RMD-13-04: Consolidated Pest Risk Management Document for pest plants regulated by Canada
Appendix 8A: Pest Risk Assessment Summary for Microstegium vimineum (Japanese stiltgrass)

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Identity of Organism

Name: Microstegium vimineum

Synonyms: Andropogon vimineum, Eulalia viminea, Eulalia viminea, (USDA-NRCS 2008). Microstegium imberbe, Microstegium willdenovianum, Pollinia imberbis, Pollinia viminea, Pollinia willdenoviana (Tu 2000).

English common names: Japanese stiltgrass, Japanese grass, Nepalese browntop, Nepalgrass (USDA-NRCS 2008).

French common names: None found

Description: Microstegium vimineum is an annual grass with a sprawling habit. It germinates in spring and grows slowly through the summer months, ultimately reaching heights of 0.6-1 m with the reclining stems growing up to 1 metre long. Slender stalks of tiny flowers are produced in August through to September or early October. In late fall they fade to pale greenish-yellow or turns purple in colour (Swearing 2004, Mehrhoff et al. 2003).

Microstegium vimineum grass has become a significant problem in forests in many eastern and Midwestern states. It spreads rapidly due to high seed production and rooting at nodes along the stem and is able to out-compete native vegetation. It often establishes in locations where moist soils are scoured such as along streambanks, floodplains, ditches and trails. Typical habitats include river corridors, forested wetlands, moist woodlands, old fields and thickets, utility rights-of-way, roadsides and lawns (Tu 2000).

Organism Status

Microstegium vimineum is not reported to occur in Canada (CFIA 2008), and no evidence was found that it is cultivated as an ornamental in Canada (CNLA 2009). Based on this information, Microstegium vimineum is considered absent from the PRA area.

Current Regulatory Status

Microtegium vimineum is not currently regulated in Canada. It is not regulated federally in the United States but is regulated in the following states: Alabama, Connecticut, and Massachusetts (USDA-NRCS 2008). It is not known to be regulated in other countries.

Probability of Entry

Microstegium vimineumis native to India, Nepal, China, and Japan and was introduced to North America in 1919 in Tennessee. By 1960 had it had spread (probably in hay and soil) to Ohio and Pennsylvania, and all Atlantic coastal states from Florida to New Jersey (see figure 1) (Howard 2005).

The most likely pathway for entry however is attached to the fur of animals or on human clothes or in mud on tires or hiking boots. Mehrhoff (2000) postulates that some populations along popular hiking trails in the US were likely introduced by way of fruits that adhered to people's clothing, shoes, hiking equipment or dogs.

Table 1: Summary of Pathways for Microstegium vimineum (Japanese stiltgrass)
Type of pathway Specific Pathways
Natural dispersal

Microstegium vimineum has high seed production which is its natural means of dispersal. It is also able to root at the nodes, which helps it to spread locally. Seeds are able to be dispersed in surface runoff and floodwaters, rivers and ditches are primary corridors for population expansion (Howard 2005)

Natural spread is an unlikely pathway into Canada due to the distances between established populations and the Canadian border.

Intentional Introduction Microstegium vimineum has not been documented as being intentionally planted as an ornamental, for erosion control or for forage (Howard 2005; EPPO 2008). No intentional pathways were identified.
Unintentional Introduction

According to an EPPO alert, Japanese stiltgrass can be introduced involuntarily as a contaminant of bird feed, soil, nursery stock and hay. No North American references were found which list birdseed as a pathway for spread. CFIA Seed Lab data show no reports of the species in marketplace monitoring samples of imported seed since 2000.

It was widely used as a packing material for porcelain from China which was likely its pathway of introduction into the U.S. (Tu 2000). It is not clear whether this pathway still exists

It also attaches readily to the fur of animals, human clothes and boots, and in mud on vehicle tires. It can be spread by hikers (Mehrhoff2000).
Unintentional introduction is the most likely pathway for entry into Canada

Probability of Establishment

Microstegium vimineum is hardy to NAPPFAST zone 6 and also appears to be establishing populations in zone 5 in the United States. This gives it a potential range in Canada of parts of Atlantic Canada, southern Ontario and coastal and southern BC (see figure 2). In the New England States, Microstegium vimineum has invaded floodplain forests, early and late successional forests, abandoned fields, roadsides and other habitats (Mehrhoff et al. 2003. Microstegium vimineum also does well in many disturbed areas. These habitats are abundant within the endangered areas within the PRA area.

