RMD-13-04: Consolidated Pest Risk Management Document for pest plants regulated by Canada
Appendix 10A: Pest Risk Assessment Summary for Paspalum dilatatum (Dallis grass)

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Identity of Organism

Name: Paspalum dilatatum (family Poaceae, subfamily Panicoideae, tribe Paniceae) (USDA-ARS 2009)

Synonyms: Digitaria dilatata, Panicum platense, Paspalum eriophorum, Paspalum lanatum, Paspalum ovatum, Paspalum pedunculare, Paspalum platense, Paspalum selloi, Paspalum velutinum (Tropicos 2009)

Common names: Dallis grass, Dallisgrass, water grass, paspalum, CAB International 2007; Hitchcock and Chase 1950; USDA-ARS 2009)

French common names: Herbe de Dallis (CAB International 2007; Hitchcock and Chase, 1950; USDA-ARS 2009)

Description: Dallis grass is a perennial, cespitose, C4 grass. The plants have rhizomes, but they are so short (less than 1 cm) that the stems form tufts. The stems are 50-175 cm tall, and erect. The leaf blades are up to 35 cm long, 2-16.5 mm wide, flat, mostly glabrous, with a few long hairs near the base on the upper surface. The panicles are terminal, with 2-7 racemose, divergent branches. The spikelets are 2.3-4 mm long and 1.7-2.5 mm wide. They are paired, and appressed to the branch axes. The lower glumes are absent; the upper glumes and lower lemmas are 5-7-veined, with pilose margins. The caryopses are 2-2.3 mm and white to brown (Barkworth 2006).

Dallis grass is considered a weed of 14 crops in 28 countries (Holm et al. 1977).

Organism Status

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

Current Regulatory Status

Canada:
Paspalum dilatatum was not regulated in Canada.
United States:
It is not regulated as a federal or state noxious weed.
Other countries:
In 28 countries it is listed as a serious or principal weed

Probability of Entry

Dallis grass was deliberately introduced into the USA as a forage grass species in the mid-1800's (Hitchcock and Chase 1950). Deliberate introduction seems to have been the main pathway for long distance spread around the world.

In parts of the southern USA, dallis grass is a perennial weed problem on lawns, golf courses and other turf areas. Control in turf is very difficult (Breeden and Brosnan 2009). This suggests that seeds have been introduced as contaminants in grass seed in that region at least. This is probably the most likely pathway for introduction into Canada, as much of the lawn and turf grass seed sold in Canada is imported from the USA (Table 1). The species has also reportedly been used as a turf grass (Barkworth 2006).

Table 1: Summary of Pathways for Paspalum dilatatum (Dallis grass)
Type of pathway Specific Pathways
Natural dispersal

Paspalum dilatatum is dispersed by means of seed.

It is unlikely that natural dispersal will allow the species to enter Canada, as established populations are in the southern states.

Intentional Introduction

Deliberate introduction as a forage plant seems to have been the main pathway for long distance spread around the world.

This is not a likely pathway for entry of Paspalum dilatatum into Canada, since it is grown intentionally only in warmer climates.

Unintentional Introduction

Seeds could be carried as contaminants in seed lots, especially turf grasses. However, no seeds of any Paspalum species have been reported from samples analyzed at the CFIA Seed Laboratory in the past nine years. This could change if more turf grass seed was imported from infested regions.

Seeds could be carried as contaminants in grain lots. Paspalum dilatatum is a weed of rice, so it could be introduced into Canada as a contaminant in rice grain lots.

This is a likely pathway for entry, but is unlikely to introduce seeds into suitable habitats.

Seeds could be carried as contaminants in commodities. Paspalum dilatatum is a weed of tropical fruit crops, including pineapples. Imported pineapples have been reported to be contaminated with weed seeds such as itchgrass, so it could be possible for Paspalum dilatatum to enter by this pathway.

This is a likely pathway for entry, but less likely to introduce seeds into suitable habitats.

Figure 1: Range of Paspalum dilatatum (Dallis grass) in North America

Figure 1. Description follows.
Description of Figure 1:

This image is a map of North America distinguishing the range of Paspalum dilatatum (Dallis grass) 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 , the most predominant colour on the map, represents contributors; light green signifies state flora/atlas; dark green represents regional fora/atlas; light blue stands for reliable reports; turquoise indicates other publication on Paspalum dilatatum while grey shows unverified reports; dark green signifies a database; and mustard yellow denotes specimen by MEB, while dark purple represents herbarium database. Large amounts of light pink are condensed in southeast states from West Virginia extending down to Texas. Light green and other colours such as yellow and red are present in the coastal states of Virginia, North Carolina and South Carolina. Magenta, blue and purple as well as turquoise are apparent along the west coastal states in California and parts of Arizona as well as in New Mexico although more spread out in nature than the eastern representation.

