Microstegium vimineum
(Nepalese browntop)

Microstegium vimineum is a cosmopolitan annual (therophyte) C4 shade-tolerant grass of forest margins and moist grassy areas. It is native to Asia and is commonly referred to as Nepalese browntop or Japanese stilt grass (Raunkiær, 1934; Flora of China Editorial Committee, 2012). It was used as packing material for porcelain shipments from Asia to North America in the early twentieth century and was first collected in the USA in 1919 (Gage et al., 2010; Fryer, 2011; EPPO Executive Committee, 2012). The grass can form dense monocultures that crowd out native vegetation and proliferate in low light conditions. Once established, populations rapidly increase easily displacing native species within three to five years (Osborne and Edgin, 2007; EPPO Executive Committee, 2012). M. vimineum is currently listed as a Class C noxious weed in Alabama, an invasive, banned species in Connecticut, and a prohibited species in Massachusetts (Geosystems Research Institute, 2012). It is a regulated species in Wisconsin (IPAW, 2012) and was added to the EPPO Alert List in 2008 where it has since been transferred to the List of Invasive Alien Plants in 2012 (EPPO Executive Committee, 2012).

M. vimineum is a cosmopolitan grass species native to India, Indo-China, Nepal, China, Korea, Far-east Russia, Philippines, and Japan. There are herbaria records, reports in floras and other documentation enumerating the presence of the species on every continent except Antarctica (ISSG, 2012; USDA-ARS, 2012a).

M. vimineum is a very serious invasive species and considered one of the most destructive introduced plants in the United States. It forms extensive and dense patches, outcompeting and eliminating nearly all other herbaceous plants, with control and eradication being difficult at best (Fryer, 2011; Weakley, 2011; ISSG, 2012). The plant establishes itself most successfully with shallow leaf litter, and reproduces best at high temperatures and high light levels. It uses water as a dispersal vector and so an increased rate of spread is associated with mesic conditions as opposed to dry conditions. Areas associated with flooding and areas with low slope or timber harvest are also beneficial for rapid spread of M. vimineum (Gage et al., 2010).

M. vimineum grows along mesic roadsides, railroad right-of-way ditches, utility right-of-way, logging roads, roadsides floodplain forest, forest wetland, herbaceous and shrub wetland, early and late successional forest, planted forest, forest edges and margins, woodland borders, floodplains, grassy areas, vacant lot, managed landscapes, and stream sides; in mesic upland sites, usually in moderate to dense shade. The findings of Cheplick (2010) state that "The large size and subsequent greater reproduction of plants in populations of open, sunny habitats such as along roadsides and disturbed drainage ditches or streams, provides a major source of propagules able to colonize new areas following dispersal."     

M. vimineum will not grow in areas with periodic standing water, nor in full, direct sunlight (Fairbrothers and Gray, 1972; Hunt and Zaremba, 1992; Redman, 1995; Flora of China Editorial Committee, 2012).

In mountainous regions, M. vimineum is found below elevations of 4000 feet (Evans et al., 2006). Sunlight and moist soil, therefore, increase the chances of Japanese stiltgrass establishment and favour its growth. Establishment and spread are limited in shaded environments (Huebner, 2003; Fryer, 2011).

M. vimineum impedes regeneration of native woody species and lowers overall species diversity and stem densities (Oswalt et al., 2007). One study found that it inhibited the success of small-seeded trees and reduced tree survival after one growing season by more than 20%, negatively impacting on Acer negundo (box elder), Platanus occidentalis (sycamore) and Liriodendron tulipifera (tulip poplar) (Flory and Clay, 2010).

An infestation of M. vimineum documented in 2002 in Chapman State Park , Maryland, USA showed adverse impact on Myosotis macrosperma Engelm (largeseed forget-me-not), Solidago bicolor (white goldenrod), Arisaema dracontium (green dragon) and Nemophila aphylla (smallflower baby blue eyes) (Imlay, 2012).

Genetics

Chromosome base number of M. vimineum is x = 10, 2n = 20 and 40; chromosomes are small (Watson and Dallwitz, 1992).

There is evidence of genetic differences among M. vimineum populations, conferring higher shade tolerance in some populations that have larger leaves. One experiment showed that two test populations significantly increased biomass production under favourable conditions, unlike a third population. In this experiment the most productive populations also responded to shade stress via greater specific leaf area (SLA) while the third population did not (Droste et al., 2009).

