Bromus tectorum (downy brome)
- Summary of Invasiveness
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Plant Type
- Distribution Table
- History of Introduction and Spread
- Risk of Introduction
- Habitat List
- Hosts/Species Affected
- Host Plants and Other Plants Affected
- Biology and Ecology
- Air Temperature
- Rainfall Regime
- Soil Tolerances
- Notes on Natural Enemies
- Means of Movement and Dispersal
- Pathway Vectors
- Plant Trade
- Impact Summary
- Environmental Impact
- Impact: Biodiversity
- Threatened Species
- Social Impact
- Risk and Impact Factors
- Similarities to Other Species/Conditions
- Prevention and Control
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Bromus tectorum L.
Preferred Common Name
- downy brome
Other Scientific Names
- Anisantha tectorum (L.) Nevski
International Common Names
- English: bronco grass; cheat grass; cheatgrass; drooping brome grass; nodding brome grass; six weeks grass
- Spanish: bromo velloso; espiguilla colgante
- French: brome des toits
Local Common Names
- Canada: nodding brome
- Germany: Dach- Trespe
- Italy: forasacco dei tetti
- Japan: umanochahiki
- Netherlands: muurdravik; zwenkdravik
- Sweden: taklosta
- BROTE (Bromus tectorum)
Summary of InvasivenessTop of page B. tectorum has tremendous phenotypic plasticity, with self fertilization permitting continuous replication of successful genotypes, and occasional out-crossing inducing genotypic variability and adaptation to new environments. Seeds have the advantage of simultaneous germination and through dormancy acquired in the seedbed, continuous germination and the development of large, persistent seedbanks. B. tectorum has an excellent long-distance seed dispersal mechanism through contamination of seed lots of crop species and through wool, hair, clothing, and vehicle contamination. B. tectorum is a highly invasive exotic weed that truncates succession to remain a dominant species for prolonged periods of time. B. tectorum is one of the few invasive annual exotic species that is a major weed of rangelands and agronomic fields in North America.
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Plantae
- Phylum: Spermatophyta
- Subphylum: Angiospermae
- Class: Monocotyledonae
- Order: Cyperales
- Family: Poaceae
- Genus: Bromus
- Species: Bromus tectorum
Notes on Taxonomy and NomenclatureTop of page In Russia, Anisantha tectorum (L.) Nevski is used by some authors as the taxon for this species, but Bromus tectorum L. is universally used in publications in English (Kostivkovsky and Young, 2000). If near universal agreement is found on the scientific name, chaos prevails on common names. The Weed Science Society of America has assigned the common name of downy brome for B. tectorum. In the western USA it is widely known as cheatgrass. Other common names in the USA include bronco grass, six weeks grass and military grass. In western Canada it is known as nodding brome (Taylor and MacBryde, 1977). The Latin root for the specific name tectorum refers to the roof in apparent reference to B. tectorum growing on thatched roofs (Kostivkovsky and Young, 2000).
DescriptionTop of page B. tectorum plants have a soft hispid pubescence throughout the leaves and stems. The leaf blades are flat and rather narrow, 2-3.5 mm broad. The stature of the plant and the number of tillers is highly variable. Plant height ranges from 2.5 to 50 cm. In dense stands, 8500 to 11,000 plants per m², B. tectorum may produce a single stem and a single diminutive panicle. In very sparse stands (10 per m²), as would occur the season after a wildfire, B. tectorum plants may reach 0.5 m in height and have a multitude of robust panicles of florets (Young et al., 1987). The panicle ranges from 6 to 20 cm in length and usually, but not always, nodding (Wilken and Painter, 2003). The spikelets are sub-cylindrical to slightly compressed. Glumes generally glabrous, but for all the inflorescence components, the presence and amount is highly variable. The upper glume is 7-12 mm long with three veins. The lower glume is 5-6 mm long with one vein. The florets are 3-7 mm long, the lemma body 9-13 mm long with 5-7 veins and the tip has two teeth 2-3 mm long. The caryopsis is about 2 cm long with a recurved, sharp-tipped awn. The colour of the caryopsis ranges from light straw to reddish brown, and is covered with silicon barbs that aid in attachment to wool, hair and clothing. The awn moves with wetting and drying forcing the sharp point of the caryopses deeper into wool or hair and even into the flesh, eyes, ears or toes of animals. B. tectorum rosettes often have bright red leaves during mid-winter, which apparently is due to nitrogen stress.
Plant TypeTop of page Annual
Grass / sedge
DistributionTop of page B. tectorum is widely distributed in central Asia (Kostivkovsky and Young, 2000), with the western edge of its native range generally given as the Balkans, Europe. The present distribution of B. tectorum in Europe extends to south-western Spain, but west of the Balkans, B. tectorum is considered to be an adventive species. B. tectorum is common in the Middle East and occurs across North Africa in areas with a Mediterranean-type climate but may be considered adventive west of Egypt (Meusel et al., 1965). To the east, the native range extends into China, probably restricted to Xinjiang province. Presence in India (USDA-ARS, 2003) is probably restricted to the north. This species has been so closely associated with winter cereal grain production and range livestock grazing for such an extended period of time, it is difficult to separate native habitat from where it has been introduced in pre-history.
Distribution TableTop of page
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.
