Invasiveness Assessment - Dwarf arrowhead (Sagittaria pygmaea) in Victoria

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Plant invasiveness is determined by evaluating a plant’s biological and ecological characteristics against criteria that encompass establishment requirements, growth rate and competitive ability, methods of reproduction, and dispersal mechanisms.

Each characteristic, or criterion, is assessed against a list of intensity ratings. Depending upon information found, a rating of Low, Medium Low, Medium High or High is assigned to that criterion. Where no data is available to answer a criterion, a rating of medium (M) is applied. A description of the invasiveness criteria and intensity ratings used in this process can be viewed here.

The following table provides information on the invasiveness of Dwarf arrowhead.

A more detailed description of the methodology of the Victorian Weed Risk Assessment (WRA) method can be viewed below:

Victorian Weed Risk Assessment (WRA) method (PDF - 630 KB)
Victorian Weed Risk Assessment (WRA) method (DOC - 1 MB)
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Common Name: Dwarf arrowhead
Scientific name: Sagittaria pygmaea

Question	Comments	Rating	Confidence
Establishment
Germination requirements?	“Seeds germinate in Spring” (Parsons & Cuthbertson, 2001)	MH	MH
Establishment requirements?	6 hours of light or less cause seeds to germinate (Murray-Goulburn Water, draft). “A large amount of stubble can…[shade] arrowhead seeds, slowing germination” (RIRDC, 2000). Require light to germinate (Flower et al, 1999). Less than 10% of seed germinated without light, compared to up to 50-65% with light (Flower et al, 1998). Can establish under moderate cover.	MH	H
How much disturbance is required?	“Usually found in warm-temperate shallowly flooded or marshy areas associated with rivers and streams…drainage ditches, rice fields and permanent swamps associated with irrigation drainage systems.” (Parsons & Cuthbertson, 1992). All evidence points towards highly disturbed aquatic environments, such as irrigation channels and rice fields.	ML	MH
Growth/Competitive
Life form?	“Emergent, broad-leafed aquatic plant” (Flower et al, 1999)	H	H
Allelopathic properties?	None found.	L	L
Tolerates herb pressure?	Spoonbills and carp have been observed feeding on the plant. Either or both of these animals may have been the cause of the plants disappearance on river flats along the River Murray but this is not certain (Parsons & Cuthbertson, 1992). The weed may still persist.	ML	MH
Normal growth rate?	Rice has been observed outgrowing S. montevidensis in California (Gibson et al, 2001) however Australian rice yields can be reduced by up to 75% (Flower et al, 1999). The ability of rice to out compete arrowhead depends on when it is sown after flooding (Flower et al, 2002). Equal competitiveness of same life forms.	MH	H
Stress tolerance to frost, drought, w/logg, sal. etc?	One study showed that draining rice bays for up to two weeks killed very young arrowhead seedlings, but didn’t kill more mature seed-grown plants (Flower et al, 1999). Survive in temporary dry ditches (Sagliocco & Bruzzese, 2005). Killed by frost and most plants killed by drought (Kaul, 1985 Angelo & Boufford (2005) record it growing in tidal mud flats of estuaries and salt marshes. The rosette form may be able to tolerate submersion for years (MGW, nd). As a low growing aquatic species (Parsons & Cuthbertson, 1992) will not be affected by fire. Tolerance of fire, waterlogging and salinity, but not frost or drought (severe enough to dry out damp areas).	MH	MH
Reproduction
Reproductive system	“Reproducing by seed and rhizomes” (Parsons & Cuthbertson, 1992)	H	MH
Number of propagules produced?	Can produce many thousands of seeds per plant (MGW, nd). 414,000 achenes per plant per year (Kaul, 1985).	H	H
Propagule longevity?	“The viability of …seeds does not reduce significantly after three years in the soil, suggesting that they remain viable for many years after being deposited” (Murray-Goulburn Water, draft). Kaul (1985) found that scarified seed viability reduced to 50% after 7 years.	ML	H
Reproductive period?	Species forms dense monocultures, as seen in photograph (Parsons & Cuthbertson, 1992).	H	H
Time to reproductive maturity?	“Seeds germinate in spring and plants mature during summer, producing flowers from January to March. Seeds mature in autumn” (Parsons & Cuthbertson, 1992). Kaul (1985) describes S. calycina (syn. for subspecies ) as a fruit-bearing annual. “Two subspecies recorded in Australia, one perennial (ssp. Montevidensis and one annual ssp. calycina)” (Murray-Goulburn Water, draft). Produces propagules in under a year.	H	H
Dispersal
Number of mechanisms?	“Under Australian field conditions, spread of arrowhead is by seed. They move in water along channels and ditches and also in mud sticking to hooves and fur of animals…Some seeds are eaten by ducks and believed to be viable when excreted” (Parsons & Cuthbertson, 1992).	H	MH
How far do they disperse?	“Under Australian field conditions, spread of arrowhead is by seed. They move in water along channels and ditches and also in mud sticking to hooves and fur of animals” (Parsons & Cuthbertson, 1992) “due to a slightly sticky surface” (Murray-Goulburn Water, draft). “The achenes produced by Altismataceous species have a pericarp that provides buoyancy…those of S. montevidensis float for a little under two weeks.” (Murray-Goulburn Water, draft). Very likely that some propagules will disperse greater than 1 km by sticking to animal fur, or transport by water.	H	MH

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