Andropogon virginicus
(broomsedge)

A. virginicus is a perennial member of the grass family (Poaceae). It is an abundant species within its large native range extending from most of eastern North America to northern South America (Colombia). It has expanded its range through accidental introductions to the west coast of the USA, Asia, Oceania and Europe. It is weedy within its native range and behaves similarly where introduced. It is an aggressive colonizer of disturbed, low nutrient dry soils. The species is fire tolerant, and minimizes competition via allelopathy. It is a serious weed in Hawaii where it threatens endangered and threatened plant species, and it is listed on the state’s noxious weed list. It alters successional processes, changes fire regimes, causes erosion, and alters hydrology. It is also a threat to agricultural systems where it invades pastures.

A. virginicus has a broad native range in the Americas. It reaches its northern distributional limit in Ontario, Canada. It is found through the eastern USA from Michigan, New York, and Massachusetts, south to Florida, and west to Kansas, Oklahoma, and Texas. To the south it occurs from Mexico to Colombia, apparently absent only from El Salvador and Nicaragua. It also occurs naturally in the Caribbean, the Greater Antilles, Trinidad and Tobago, the Bahamas and as far as Bermuda (Campbell, 1983; Campbell, 2003).

The species has been introduced outside of its natural range to the western USA in California, in Asia, Europe and Oceania. In Europe it has been found at a military base in the department of Gironde in France (Granereau and Verloove, 2010) and also in Landes department (Fried and Mandon-Dalger, 2014), and in the Caucasus region (Zazanashvili, 1999; Akhalkatsi and Kimeridze, 2012). In Asia it is known from Japan (Koyama, 1987) and South Korea (Jong-Cheol et al., 2008). In Oceania it has been introduced to and is reported as invasive in Hawaii (Snow and Lau, 2010), Australia (AVH, 2016), and New Zealand (Gardner et al., 1996).

However, reports from French Polynesia (ISSG, 2016) appear to be in error. French Polynesia has placed the species in its exclusion list, and this may have been misinterpreted. No documentation of the occurrence could be found.

Risk assessments of A. virginicus have been conducted for Australia and the Pacific Islands (ISSG, 2016). Both found the species to be a high risk. It has been placed on the exclusion list by French Polynesia because of the threat. 

In its native range in the Americas A. virginicus occupies a wide range of dry to wet open habitats, particularly following disturbance, including old fields and roadsides (Campbell, 1983; Weakley, 2015). In the USA it is characteristic of a post-disturbance successional stage termed the “broomsedge stage” (Campbell, 1983), colonizing fields 3-5 years post-disturbance and persisting due to allelopathic properties (Rice, 1972).

The preference for dry to wet, disturbed, sunny habitats continues where it is introduced. In Japan the species colonized a burned watershed 2 years post fire, following behaviour in North America (Angara et al., 2000). It is a dominant species of savannah vegetation on the windward sides of Oahu and Hawaii (Sorenson, 1991), and is the one of the three most prevalent grasses in the seasonal submontane zone in Hawaii (Hughes et al., 1991). In Australia it has invaded nutrient poor habitats and altered fire regimes (Cronk et al., 2001; PIER, 2016). In France, the species is an invasive in dry heath, moor, and sphagnum pond, as well as disturbed areas (Fried and Mandon-Dalger, 2014). In the Caucasus region it is reported as invasive in broad-leaved forest (Akhalkatsi and Kimeridze, 2012), an atypical habitat that should be verified.

Genetics

A. virginicus has a chromosome number of 2n=20 (Campbell, 1983).

Reproductive Biology

Like most grass species, A. virginicus is wind pollinated, although precocious maturation of spikelets precludes wind pollination in some spikelets. The percentage of spikelets that are cleistogamous in A. virginicus is small compared to other closely related species (Campbell, 1982). The capacity for autogamy increases homozygosity, making inbred forms more adaptable to specific environments, increasing success as a colonizer (Campbell, 1983).

Campbell (1983) found that A. virginicus hybridizes with A. longiberbis and A. glomeratus var. pumilus. These hybrids are much less fertile than their parents.

Physiology and Phenology

In the Americas this species flowers in the autumn, from about September to October (Weakley, 2015). In Hawaii flowering is stimulated in the autumn by shortening daylight (Sorenson, 1991). Seeds of A. virginicus have been found to form persistent seed banks (Baskin and Baskin, 1998). It can resprout within 96 hours after a fire (Hughes et al., 1991).

