbicides which often have increased efficacy following thatch removal.

Physical/mechanical control

Mechanical control is more effective in the low-elevation annual grasslands than in the Great Basin. In the low-elevation annual grasslands mowing is most effective if used in the late spring after desirable species have set seed and before T. caput-medusae has produced viable seed (Kyser et al., 2014). Fuel costs and rocky or steep terrain may limit the use of mowers, and rocks striking the mower blade may produce sparks which can start fires (Mattise and Scholten, 1994; Kyser et al., 2014). Deep tillage using disks is used to bury T. caput-medusae seeds, control existing plants and remove thatch in preparation for herbicide application and should be followed by reseeding with more desirable species (Young, 1992; Kyser et al., 2014). Shallow tillage, using a harrow, is similarly used to remove thatch. In contrast, however, use of a harrow does not control existing T. caput-medusae plants. Deep tillage is impractical in most locations due to rocky soil, slopes and shrub and tree presence. Deep tillage can also increase the risk of soil erosion and cause reductions in soil moisture, organic matter and biological soil crusts as well as damaging remnant vegetation.  Shallow tillage causes less soil damage than deep tillage and can be used on rockier terrain (Kyser et al., 2014). Spring ploughing, disk tillage, harrow tillage, mowing or raking can reduce T. caput-medusae by 65-95% for up to a year following treatment. However, T. caput-medusae can quickly reinvade if subsequent treatment actions are not taken (Harwood, 1960; Young et al., 1969; Kyser et al., 2007; Cox and Allen, 2008).

Mechanical treatments are not recommended in high-elevation Artemisia communities since these activities typically favour exotic annual species (Davies et al., 2011; 2012) and may be destructive to soil and remnant native plants (Mattise and Scholten, 1994; Pierson et al., 2007).

Movement control

Most T. caput-medusae seeds do not self-disperse more than 2 m from the parent plant. Planting competitive taller vegetation such as Agropyron cristatum in 6-m containment barriers around infestations can prevent up to 98% of T. caput-medusae seeds from moving out of infested areas (Davies et al., 2010). Once the barriers are established, vehicle, human and livestock travel from infested to uninfested areas should be restricted as much as possible, especially during seed dispersal (Davies et al., 2010; Nafus and Davies, 2014).

Biological control

Biological control agents are not currently commercially available for T. caput-medusae. Fungi that successfully reduced seed production of T. caput-medusae were also detrimental to desirable native vegetation and/or seed grain (Grey et al., 1995; Siegwart et al., 2003; Berner et al., 2007). A rhizobacterium, Pseudomonasfluorescens strain D7 (Pf D7), was successful at reducing T. caput-medusae, Bromus tectorum and Aegilops cylindrica density in the laboratory (Kennedy et al., 2001). Field trials are currently ongoing and, three years after application, have shown promising success at reducing target species without negatively affecting more desirable species.

Chemical control

Pre-emergent and post-emergent herbicides have been used to successfully reduce T. caput-medusae. Glyphosate, a post-emergent, nonselective, foliar applied herbicide has been used to successfully treat T. caput-medusae in the Great Basin without significantly harming non-target species. It is most successful if applied at low rates during the tillering stage of T. caput-medusae (Kyser et al., 2012). In low-elevation annual rangelands, it can be applied at higher rates during T. caput-medusae’s early flowering stage so that more desirable annual grasses have already successfully produced seed (Kyser et al., 2014).

There are a number of pre-emergent herbicides that have been successfully used to reduce T. caput-medusae. Soil type, moisture availability and the amount of thatch can affect the success of control efforts. Burning prior to herbicide application can improve herbicide efficacy when using Imazapic. 

Risks of improperly used herbicides include water contamination, spray or vapour drift to non-target areas, water contamination, human and animal toxicity, herbicide resistant weed selection and decreased vegetation diversity (Kyser et al., 2014). To minimize these risks, herbicides should always be applied following label directions in appropriate weather conditions.

Control by utilization

Timing and intensity are important when using grazing to control T. caput medusae (DiTomaso et al., 2008; Sheley et al., 2008; Sheley and Svejcar, 2009). In low-elevation annual grasslands, high intensity, short-duration grazing done in the late spring (April/May) after T. caput-medusae stem elongation but before the seed milk stage reduced seed production by 86-100% and eventually reduce the seedbank (Lusk et al., 1961; DiTomaso et al., 2008; DiTomaso and Smith, 2012; Kyser et al., 2014). Early spring (March) or autumn (October/November) grazing did not reduce T. caput-medusae cover (DiTomaso et al., 2008) and year round grazing was associated with greater medusahead frequency (Harrison et al., 2003). James et al. (2015) report that grazing is the preferred control method among stakeholders, and that on annual rangeland an almost twofold reduction in medusahead abundance was achieved by timing high stocking rates with phenological stages when the plant is most susceptible to defoliation.