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Reference||Notes|
|Afghanistan||Widespread||Native||Not invasive||Kostivkovsky and Young, 2000|
|Azerbaijan||Widespread||Native||Not invasive||Kostivkovsky and Young, 2000|
|China||Restricted distribution||Native||Not invasive||USDA-ARS, 2003|
|Georgia (Republic of)||Present||Native||Not invasive||Kostivkovsky and Young, 2000|
|India||Restricted distribution||Native||USDA-ARS, 2003|
|Iran||Present||Native||Not invasive||Kostivkovsky and Young, 2000|
|Iraq||Present||Native||Not invasive||Kostivkovsky and Young, 2000|
|Israel||Present||Native||Not invasive||Kostivkovsky and Young, 2000|
|Jordan||Present||Native||Not invasive||Kostivkovsky and Young, 2000|
|Kazakhstan||Present||Native||Not invasive||Kostivkovsky and Young, 2000|
|Kuwait||Present||Native||Not invasive||Kostivkovsky and Young, 2000|
|Kyrgyzstan||Present||Native||Not invasive||Kostivkovsky and Young, 2000|
|Lebanon||Present||Native||Not invasive||Kostivkovsky and Young, 2000|
|Pakistan||Present||Native||Not invasive||Kostivkovsky and Young, 2000|
|Saudi Arabia||Present||Native||Not invasive||Kostivkovsky and Young, 2000|
|Syria||Present||Native||Not invasive||Kostivkovsky and Young, 2000|
|Tajikistan||Present||Native||Not invasive||Kostivkovsky and Young, 2000|
|Turkey||Present||Native||Not invasive||Tutin et al., 1980; Kostivkovsky and Young, 2000|
|Turkmenistan||Present||Native||Not invasive||Kostivkovsky and Young, 2000|
|United Arab Emirates||Present||Native||Not invasive||Kostivkovsky and Young, 2000|
|Uzbekistan||Present||Native||Not invasive||Kostivkovsky and Young, 2000|
|Algeria||Present||Not invasive||Meusel et al., 1965|
|Egypt||Present||Native||Not invasive||Meusel et al., 1965|
|Libya||Present||Introduced||Not invasive||Meusel et al., 1965|
|Morocco||Present||Introduced||Not invasive||Meusel et al., 1965|
|South Africa||Present||Introduced||USDA-ARS, 2003|
|-Canary Islands||Present||Introduced||USDA-ARS, 2003|
|Canada||Present||Present based on regional distribution.|
|-Alberta||Present||Introduced||Invasive||Cronquist et al., 1977|
|-British Columbia||Widespread||Introduced||Invasive||Cronquist et al., 1977|
|-Manitoba||Present||Introduced||Invasive||Cronquist et al., 1977|
|-Saskatchewan||Present||Introduced||Invasive||Cronquist et al., 1977|
|USA||Present||Present based on regional distribution.|
|-Alabama||Present||Introduced||Not invasive||Hitchcock, 1950|
|-Alaska||Present||Introduced||Not invasive||Cronquist et al., 1977|
|-Connecticut||Present||Introduced||Not invasive||Hitchcock, 1950|
|-Delaware||Present||Introduced||Not invasive||Hitchcock, 1950|
|-Florida||Present||Introduced||Not invasive||Cronquist, 1977|
|-Georgia||Present||Introduced||Not invasive||Hitchcock, 1950|
|-Illinois||Present||Introduced||Not invasive||Hitchcock, 1950|
|-Indiana||Present||Introduced||Not invasive||Hitchcock, 1950|
|-Iowa||Present||Introduced||Not invasive||Hitchcock, 1950|
|-Kentucky||Present||Introduced||Not invasive||Hitchcock, 1950|
|-Louisiana||Present||Introduced||Not invasive||Hitchcock, 1950|
|-Maine||Present||Introduced||Not invasive||Hitchcock, 1950|
|-Maryland||Present||Introduced||Not invasive||Hitchcock, 1950|
|-Massachusetts||Present||Introduced||Not invasive||Hitchcock, 1950|
|-Michigan||Present||Introduced||Not invasive||Hitchcock, 1950|
|-Minnesota||Present||Introduced||Not invasive||Hitchcock, 1950|
|-Mississippi||Present||Introduced||Not invasive||Hitchcock, 1950|
|-Missouri||Present||Introduced||Not invasive||Hitchcock, 1950|
|-New Hampshire||Present||Introduced||Not invasive||Hitchcock, 1950|
|-New Jersey||Present||Introduced||Not invasive||Hitchcock, 1950|
|-New Mexico||Present||Introduced||Invasive||Hitchcock, 1950|
|-New York||Present||Introduced||Not invasive||Hitchcock, 1950|
|-North Carolina||Present||Introduced||Not invasive||Hitchcock, 1950|
|-North Dakota||Present||Introduced||Invasive||Hitchcock, 1950|
|-Ohio||Present||Introduced||Not invasive||Hitchcock, 1950|
|-Rhode Island||Present||Introduced||Not invasive||Hitchcock, 1950|
|-South Carolina||Present||Introduced||Not invasive||Hitchcock, 1950|
|-South Dakota||Present||Introduced||Invasive||Hitchcock, 1950|
|-Tennessee||Present||Introduced||Not invasive||Hitchcock, 1950|
|-Vermont||Present||Introduced||Not invasive||Hitchcock, 1950|
|-Virginia||Present||Introduced||Not invasive||Hitchcock, 1950|
|-West Virginia||Present||Introduced||Not invasive||Hitchcock, 1950|
|-Wisconsin||Present||Introduced||Not invasive||Hitchcock, 1950|
|Albania||Present||Native||Not invasive||Tutin et al., 1980|
|Austria||Present||Introduced||Not invasive||Tutin et al., 1980|
|Belgium||Present||Introduced||Not invasive||Tutin et al., 1980|
|Bosnia-Hercegovina||Present||Native||Not invasive||Tutin et al., 1980|
|Bulgaria||Present||Native||Not invasive||Tutin et al., 1980|
|Croatia||Present||Native||Not invasive||Tutin et al., 1980|
|Cyprus||Present||Native||Not invasive||Tutin et al., 1980|
|Czech Republic||Present||Native||Not invasive||Tutin et al., 1980|
|Denmark||Present||Introduced||Not invasive||Tutin et al., 1980|
|Estonia||Present||Introduced||Not invasive||Tutin et al., 1980|
|Finland||Present||Introduced||Not invasive||Tutin et al., 1980|
|France||Present||Introduced||Not invasive||Tutin et al., 1980|
|-Corsica||Present||Introduced||Not invasive||Tutin et al., 1980|
|Germany||Present||Introduced||Not invasive||Tutin et al., 1980|
|Gibraltar||Present||Introduced||Not invasive||Tutin et al., 1980|
|Greece||Present||Native||Not invasive||Tutin et al., 1980|
|Hungary||Present||Native||Not invasive||Tutin et al., 1980|
|Italy||Present||Introduced||Not invasive||Tutin et al., 1980|
|Latvia||Present||Introduced||Not invasive||Tutin et al., 1980|
|Liechtenstein||Present||Introduced||Not invasive||Tutin et al., 1980|