Environmental Requirements

Several studies have focused on the ability of A. virginicus to tolerate extreme soil conditions, including contaminated soils. It has a high tolerance to aluminium, extreme acidity, low nutrients, lead, and zinc, often in abandoned coal or lead/zinc mines (Gibson and Risser, 1982; Ning and Cumming, 2001; Cumming and Ning, 2003; Ezaki et al., 2013). The ability to tolerate aluminium is conferred by mychorrhizal fungi (Ning and Cumming, 2001; Cumming and Ning, 2003).

This species is tolerant of low nutrient conditions. The ability to survive in harsh conditions, such as with high aluminum concentrations, is conferred by mychorrizal fungi (Ning and Cumming, 2001; Cumming and Ning, 2003). It is considered a fire hazard and known to alter the fire regime in areas where it has invaded (PIER, 2016). 

A. virginicus has been found to host several species of rust and smut fungi. These include the rusts Puccinia andropogonis, P. ellisiana, Uromyces andropogonis, Sorosporium ellisii (a head smut), S. everhartii (a seed smut), Sphacelotheca seymouriana and S. occidentalis (head or seed smuts), and Ustilago striiformis (stripe smut) (Gardner and Davis, 1982). Beckham et al. (1971) report nine species of thrips on A. virginicus in northern Georgia. Agindotan et al. (2013) report a new marfavirus, tentatively named Switchgrass mosaic virus (SwMV), on A. virginicus.

A. virginicus is very similar to other species of Andropogon, especially those in Section Leptopogon. The ability to identify members of the complex requires carefully collected mature specimens, and careful examination of inflorescence and spikelet morphology (Campbell, 1983; Campbell, 1986). In areas where introduced it could be most easily confused with the closely related A. glomeratus which, like A. virginicus, is naturalized in Hawaii and Japan, and the two are sympatric in their native ranges in North America and the Caribbean. A. glomeratus was misidentified as A. virginicus at Midway Atoll (Snow and Lau, 2010). A. glomeratus is in overall aspect a more robust species with a broader, bushier inflorescence. Positive identification requires examination of spikelets. In A. glomeratus the keel of the first glume is usually scabrous below the middle. Its leaves are usually more than 44 cm long. In contrast, in A. virginicus keels of the first glume are scabrous only above the middle, and leaves are usually less than 31 cm long (Weakley, 2015). A. virginicus could also be confused with other members of the grass family in Section Andropogoneae. For example, Jong-Cheol et al. (2008) note that in Korea it can be confused with Themeda triandrus, as well as an annual congener A. brevifolius.  

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.

Control of A. virginicus should focus on a combination of habitat management to prevent invasion, and chemical control if invasions do occur. As the species is an aggressive invader of disturbed soils, steps should be taken to minimize disturbances near A. virginicus colonies. This may include steps such as the control of wild boar populations because of associated soil disturbances. Where the species invades pastures, a combination of grazing and herbicide application may be effective, or these can be used in conjunction with prescribed burning.          

Physical/Mechanical Control

Peters and Lowance (1974) found that control could be achieved in pastures by drilling in tall fescue, with the supplementation of nitrogen, phosphate and potassium. Coupled with mowing in winter and grazing of new growth of spring by cattle, A. virginicus was nearly eliminated after five years.

Biological Control

Options for biocontrol of grasses are generally limited because of the potential threat to many important crop species such as sugarcane, rice and wheat. An assessment of the suitability of biocontrol in Hawaii concluded that the threat to sugarcane would be a major impediment (Gardner and Davis, 1982).  

Chemical Control

Herbicides can be effective in the control of A. virginicus, but only when applied on new growth. Use of glyphosate (1% in water) applied to new growth is recommended (Motooka, 2003).

Butler et al. (2002) found that in pastures a combination of herbicide (particularly glyphosate) with burning, followed by “sound forage management practices” was effective. Both MSMA and DSMA herbicides were shown to be effective (Lowance et al., 1975). In agricultural habitats a combination of bromacil, hexazinone, tebuthiuron, bromacil and diuron, and buthidazole have been used as an effective control (Cronk et al., 2001).

Further data should be sought on the occurrence A. virginicus in the Caucasus region of Eurasia. Little data could be found on the report of this species there, such as in Georgia (Zazanashvili, 1999; Akhalkatsi and Kimeridze, 2012).

30/03/16 Original text by:

Keith Bradley, Consultant, South Carolina, USA