Figure 1: North American range of Microstegium vimineum (Japanese stiltgrass)

Figure 1. Description follows
Description of Figure 1:

This image is contains a map of eastern North America distinguishing the range of Microstegium vimineum (Japanese stiltgrass) through the use of colours designated to a specific method. Red dots signify a UTC specimen; green dots equate to other herbaria; blue dots represents manual data; dark blue denotes UTC counties; magenta indicates other herbaria counties; light pink represents contributors; light green, the most predominant colour on the map, signifies state flora/atlas; dark green represents regional fora/atlas; light blue stands for reliable reports; turquoise indicates other publication on Microstegium vimineum while grey shows unverified reports; dark green signifies a database; and mustard yellow denotes specimen by MEB while dark purple represents herbarium database. While small light green pieces are predominant, they are heavily condensed in the following states and their surrounding areas: Virginia, New Jersey, Washington DC; parts of Pennsylvania, South Carolina, Kentucky, Tennessee and West Virginia.  Moreover, they also have the presences of other colours, such as magenta, light blue, dark green and dark purple, though none of them pervasive.

Source: Barkworth 2006

Figure 2: Potential range of Microstegium vimineum (Japanese stiltgrass)

Figure 2. Description follows
Description of Figure 2:

This image shows the potential range of Microstegium vimineum in Canada and northern America through the use of a map. Red is used to indicate the regions in which Microstegium vimineum could survive according to the Canadian Plant Hardiness Zones map, in this case NAPPFAST Hardiness Zones 5 and 6. In Canada, the potential range of Microstegium vimineum includes parts of Atlantic Canada, southern Ontario and coastal and southern BC. Most of the red can be found in the U.S., this includes the greater parts of the east and west coasts extending inwards reaching most of the central states as well as areas located near the Great Lakes extending downwards beyond the image

Source: Barkworth 2006

Probability of Spread

Although often associated with forested and wetland areas, Microstegium vimineum spreads rapidly into disturbed areas (Weber 2003). Its seeds can be spread by hikers (Mehrhoff et al. 2003), and are dispersed by surface runoff, floodwaters, on human and animal feet, and are carried in contaminated hay, soil or potted plants (Swearingen 2004). Ditches and rivers are primary corridors for population expansion (Howard 2005). It is unpalatable to grazing animals but they have been reported to spread the seeds (Swearingen 2004). By avoiding it and grazing other, more palatable, species wildlife may be indirectly contributing to its spread (Howard 2005). In suitable habitats it quickly spreads by rooting from its prostrate culms, forming dense, monospecific stands (Barkworth 2006).

Potential Economic Consequences

Microstegium vimineum has not been documented as being intentionally planted as an ornamental, for erosion control, or for forage (Tu 2000). It is mainly reported as a weed of natural areas, wetlands and forest understories. There have, however, been reports of it encroaching into crop production areas, landscape plantings, and turfgrass (Judge et al. 2008).
In 2008, a value of $24 million of field crops possibly intended for birdfeed in Canada was imported from areas where Microstegium vimineum is present as well as a value of $746,000 of hay and straw.

Potential Environmental and Social Consequences

Microstegium vimineum can form dense monotypic stands that replace natural communities and can dominate entire habitats, including forest understories and wetland areas. It can tolerate low light conditions allowing it to threaten forest vegetation as well as edge habitats. Once established, it is able to crowd out native herbaceous vegetation in wetlands and forests within three to five years (EPPO 2008).

Populations of Microstegium vimineum disrupt the quality of nesting habitats for birds (e.g. quails). However, it creates an excellent habitat for rats, such as cotton rats, a predator of birds. Microstegium vimineum also may be responsible for altering natural soil conditions, creating an inhospitable environment for many native species. Kourtev et al. (1998) reported that in areas that have been invaded by Microstegium vimineum, both litter and organic soil horizons were thinner than in uninvaded areas, and the pH of such soils was significantly higher (Tu 2000).

Following canopy disturbance Microstegium vimineum responds with a sudden increase in biomass allowing it to rapidly invade forests and impede regeneration of native woody species. Rapid increases in growth of Microstegium vimineum lead to the development of "mats'' on the forest floor that negatively affect native woody species regeneration in multiple ways: directly, through competition for sunlight, nutrients, and water and indirectly, by reducing the likelihood of successful seed to soil contact for germination (Oswalt et al. 2007).

Uncertainty

The climatic tolerance of Microstegium vimineum is uncertain. It is becoming increasingly widespread in the Eastern U.S. in NAPPFAST zone 6, however, there also appear to be populations in zone 5. Its native range is mainly zone 6 but there are some areas of zone 5 included in its distribution. For the purposes of this risk assessment it was assumed that it would be able to establish in at least parts of zone 5.

Conclusion

Based on the outcome of this pest risk assessment, Microstegium vimineum is likely to establish and become invasive in parts of Canada including southern and coastal BC, southern Ontario and parts of Atlantic Canada if it is introduced to these areas. This plant should be considered for regulation under the Plant Protection Act. It should be noted that one of the potential pathways into Canada is as a "hitch-hiker" on human clothing, muddy vehicles or pet fur, which may be difficult to regulate.

Technical Issues for Consideration

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