|Lithuania||Present||Introduced||Not invasive||Tutin et al., 1980|
|Luxembourg||Present||Introduced||Not invasive||Tutin et al., 1980|
|Macedonia||Present||Native||Not invasive||Tutin et al., 1980|
|Monaco||Present||Introduced||Not invasive||Tutin et al., 1980|
|Netherlands||Present||Introduced||Not invasive||Tutin et al., 1980|
|Norway||Present||Introduced||Not invasive||Tutin et al., 1980|
|Poland||Present||Introduced||Not invasive||Tutin et al., 1980|
|Portugal||Present||Introduced||Not invasive||Tutin et al., 1980|
|Romania||Present||Native||Not invasive||Tutin et al., 1980|
|Russian Federation||Present||Present based on regional distribution.|
|-Central Russia||Present||Native||Not invasive||Tutin et al., 1980; Kostivkovsky and Young, 2000|
|-Southern Russia||Present||Native||Not invasive||Tutin et al., 1980; Kostivkovsky and Young, 2000|
|-Western Siberia||Present||Native||Not invasive||Tutin et al., 1980; Kostivkovsky and Young, 2000|
|Serbia||Present||Native||Not invasive||Tutin et al., 1980|
|Slovakia||Present||Native||Not invasive||Tutin et al., 1980|
|Slovenia||Present||Introduced||Not invasive||Tutin et al., 1980|
|Spain||Present||Introduced||Not invasive||Tutin et al., 1980|
|Sweden||Present||Introduced||Not invasive||Tutin et al., 1980|
|Switzerland||Present||Introduced||Not invasive||Tutin et al., 1980|
|UK||Present||Introduced||Not invasive||Tutin et al., 1980|
|Ukraine||Present||Native||Not invasive||Tutin et al., 1980|
|Yugoslavia (former)||Present||Native||Not invasive||Tutin et al., 1980|
|Australia||Restricted distribution||Introduced||Royal Botanic Gardens Sydney, 2003; USDA-ARS, 2003|
|-New South Wales||Present||Introduced||Royal Botanic Gardens Sydney, 2003|
|-Tasmania||Present||Introduced||Royal Botanic Gardens Sydney, 2003|
|-Victoria||Present||Introduced||Royal Botanic Gardens Sydney, 2003|
|New Zealand||Present||Introduced||1870||Forde and Edgar, 1995; Owen, 1996; USDA-ARS, 2003|
History of Introduction and SpreadTop of page Based on herbarium specimens in the USA, B. tectorum was first collected in Pennsylvania in 1861, Washington in 1893, Utah 1894, Colorado in 1895, and Wyoming in 1900 (Yensen, 1981). In North America in 1950, B. tectorum occurred in Alberta, Canada, Mexico, and all the mainland USA except for the far south-eastern portion of the country (Hitchcock, 1950). Today, it is found in all states including Alaska and all of the Canadian Prairie Provinces (Cronquist et al., 1977). It has also spread into northern and western Europe and North Africa. B. tectorum was first noted as an exotic species in New Zealand in 1870 (Owen, 1996), and it is also present in parts of Australia and South Africa though dates of introduction are not known. USDA-ARS (2003) notes presence in South America but without specific locations and no other reports could be located.
The history of introduction in North America is a comprehensive one. The completion of the trans-continental railroad in 1868 and the subsequent development of regional railroad networks greatly enhanced the rate of spread of exotic weed species in western North America (Young and Longland, 1996). The widespread adoption of steam powered grain threshing equipment which was moved from farm to farm was disastrous for spreading weeds. They were not cleaned between farms and the farmers saved their own grain, often contaminated with their neighbours weeds, for seed grain (Morrow and Stahlman, 1984). This process has been documented for the spread of the exotic annual Russian thistle (Salsola targus) (Young, 1988). The search for winter hardy strains of alfalfa in the late 1800s led to widespread importation of seed from central Asia under the general name of 'Turkestan' seed. These unregulated, uninspected importations were probably significant contributors to the exotic weed flora of western North America.
Yensen (1981) recreated the historical developmental of B. tectorum for southern Idaho from it being first noted as a weed in cereal grain and alfalfa fields in the early 1900s. Rural roads were little more than dirt tracks throughout the big sagebrush steppe and B. tectorum spread along these roads as a ruderal species. Periodic grading of the road surface with soil brought up from boarding burrow pits assured sufficient disturbance to provide habitats for exotic invasive weeds. After a considerable lag period as a strict ruderal species, B. tectorum was suddenly noticed to be invading degraded stands of big sagebrush where the native perennial grasses had been killed by excessive, improperly timed, and continuous grazing. When the native perennial grasses had been killed, the density of big sagebrush, which is not preferred as a browse source by domestic livestock, increased, effectively closing the sites to establishment of perennial grass seedlings even if grazing was excluded (Robertson and Pearse, 1947). B. tectorum invaded these brush stands that had virtually no herbaceous understorey and provided the fine-textured fuel with sufficient continuity of cover to ignite and allow wildfires to spread from shrub to shrub. B. tectorum populations exploded in the burned areas and effectively truncated plant succession to continued dominance by the exotic annual weeds. The agricultural economic depression that occurred after World War I brought the abandonment of many sub-marginal farms in the intermountain area of the USA. These abandoned farms were rapidly colonized by invasive exotic annual weeds which led to B. tectorum dominance. The only break in this process during the first half of the 1900s in the western USA was that the ranges were so excessively grazed that herbivory by domestic animals served to biologically suppress annual grasses (Emmerich et al., 1993).