Source: Barkworth 2006

Figure 2: Potential range of Paspalum dilatatum in Canada

Figure 2. Description follows.
Description of Figure 2:

This image shows the potential range of Paspalum dilatatum in Canada and northern America through the use of a map. Red is used to indicate the regions in which Paspalum dilatatum could survive according to the Canadian Plant Hardiness Zones map, in this case NAPPFAST Hardiness Zones 6-9. In Canada, the potential range of Paspalum dilatatum is limited to coastal and southwestern BC, and, possibly, southern Ontario and parts of the Maritime Provinces if it is introduced to these areas. Most of the red can be found in the U.S., this includes the greater parts of the east and west coasts as well as areas located near the Great Lakes extending downwards beyond the image.

NAPPFAST zones 6-9

Probability of Establishment

Paspalum dilatatum is thought to be native to Bolivia, Brazil, Chile, Paraguay, Uruguay and Argentina (USDA-ARS 2009). The exact native range is now obscure because of the species' pantropic distribution (Weber 2003). It is now well established across the southern U.S., including Hawaii, as a weed in waste places (Barkworth 2006). Paspalum dilatatum is also naturalized in s. Europe, tropical and s. Africa, Asia, Australia, New Zealand, Macaronesia, the Mascarenes, Melanesia, and Polynesia (USDA-ARS 2009).

Based on its current range in the U.S., Paspalum dilatatum will survive to NAPPFAST zone 6. This would suggest a potential range in Canada that includes coastal and southern British Colombia, extreme southwestern Ontario and coastal parts of Nova Scotia and Newfoundland (see figure 2).

In its native range, Paspalum dilatatum is found in sandy or muddy soil near the seashore or in saline soil and swamps. In its introduced range it invades heaths, shrubland, riparian habitats and freshwater wetlands (Weber 2003). In Western Australia it is a serious weed in disturbed and natural clay pans, as well as swamps, lawns, roadsides and pastures (Hussey et al. 2007).

There would be no shortage of suitable habitats in the limited potential Canadian range.

Probability of Spread

Locally, Paspalum dilatatum plant fragments can be moved around by farm equipment, but seed is the main mode of spread. Large quantities of seed are produced, often by apomixis (Holm et al. 1977). Seeds are spread by attaching to animals and people (Holding and Bowcher 2007).

Potential Economic Consequences

Paspalum dilatatum is listed as a serious or principal weed in Australia, Philippines, Brazil, Columbia, the former Soviet Union and Taiwan (Holm et al. 1991). It can be a problem when it encroaches on irrigation ditches (Holm et al. 1977). It is particularly a weed problem in bananas, papayas, pineapples and rice in the Philippines, sugarcane, bananas, orchards and vineyards in Australia, in bananas, pineapples, papayas and sugarcane in Hawaii, tea and citrus in the former Soviet Union, rice in Brazil and India and potatoes, vegetables and pasture in New Zealand. In the United States it is a pasture weed (Holm et al. 1977). In Tennessee, Paspalum dilatatum is now a weed problem throughout the state on lawns, golf courses and other turf areas where it is very difficult to control (Breeden and Brosnan 2009). It is listed as a serious turf weed on websites in a number of other southeastern states and researchers are looking for cost-effective control measures (Henry and Yelverton 2005).

Potential Environmental and Social Consequences

The dense growth habit smothers other low-growing plants and prevents recruitment of native woody species in Western Australia (Hussey et al. 2007).

Uncertainty

It is quite uncertain that Paspalum dilatatum will actually survive to NAPPFAST zone 6. It is certainly widespread in zone 7 in the United States, but if it could only survive to zone 7, its potential range in Canada would be limited to coastal and southwestern British Colombia.

Conclusion

Based on the outcome of this pest risk assessment, Paspalum dilatatum is likely to establish and become invasive in parts of Canada including southern and coastal British Colombia, and, possibly, southern Ontario and parts of the Maritime Provinces if it is introduced to these areas. This plant should be considered for regulation under Plant Protection Act and Seeds Act. It is recommended that the pest risk analysis process continue for this plant with the completion of a Risk Management Document.

Technical Issues for Consideration

There are 320 species of Paspalum in the warm regions of the world (Watson and Dallwitz 1992 onwards), so there will always be identification issues. However, dallis grass is identifiable by trained personnel.

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