Reproductive Biology

M. vimineum produces abundant seed and relies entirely on its seed bank for its annual recruitment. The seeds may need a period of stratification (cool temperatures and high moisture) before they will germinate (Woods, 1989) and seeds stored in the soil may remain viable for as long as five years (Barden, 1991). The seeds may have low germination rates but many seeds are produced by each plant, resulting in an estimated 0.1– 4 million seeds per m² (Barden, 1987; Woods, 1989; Gibson et al., 2002; Judge et al., 2008; Warren et al., 2010). Each tiller of M. vimineum typically produces one terminal raceme and two to seven axillary racemes; each tiller may produce 100 to 1000 seeds per year (Cheplick, 2010). 

It grows quickly in low light conditions, sometimes forming dense monocultures (monospecific stands), fruits within a single season, and is moved easily into disturbed habitats by natural means (e.g., flood scouring) and artificial means (e.g., mowing, tilling, foot traffic, and other soil disturbing activities) (Swearingen, 2000; EPPO Executive Committee, 2012). M. vimineum seeds reportedly have short-term persistence in soil ranging from one to five years (Barden, 1987; Gibson et al., 2002; Vidra et al., 2007). There are few reports or studies on M. vimineum germination though open sites and little to no litter favour germination (Fryer, 2011).

M. vimineum seed matures until fall frosts followed by plant death (Gibson et al., 2002). Seeds are also able to survive submersion in water for periods of up to 10 weeks. Barden (1991) reports that seeds can germinate while under water, but the plants do not grow. If standing water is removed, more seeds will germinate shortly afterwards.

The plant is both cleistogamous and chasmogamous (self- and cross-pollinated) (Huebner, 2003; Kuoh, 2003).

Physiology and Phenology

M. vimineum shows "extreme plasticity" in morphology, producing both flowers and stolons under a wide range of nutrient and light conditions (Fryer, 2011). Flowering and fruiting occurs in late summer (August–November); senesces in early fall (Evans et al., 2006; Flora of China Editorial Committee, 2012).

The reproductive success and establishment of the plant in woodlands "may be due to high tolerance to a range of intraspecific densities and to an ability to set seed under shady conditions even when densities are high" (Cheplick, 2010).

It is a variable species, usually with apparently awnless spikelets, where in fact a weakly developed awn is enclosed within the glumes. Sometimes the awn is exserted and obvious; rarely is it completely absent (Flora of China Editorial Committee, 2012). This species variability may have led to attempts to circumscribe varieties, subspecies, forms and novel species leading to taxonomic confusion.

Being a shade tolerant plant, it is adapted for low-light conditions and uses the C4 pathway for photosynthesis (Barden, 1987; Horton and Neufeld, 1998). M. vimineum lacks the high density of minor veins found in most C4 grasses, and forms, instead, files of distinctive cells (Raghavendra, 2010). 

M. vimineum community and species responses to a future carbon dioxide enriched atmosphere may be mediated by other environmental factors and will depend on individual species responses (Belote et al., 2008). High carbon dioxide levels may negatively affect M. vimineum compared to plant species better able to assimilate extra carbon dioxide (Fryer, 2011). 

Associations

M. vimineum is associated in North America with several other non-native species in the United States: garlic mustard (Alliaria petiolata), Japanese honeysuckle (Lonicera japonica), sericea lespedeza (Lespedeza cuneata), multiflora rose (Rosa multiflora), Japanese barberry (Berberis thunbergii) and Norway maple (Acer platanoides) (Gibson et al., 2002; Morrison and Mauck, 2007; Dogra et al., 2010; Fryer, 2011).

Environmental Requirements

It grows in temperate to warm continental climates. There is little available specific information about temperature ranges for the species. The coldest reported winter temperatures for a seed bank of M. vimineum are approximately -21 to -23 °C (Redman, 1995). This low temperature would equate with USDA Plant Hardiness Zone of 6b. There are no specific studies of precipitation requirements. 

In the United States it is found in acidic soils (pH 5.8 to 4.8); some populations are established, however, on limestone or marble derived soils. Where it occurs, the soils typically have average levels of potassium and phosphorus but have high levels of nitrogen (Redman, 1995; Tu, 2000). It is commonly found on silty to sandy loams and on clays (red clays in the Piedmont), and prefers damp or wet soils but does not tolerate standing water (Barden, 1987; Hunt and Zaremba, 1992; Gibson et al., 2002; ISSG, 2012). Dry upland soils or “highly disturbed" soils such as gravel and dirt mounds by roadsides may also provide a suitable habitat (Swearingen, 2000; Touchette and Romanello, 2009; Fryer, 2011). 