B. tectorum on rangelands was largely confined to the big sagebrush zone from its introduction until the 1980s. This zone is characterized by annual precipitation of 200-350 mm and loam-textured surface soils that are not affected by accumulations of soluble salts. During the 1980s, B. tectorum suddenly spread to the salt deserts of the intermountain area (Young and Tipton, 1990). As the name implies, these areas are characterized by salt-affected soils and annual precipitation of 100-150 mm. The spread of B. tectorum into these areas made possible wildfires as a stand renewal process for the first time. At about the same time, B. tectorum also spread into higher elevation, higher precipitation coniferous woodlands. Within the sagebrush zone, B. tectorum became a much more dominant species as grazing management systems were implemented that included rotational deferment until after seed ripening or a complete year-long rest from grazing. This abundance of B. tectorum fuel led to the occurrence of regional firestorms that burned huge areas of rangelands in a very short period of time.
Risk of IntroductionTop of page Planting contaminated seed and feeding contaminated hay or grain to livestock are common means of dispersal of B. tectorum. In most of the USA, B. tectorum is not a regulated noxious weed meaning that it can occur in seed lots as long as it does not exceed the 'other weed species' limit, applying even to certified seed lots. As such, it is likely to spread to other areas where it is not yet present as a seed contaminant. For example, the revegetation of the right-of-way for a natural gas pipe line in Alberta, Canada by seeding certified crested wheatgrass seed grown in Idaho, USA, resulted in a strip of B. tectorum across the south-eastern Alberta prairie. The transactions involved in purchasing the seed were all perfectly legal because B. tectorum fell under the 'other weed species' category in the USA. Even if you purchase certified seed, it is worthwhile to have the seed sampled to identify the non-legally noxious weed seeds that may contaminate the seed lot.
HabitatTop of page It is an opportunistic species with few absolutely restrictive requirements. B. tectorum is such a widespread species you should not be surprised where one may encounter the species, even in your socks (WSSA, 2003). B. tectorum is an exotic invasive annual grass in North America (Klemmedson and Smith, 1964). In the semi-arid to arid environments of western North America it is found in environments similar to central Asia where it originally evolved. B. tectorum is often described as a classic ruderal species found along roads, fence lines, and irrigation structures. B. tectorum currently dominates millions of hectares of degraded rangelands in the intermountain area between the Sierra-Cascade and Rocky Mountains in western North America. The bulk of this area was formerly big sagebrush (Artemisia tridentata)/native perennial bunch (caespitose) grass plant communities (Hull and Pechanec, 1947). B. tectorum dominance ranges from portions of the salt desert shrub steppe through the big sagebrush/bunchgrass zone to coniferous woodlands and even to true native perennial grasslands. Generally, the habitat of B. tectorum is described as disturbed plant communities, Daubenmire (1940) conclusively demonstrated that this exotic annual grass could invade high ecological condition bluebunch wheatgrass (Pseudoroegneria spicatum) communities that had always been protected by grazing by large herbivores.
Habitat ListTop of page
|Cultivated / agricultural land||Present, no further details||Harmful (pest or invasive)|
|Disturbed areas||Present, no further details||Harmful (pest or invasive)|
|Managed forests, plantations and orchards||Present, no further details||Harmful (pest or invasive)|
|Managed grasslands (grazing systems)||Present, no further details||Harmful (pest or invasive)|
|Rail / roadsides||Present, no further details||Harmful (pest or invasive)|
|Urban / peri-urban areas||Present, no further details||Harmful (pest or invasive)|
|Deserts||Present, no further details||Harmful (pest or invasive)|
|Natural forests||Present, no further details||Harmful (pest or invasive)|
|Natural grasslands||Present, no further details||Harmful (pest or invasive)|
|Riverbanks||Present, no further details||Harmful (pest or invasive)|
Hosts/Species AffectedTop of page It is a very serious agronomic weed in winter wheat (Triticum aestivum) fields of the Pacific Northwest and the central and southern Great Plains of the USA (Young et al., 1984). Although the crop and season of planting are the same, the Pacific Northwest and the Great Plains of the USA have grossly different climates. The weed causes tremendous yield losses in winter wheat and barley (Hordeum vulgare) production and is a spring weed problem in alfalfa (Medicago sativa) hay production. It is a very troublesome species in bluegrass (Poa pratensis) seed production fields. B. tectorum is also a serious problem on rangelands, and often encountered as a weed of orchards and vineyards.
Host Plants and Other Plants AffectedTop of page
Biology and EcologyTop of page Reproductive Biology
The breeding system of B. tectorum is apparently an example of that theorized by Allard (1965) for largely self-pollinated species of annual grasses that are environmentally conditioned to occasional out-crossing. Under this breeding system, if an individual B. tectorum plant is introduced to a site where genotypically it is a good fit to the environmental potential of the site, it populates the site with offspring with stable duplicates of the desirable genotype through self fertilization. A healthy amplitude for phenotypic plasticity helps this one-fits-all genotype to be very successful. Occasionally, environmental conditions would be adequate to allow these self-pollinated species to cross-pollinate and produce hybrid offspring (Allard, 1965). In applying this concept to B. tectorum, Young and Evans (1976) took an additional step and suggested that for the first generation after hybridization, the offspring would express heterosis because essentially two inbred lines selected for survival in the environment of residency were being crossed. The population density of B. tectorum on a range site that has not burned for several years is 5000-10,000 plants per m² (Young et al., 1969), but which may shrink to 10 plants per m² following a wildfire. The reduction in herbaceous plant density coupled with the loss of perennial shrubs after fire, combine to enable each B. tectorum plant to have a much greater potential to spread and the resulting B. tectorum plants are huge in comparison to those that existed in dense stands before the wildfire. The post-fire B. tectorum plants produce multiple tillers that extend flowering over a prolonged period and the decreased plant density allows a greater amount of soil moisture per B. tectorum plant. Improved moisture relations, which normally limits B. tectorum growth, increases the chances that the floret will be sufficiently open to allow the anthers to be exerted.