Integrated management

Combining treatment methods tends to increase success of T. caput-medusae control efforts (Monaco et al., 2005; Kyser et al., 2007; Davies, 2010; Davies and Sheley, 2011). Removing the thatch layer via fire, mechanical methods or grazing prior to herbicide application not only reduced the amount of herbicide necessary for successful control, but improved success of T. caput-medusae control, increased the longevity of T. caput-medusae suppression and improved bunchgrass establishment compared to using any of the treatments alone (Kyser et al., 2007; Sheley et al., 2007; Davies and Sheley, 2011; Sheley et al., 2012a). Removing thatch prior to sheep grazing improved T. caput-medsae control (Lusk et al., 1961). James et al. (2015) review the effects of combinations of herbicide, burning, seeding and grazing on rangeland dominated by either annual or perennial vegetation.

Ecosystem restoration

Seeding desirable species is generally a critical component of successful T. caput-medusae control efforts (Seabloom et al., 2003). In nearly all cases, revegetation to restore ecosystem services must be preceded by T. caput-medusae control prior to seeding of desirable species. However, even if T. caput-medusae control is successful, revegetation of invaded plant communities is expensive and often unsuccessful, especially when using native perennial species (James et al., 2011). In order to reduce reinvasion, seeded species, such as perennial bunchgrasses, must be functionally similar to T. caput-medusae in their resource acquisition patterns to best limit the resources available to T. caput-medusase (Davies, 2008; James, 2008a; b; Nafus and Davies, 2014). Successfully establishing seeded species can be difficult as T. caput-medusae is more competitive than perennial bunchgrasses, especially during the seedling stage (Harris and Wilson, 1970; Hironaka and Sindelar, 1973; Young and Mangold, 2008). Therefore, it is critical to successfully control T. caput-medusae prior to seeding more desirable species (Davies, 2010; Nafus and Davies, 2014).

Success of revegetation efforts will vary by species, herbicide type, and site characteristics as well as the type and timing of pretreatment actions. In the Great Basin, season of burn and herbicide application can affect success. Autumn herbicide application resulted in higher perennial grass cover than spring application for both imazapic and sulfometuron. Imazapic application resulted in higher perennial grass cover than sulfometuron application (Monaco et al., 2005). Combining pre-emergent herbicide treatment with burning resulted in greater perennial grass cover and plant diversity than either treatment alone, regardless of whether burning occurred in the spring or autumn. Spring burning, however, was associated with slightly higher perennial grass cover while autumn burning was associated with higher perennial forb cover (Davies and Sheley, 2011; Sheley et al., 2012a).

Revegetation can be implemented following treatment by broadcast seeding or by drill seeding. Revegetation efforts using a drill seeding method tend to be more successful than a broadcast seeding method. The success of using broadcasted seeds can be improved by incorporating seeds into the soil using a harrow method (Kyser et al., 2014). When using a pre-emergent herbicide, it is recommended that seeding of desirable species be delayed for one year following herbicide treatment to improve perennial grass establishment, as a single application approach (in which seed and herbicide are applied simultaneously) is associated with higher seedling mortality (Wilson et al., 2010; Davies et al., 2014; Madsen et al., 2014; Davies et al., 2015). The single entry approach, however, may save time and reduce treatment costs on sites where there is sufficient remnant native vegetation and may be more successful when utilized with a post-emergent herbicide, although results are likely to be heterogeneous across large landscapes (Sheley et al., 2012a; Sheley et al., 2012b; Kyser et al., 2014).

In low-elevation annual grasslands, carefully timed burning and grazing can be utilized to favour more desirable species (MacLauchlan et al., 1970; Menke, 1992; Meyer and Schiffman, 1999; DiTomaso et al., 2008; Kyser et al., 2008). Perennial grass responds most positively to early spring, high intensity grazing treatments that reduce annual grass abundance and seed production (Menke, 1992; Stein et al., 2014). Lower grazing intensity may enable limited establishment of more desirable exotic annual grass species (Stein et al., 2014).



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Missouri Botanical Garden, 2015. Tropicos database., St. Louis, Missouri, USA: Missouri Botanical Garden.

USDA-ARS, 2015. Germplasm Resources Information Network (GRIN). Online Database. Beltsville, Maryland, USA: National Germplasm Resources Laboratory.

USDA-NRCS, 2015. The PLANTS Database. Greensboro, North Carolina, USA: National Plant Data Team.

Links to Websites

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GISD/IASPMR: Invasive Alien Species Pathway Management Resource and DAISIE European Invasive Alien Species Gateway source for updated system data added to species habitat list.
Global register of Introduced and Invasive species (GRIIS) source for updated system data added to species habitat list.


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06/04/15 Original text by:

Aleta Nafus, Oregon State University, Corvallis, Oregon,  USA

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