The species thrives and reproduces most successfully in North America with canopy openness, and the species occurs less frequently and grows poorly in shade (Barden, 1987; Gibson et al., 2002; Claridge and Franklin, 2003; Cole and Weltzin, 2004; Glasgow and Matlack, 2007; Marshall and Buckley, 2007; Eschtruth and Battles, 2009; Warren et al., 2010).

When provided with adequate leaf-litter, it is highly competitive, especially in low light conditions (Leicht et al., 2005; Marshall and Buckley, 2007). M. vimineum has been shown to raise soil pH and have an associated increase in N-mineralization and nitrification, as well as decreased litter thickness (Kourtev et al., 1998; Ehrenfeld et al., 2001).

Thirteen species of fungi and eight arthropod species are reported for the genus Microstegium(Zheng et al., 2006).

Animal feed, fodder, forage

• Forage

Materials

• Baskets

The grass is similar in appearance to the North American native whitegrass Leersia virginica with which it often co-exists (Mehrhoff, 2000). It is also similar to the North American invasive Oplismenus hirtellus ssp. undulatifolius (basketgrass), as well as the native Oplismenus hirtellus subsp. setarius. It can be readily confused with another North American native, Dichanthelium clandestinum (deertongue) and may also be mistaken for Arthraxon hispidus (small carpgrass).

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

Manual, mechanical, environmental/cultural, and chemical methods are all useful to varying degrees in controlling M. vimineum(Tu, 2000). Various private and public entities in the United States as well as the European Union have developed an early detection and rapid response program (integrated vegetative management, IPM) to deal with the introduction, spread, control, education (public awareness), and management of M. vimineum(EPPO Executive Committee, 2012; USDA-ARS, 2012b).

Control should be directed to propagule source reduction and eradication programs along roadways and other sunlit, disturbed areas cutting off shaded ecosystem sink populations (Pulliam, 1988; Warren et al., 2010).

Physical control methods may include hand pull or mechanical cutting of plants using a mower or "weed whacker" on vegetative shoots of small infestations (Judge et al., 2008). No biological control agent is available at this time.

Fenoxaprop-p-ethyl, has been shown to be the best chemical for controlling M. vimineum populations and maintaining native understory flora in mixed hardwood stands of eastern North America (Jacques, 2007; Judge et al., 2008; Flory, 2010; Pomp et al., 2010).  However, using systemic herbicides such as glyphosate (e.g. Roundup), a herbicidal soap like pelargonic acid that kills the plants (e.g. Scythe) or herbicides specific to annual grasses may be a more effective choice. If applying glyphosate to M. vimineum in wetland sites, use the formulation labelled for wetland areas (Swearingen, 2000; Judge et al., 2008; Swearingen et al., 2010).

There is some ongoing research which suggests that invasions of M. vimineum can be controlled or eliminated using post-emergent grass specific herbicide followed by a spring application of pre-emergent herbicide (Flory, 2010).

Ecosystem Restoration 

M. vimineum control and management is difficult because invaded areas are often very large and the seed can persist in the soil for several years. The plant can be removed by hand-weeding, mowing, or selective herbicides, however, the re-establishment of a resilient, self-sustaining ecosystem or community will depend upon the variables of the control method. Purdue University Extension observes that "although multiple methods may be used to kill M. vimineum, there are few practical techniques. Hand-weeding can be effective for small invasions and mowing can help to reduce seed production in flat, easily accessible areas. For large invasions in areas with trees or steep topography, selective herbicides are preferred" (Kleczewski et al., 2011). Further complicating restoration plans, for example, are the biological and physical alterations such as the differences in non-native earthworm densities in M. vimineum infested soils that have been reported (Kourtev et al., 1999). In addition, M. vimineum restoration activities are increasing available nitrogen and favoring invasive species, which in turn detracts from restoration success (DeMeester and Richter, 2009). 

M. vimineum dispersal is an area for research as well as a potentially effective weak point for targeted management (Warren et al., 2010).

Precipitation requirements and temperature extrema are also areas of needed investigation. Additional research should target M. vimineum's impact on rare and endangered species.

Furthermore, research on indigenous and non-indigenous pathogens (both general and host specific) is scarce and invites additional study.

23/07/12 Original text by:

John Peter Thompson, Maryland, USA