Physiology and Phenology
B. tectorum can either be a true annual with germination in early spring and reaching maturity in the early summer of the same year, or a winter annual with germination in the autumn, over-wintering as a flat rosette of leaves on the soil surface and sending up flowering tillers the next spring (Harris, 1967). In the Pacific Northwest, USA, autumn germination occurs every year and B. tectorum is a true winter annual which is why it is so competitive with winter wheat. In the Great Basin, USA, B. tectorum germinates in the autumn about once every 5 years. With either autumn or early spring germination, B. tectorum tillers begin rapid elongation from mid-April to mid-May. Most accessions of B. tectorum plants require vernalization before they will flower (Hulbert, 1955; Finnerty and Klingman, 1962), achieved by germinating seeds at 5°C. B. tectorum plants in the field will always flower if soil moisture is available; and most accessions collected from salt desert environments will flower in the greenhouse without vernalization. Phenology of B. tectorum plants is extremely variable depending on field environmental conditions. Flowering can initiate as early as late April or as late as early July on the same site in different years.
B. tectorum seeds are initially not dormant at maturity, or may have short-term after-ripening requirements (Young et al., 1969; Milby and Johnson, 1987). Only a small portion of the annual seed production is required to provide plants that completely occupy a given area the next season. B. tectorum seeds that do not find safe sites for germination acquire a dormancy that permits the building of seedbanks and it is the development of these seedbanks that makes the control of B. tectorum such a prolonged and difficult problem. The acquired seed dormancy breaks down gradually over 3-5 years. Germination of seeds with the acquired dormancy can be enhanced by enrichment of the germination substrate with nitrate or gibberellin. B. tectorum seeds have specific requirements for safe sites for germination. The seeds have very low germination rates on the surface of seedbeds in semi-arid or arid environments. Litter coverage or micro-topography in the seedbed surface is required for successful germination under field conditions (Evans and Young, 1970, 1972). B. tectorum seeds can germinate at very cold seedbed temperatures (Evans et al., 1970) and germination will occur at a constant 0°C or alternating temperatures to as low as 0°C (Young and Evans, 1982).
Very small amounts of nitrogen have large influences on the dynamics of B. tectorum populations. Fertilization with ammonium sulphate over a stand of established perennial grasses, with B. tectorum as a component of the community, can result in the death of the perennials as B. tectorum out-competes the perennials for soil moisture (Kay and Evans, 1965). Fertilization of B. tectorum stands without perennial grasses can result in extreme increases in herbage production (Kay, 1966). Immobilization of nitrogen with a carbon source or the inhibition of nitrification severely decreases the establishment and growth of B. tectorum populations (Young et al., 1998a). This aspect of B. tectorum physiology has not been developed into a commercially successful control measure, but the reciprocal (fertilization with nitrogen) certainly enters into the management of this species. In the reclamation of mining spoils in the USA, it appears to be desirable to add nitrogen fertilizers to raw rock dump spoils but such fertilization will always create a B. tectorum problem (Young et al., 1998b).
B. tectorum has successfully invaded so many contrasting environments in North America it is obviously a generalist in terms of environmental requirements. This is accomplished through great phenotypic plasticity and secondly through the apparent potential to rapidly evolve new genotypes. Generally, B. tectorum thrives as a winter annual, but germination in the autumn is not essential for the annual grass to persist. It is not a dominant species in truly warm deserts, even those with some winter precipitation. B. tectorum occurs, but is not a dominant annual grass in the mild Mediterranean climates of cis-montane California and southwestern Oregon, USA. B. tectorum has invaded the salt deserts of the Intermountain Area of western North America, but the true level of tolerance to salt-affected soil is not known with precision. Precipitation, both the amount received and the periodicity of moisture events are critical factors in the population density, herbage and seed production. On rangelands, B. tectorum populations can disappear over vast areas for as long as 3 years during droughts, but they will return when the drought ends.
On rangelands, B. tectorum is closely associated with the seral continuum it often culminates. This continuum is largely composted of exotic annual herbaceous broadleaved and grass species. B. tectorum can truncate succession in this continuum for extended periods of time (at least 75 years). However, the B. tectorum-dominated sites are open to the invasion by other exotic annuals, biannual, or perennial weed species. B. tectorum can be grazed to the point it is replaced by lower seral stages dominated by broadleaf herbaceous exotic weeds, but if the grazing pressure is relaxed, B. tectorum returns as the dominant species (Young et al., 1969).
Air TemperatureTop of page
|Parameter||Lower limit||Upper limit|
|Absolute minimum temperature (ºC)||-40|
|Mean annual temperature (ºC)||5||15|
|Mean maximum temperature of hottest month (ºC)||30||40|
|Mean minimum temperature of coldest month (ºC)||-5||0|
RainfallTop of page
|Parameter||Lower limit||Upper limit||Description|
|Dry season duration||3||5||number of consecutive months with <40 mm rainfall|
|Mean annual rainfall||100||650||mm; lower/upper limits|
Rainfall RegimeTop of page Winter
Soil TolerancesTop of page
Special soil tolerances
Notes on Natural EnemiesTop of page B. tectorum plants become infested with smut (Ustilago spp.) in certain years, but there is no evidence that natural infections have any lasting influence of the persistence or dominance of B. tectorum populations (Stewart and Hull, 1949). The seed pathogen Podosporiella verticillate is probably very significant in reducing viable B. tectorum seeds in seedbanks (Kreitlow and Bleak, 1964; Young et al., 1969).
Means of Movement and DispersalTop of page Natural Dispersal (Non-Biotic)
B. tectorum seeds are too heavy for wind to be a major factor in dispersal.
Vector Transmission (Biotic)
On rangelands, rodents collect and scatter hoarded seeds of B. tectorum (LaTourrette et al., 1971). Rodents recover seeds from some of these caches for consumption, while others germinate and produce viable seeds. The barbs on the lemma, palea, and awns of B. tectorum caryopses are very effective in aiding seed dispersal. The seeds stick in animal fur and also human clothing.
Planting contaminated seed and feeding contaminated hay or grain to livestock are common means of dispersal of B. tectorum. In most of the USA, B. tectorum is not a regulated noxious weed meaning that it can occur in seed lots as long as it does not exceed the 'other weed species' limit, applying even to certified seed lots.
Planting contaminated seed and feeding contaminated hay or grain to livestock are common means of dispersal of B. tectorum. The use of B. tectorum-infested cereal straw in erosion control during construction projects is a common means of dispersal for this species. Also, accidental dispersal by farmers or walkers is another means, because if you walk through a B. tectorum stand at seed maturity, you rapidly find your socks full of seeds unless you are wearing tall boots.
There are folk stories that B. tectorum was deliberately spread by stockmen in the Intermountain Area, USA, after the native perennial grasses were severely depleted by excessive grazing although these stories have never been documented or verified. The invasion rate of B. tectorum is sufficiently fast enough that intentional enhancement by humans was probably not necessary.
Pathway VectorsTop of page
Plant TradeTop of page
|Plant parts liable to carry the pest in trade/transport||Pest stages||Borne internally||Borne externally||Visibility of pest or symptoms|
|True seeds (inc. grain)||seeds|
Impact SummaryTop of page
|Fisheries / aquaculture||None|
ImpactTop of page It is estimated that as few as 100 B. tectorum plants/m² reduced winter wheat production by 27-36% (Young et al., 1984) and B. tectorum is a major problem in winter cereal production in North America. B. tectorum infests an estimated 5.7 million ha of cropland in the western USA and control measures cost US$350 million dollars annually (Mitch and Kyser, 1987; Ogg, 1994). Wildfires fuelled by B. tectorum are also a significant economic cost. For example, in Nevada, USA during a 10-day period in 1999, 0.64 million ha of rangeland burned in wildfires largely fuelled by B. tectorum. It cost US$38 million to suppress these fires and US$42 million in emergency restoration efforts on the burned area (Young and Sparks, 2002). Similar wildfire events are repeated in various portions of the Intermountain Area, USA annually.
Environmental ImpactTop of page B. tectorum increases the chances of ignition, rate of spread and expanding the season of wildfires, reducing the interval between re-occurring fires. The effect is to eliminate native woody species and truncate succession among herbaceous species to leave B. tectorum as the dominant species. At the same time, B. tectorum-dominated communities remain open to invasion by other exotic, invasive weeds. Accumulations of B. tectorum herbage mature in late spring and increase the chance of ignition and the rate of spread of wildfires. The natural wildfire season started in mid August when the herbage of native perennial grasses matured. The early maturity of B. tectorum extends the wildfire season through the warmest periods of the summer. B. tectorum provides the continuity fuel that allows the spread of wildfires from shrub to shrub. Such wildfires are a threat to human life and property, destroy the native sagebrush plants that do not re-sprout after the aerial portion of the plant is burned, destroy valuable forage resources, and the resulting burns are subject to accelerated wind and water erosion.
Impact: BiodiversityTop of page The noted American plant ecologist, DW Billings, termed B. tectorum a biotic cause of ecosystem impoverishment (Billings, 1989). In rangelands in the western USA, B. tectorum is a landscape level problem. B. tectorum closes plant communities to the recruitment of seedlings of native plant species (Robertson and Pearse, 1945) and this competition effectively truncates succession assuring prolonged dominance by B. tectorum.
Threatened SpeciesTop of page
|Threatened Species||Conservation Status||Where Threatened||Mechanism||References||Notes|
|Astragalus anserinus (Goose Creek milkvetch)||NatureServe NatureServe; USA ESA candidate species USA ESA candidate species||Idaho; Nevada; Utah||Ecosystem change / habitat alteration||US Fish and Wildlife Service, 2014a|
|Astragalus microcymbus (skiff milkvetch)||NatureServe NatureServe; USA ESA candidate species USA ESA candidate species||Colorado||Competition (unspecified)||US Fish and Wildlife Service, 2014b|
|Astragalus schmolliae (Schmoll's milkvetch)||CR (IUCN red list: Critically endangered) CR (IUCN red list: Critically endangered); NatureServe NatureServe; USA ESA candidate species USA ESA candidate species||Colorado||Ecosystem change / habitat alteration||US Fish and Wildlife Service, 2015|
|Castilleja cinerea (ash-grey paintbrush)||NatureServe NatureServe; USA ESA listing as threatened species USA ESA listing as threatened species||California||Competition - smothering||US Fish and Wildlife Service, 2013a|
|Centrocercus minimus (Gunnison sage-grouse)||USA ESA listing as threatened species USA ESA listing as threatened species||Colorado; Utah||Ecosystem change / habitat alteration||US Fish and Wildlife Service, 2013b|
|Centrocercus urophasianus (greater sage-grouse)||NT (IUCN red list: Near threatened) NT (IUCN red list: Near threatened)||California; Colorado; Idaho; Montana; Nevada; North Dakota; Oregon; South Dakota; Utah; Wyoming||Competition (unspecified); Ecosystem change / habitat alteration||US Fish and Wildlife Service, 2013c|
|Eremogone ursina (Bear Valley sandwort)||NatureServe NatureServe; USA ESA listing as threatened species USA ESA listing as threatened species||California||Competition (unspecified)||US Fish and Wildlife Service, 2007a|
|Eriogonum soredium (Frisco buckwheat)||NatureServe NatureServe; USA ESA candidate species USA ESA candidate species||Utah||Competition (unspecified); Ecosystem change / habitat alteration||US Fish and Wildlife Service, 2014c|
|Lepidium ostleri (Ostler's peppergrass)||NatureServe NatureServe; USA ESA candidate species USA ESA candidate species||Utah||Competition (unspecified); Ecosystem change / habitat alteration||US Fish and Wildlife Service, 2014d|
|Mirabilis macfarlanei||NatureServe NatureServe; USA ESA listing as threatened species USA ESA listing as threatened species||Idaho; Oregon||Competition - monopolizing resources; Ecosystem change / habitat alteration||US Fish and Wildlife Service, 2000|
|Penstemon grahamii (Graham's beardtongue)||NatureServe NatureServe; USA ESA species proposed for listing USA ESA species proposed for listing||Colorado; Utah||Competition - strangling||US Fish and Wildlife Service, 2005|
|Sclerocactus brevispinus||CR (IUCN red list: Critically endangered) CR (IUCN red list: Critically endangered); USA ESA listing as threatened species USA ESA listing as threatened species||Utah||Competition - monopolizing resources||US Fish and Wildlife Service, 2010a|
|Sclerocactus wetlandicus||USA ESA listing as threatened species USA ESA listing as threatened species||Utah||Competition - monopolizing resources||US Fish and Wildlife Service, 2010b|
|Silene spaldingii (Spalding's catchfly)||USA ESA listing as threatened species USA ESA listing as threatened species||Idaho; Montana; Oregon; Washington||Competition - monopolizing resources||US Fish and Wildlife Service, 2007b|
|Stephanomeria malheurensis (Malheur wire-lettuce)||USA ESA listing as endangered species USA ESA listing as endangered species||Oregon||Competition - strangling||US Fish and Wildlife Service, 1991|
|Taraxacum californicum (California taraxacum)||USA ESA listing as endangered species USA ESA listing as endangered species||California||Competition - strangling||US Fish and Wildlife Service, 2008|
|Trifolium friscanum (Frisco clover)||USA ESA candidate species USA ESA candidate species||Utah||Competition - monopolizing resources; Ecosystem change / habitat alteration||US Fish and Wildlife Service, 2014e|
|Urocitellus endemicus (southern Idaho ground squirrel)||No Details||Idaho||Ecosystem change / habitat alteration||US Fish and Wildlife Service, 2014f|
Social ImpactTop of page B. tectorum probably has the highest name recognition among exotic, invasive weeds in the Intermountain Area, USA and even suburban and urban residents fear wildfires fuelled by B. tectorum. The seeds often cause injury to the ears, eyes and mouths of pets such as dogs and horses as well as to humans. Livestock such as cattle and sheep can also be injured.
Risk and Impact FactorsTop of page Invasiveness
- Proved invasive outside its native range
- Highly adaptable to different environments
- Tolerates, or benefits from, cultivation, browsing pressure, mutilation, fire etc
- Highly mobile locally
- Has high reproductive potential
- Has propagules that can remain viable for more than one year
- Damaged ecosystem services
- Ecosystem change/ habitat alteration
- Negatively impacts agriculture
- Negatively impacts human health
- Negatively impacts animal health
- Negatively impacts tourism
- Reduced amenity values
- Reduced native biodiversity
- Competition - monopolizing resources
- Competition - smothering
- Competition - strangling
- Produces spines, thorns or burrs
- Highly likely to be transported internationally accidentally
UsesTop of page B. tectorum is the most abundant forage species on many intermountain area rangelands of the USA (Murray and Klemmedson, 1968) and this was true by at least the mid 1900s (Fleming et al., 1942). There are many disadvantages to basing a range livestock industry on B. tectorum versus perennial grasses, but grazing B. tectorum is a reality on millions of hectares of rangelands in North America.
Similarities to Other Species/ConditionsTop of page There are a host of annual Bromus spp. with more or less similar ecological requirements that can be associated with B. tectorum in specific environments. In the southern intermountain area of the USA, red brome (Bromus madritensis ssp. rubens) replaces B. tectorum and introgressive hybrids may exist. Occasionally, Japanese brome (B. japonicus), ripgut (B. diandrus), or rattlesnake brome (B. briziformis) may occur in the same communities as B. tectorum, but once in flower it is easy to distinguish the different species of Bromus spp. The only native annual grass that can be confused with B. tectorum in the seedling stage is six weeks fescue (Vulpia octoflora). The juvenile foliage of six weeks fescue is much finer in texture and the plants more diminutive than B. tectorum. Annual wheatgrass (Eremopyrun triticeum) is becoming increasingly abundant in B. tectorum stands in the intermountain area. At maturity the compact spike of this grass is so different from the nodding panicle of B. tectorum there is no problem in identification. Vegetatively, annual wheatgrass has so few leaves it does not resemble B. tectorum. Over vast areas of rangelands, B. tectorum has been replaced by medusahead (Taeniatherum caput-medusae). The seed, foliage, seed heads, and phenology of medusahead make it easy to distinguish from B. tectorum, and matures 2-4 weeks later than B. tectorum making infestations easy to distinguish from a considerable distance. In winter wheat fields in the Pacific Northwest, USA, jointed goatgrass (Aegilops cylindrica) has become a serious winter annual weed in addition to B. tectorum.
Prevention and ControlTop of page Cultural Control
On rangelands, the establishment of perennial grasses effectively suppresses B. tectorum so that it remains persistent, but only as a minor component of vegetation communities. The problem with this approach is in the establishment of perennial grass seedlings in the face of competition from B. tectorum, and which was found to be almost impossible in western North America. During the 1940s it was determined that the exotic perennial crested wheatgrass (Agropyron desertorum) could be established on sagebrush rangelands (Young and McKenzie, 1982) and although weed control is still required for its establishment, seedlings are much more competitive compared to those of the native perennial grasses. In areas of winter wheat production, if cultural, environmental, and economic restraints permit, periodic switching to spring-planted cereal grains permits suppression by herbicide application or tillage before planting of the grain crop. In addition, immobilization of nitrogen followed by planting seeds of a non-leguminous native shrub, antelope bitterbrush (Purshia tridentata) that symbiotically fixes nitrogen, has been highly successful on B. tectorum infested rangelands (Young and Clements, 2002).
On rangelands and much of the winter wheat production areas in the USA, deep mouldboard ploughing to bury the B. tectorum seedbank is not physically or economically feasible. Post-emergence mechanical tillage as a seedbed preparation is feasible, but on arid rangelands this forces a delay in planting until sufficient B. tectorum has emerged for the tillage to have sufficient efficacy before the onset of the summer drought.
On rangelands in the western USA, there are two basic approaches for the use of herbicides for control of B. tectorum as a means of establishing seedlings of perennial grasses. The first system is to wait until a significant portion of the germinable seeds of B. tectorum have emerged in the early spring and then apply a contact herbicide such as paraquat (Evans et al., 1967). Seeding of perennial grass could be carried out simultaneously with the sprayer mounted on the front of the drill. This practice never became popular because of paraquat toxicity to humans and the failure to apply it aerially, and glyphosate was not a good substitute for paraquat probably because of the delay in B. tectorum mortality during the cold spring period after application of this herbicide. The second system involves the creation of a fallow with herbicides, with atrazine applied in October or November and B. tectorum controlled during the following growing season with the site being seeded to perennial grasses the following October, one year after the application of the herbicide (Eckert and Evans, 1967). If the summer rains are good in the fallow year, lower successional levels of exotic invasive broadleaved weeds will flourish on the fallow and certainly during the seedling year of the perennial grasses.
In winter cereal production, B. tectorum germinates very close to the same time as the winter wheat or barley (Ogg, 1994). Selective control of B. tectorum during the first 3 weeks following emergence is critical in reducing competition. In conventionally tilled wheat, diclofop has been the most effective soil-applied herbicide for control of B. tectorum, killing B. tectorum as it germinates and reducing B. tectorum populations by up to 95% allowing wheat yields to increase by 30-40% (Stahlman, 1984).
Metribuzin is the only herbicide available that when applied post-emergence will control B. tectorum selectively in winter wheat (Swan and Whitesides, 1988), but to be effective it must be applied before the weed begins to tiller extensively. The margin of crop safety with metribuzin is very narrow and is based on the ability of wheat to metabolize metribuzin rapidly and its ability to root deeply early in its life cycle and thus avoid uptake of the herbicide (Devlin et al., 1987). The use of metribuzin in wheat is restricted to fine textured soils with more than 1% organic matter (Ogg, 1994). A special formulation of atrazine was used in the Pacific Northwest, USA, to reduce B. tectorum in winter wheat, and applied before the wheat was seeded with the seeding done with a deep furrow drill. The openers on the drill moved the herbicide on the soil surface to the side and B. tectorum was controlled between, but not within the rows (Ogg, 1994).
Much of the winter wheat production in the Pacific Northwest of the USA takes place on fine textured, aeolian soils, often on steep slopes and erosion is a serious problem. Conservation tillage practices that leave litter to protect the soil surface may have promoted an increase in B. tectorum, considering the seedbed ecology of the species (Evans and Young, 1970). Winter wheat production without tillage (no-till) has significant popularity in the Pacific Northwest. In no-till winter wheat a granular formulation of triallate plus trifluralin applied to the soil surface before planting has controlled B. tectorum (Ogg, 1994). The granulars are not absorbed by litter on the soil surface so they are more effective than liquid applications and a deep furrow drill is necessary to move the herbicide away from the wheat seeds.
Classical biological control of grasses such as B. tectorum poses problems regarding specificity, noting that many crops are also grasses. There has been considerable interest in using naturally occurring soil micro-organisms for weed control (Grey et al., 1995), with Kennedy (1994) having screened thousands of isolates of soil bacteria for the inhibition of B. tectorum germination or seedling growth. Several isolates were found that slow the root growth of B. tectorum giving wheat a competitive advantage (Kennedy et al., 1989, 1991), but the practical problem with application of mycoherbicides is in obtaining infection in the field. B. tectorum can be infested with smut (Ustilago spp.), and Meyer et al. (2001) proposed the use of U. bullata as a biological control agent for B. tectorum.
In the Pacific Northwest, USA, some winter wheat farmers appear to tolerate the presence of B. tectorum whereas others are destroyed by the weed. Those that successfully suppress B. tectorum maintain an active programme that identifies and maps infestations. Rotation to spring-planted cereal crops on infested fields is used to reduce seedbanks. Contact herbicides are used to control spot infestations and herbicidal control practices used correctly in winter wheat stands help to reduce competition from B. tectorum. On rangelands, ranchers can graze B. tectorum infestations or watch them eventually burn in uncontrollable wildfires. Grazing management that involves rest from grazing or deferment of grazing until after seed ripening of B. tectorum have proven to be disasters. Weed control to allow establishment of perennial grasses followed by quality grazing management relegates B. tectorum to a minor species in most rangeland communities.
ReferencesTop of page
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Young JA, Tipton FH, 1990. Invasion of cheatgrass into arid environments of the Lahonton Basin. General Technical Report 276. Odgen, USA: USDA Forest Service, 37-40
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