Invasiveness Assessment - Wards weed (Carrichtera annua) 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 Wards weed.

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: Wards Weed
Scientific name: Carrichtera annua

Question	Comments	Rating	Confidence
Establishment
Germination requirements?	Carrichtera annua needs about 15 mm of rainfall for good germination, although some seeds may germinate after 100 mm of rain (Halevy 1989). Seeds have an optimal germination temperature of 15-20°C and are thermoinhibited at 0°C and 35°C (Gat-Tilman 1995). The species requires natural seasonal disturbance for germination.	MH	H
Establishment requirements?	C. annua is documented to inhabit grassy woodland communities (Carr et al. 1992) and is therefore able to establish under moderate canopy cover. It has been reported that leaf litter greatly reduced the germination of C. annua (Barritt and Facelli 2001), although germination may still occur. C. annua can establish under moderate canopy/litter cover.	MH	MH
How much disturbance is required?	Known to invade disturbed areas, especially areas degraded by overgrazing, where it replaces pasture plants (Cooke 2003). Common plant of dry open and disturbed sites (Weber 2003). Also reported to inhabit mallee shrubland, lowland grassland, grassy woodland (Carr et al. 1992) and heathland communities (Weber 2003). C. annua can establish in healthy and undisturbed natural ecosystems.	H	MH
Growth/Competitive
Life form?	C. annua is a tap-rooted, erect winter annual herb, to 60 cm in height (Cooke 2003: Weber, 2003). Life form: Other	L	MH
Allelopathic properties?	No allelopathic properties described in the literature reviewed.	M	L
Tolerates herb pressure?	Stock will only eat the weed when the absence of other feed forces them to do so, due to its hairiness and pungent, penetrating odour (Orchard 1946). The plant is consumed but not preferred.	MH	MH
Normal growth rate?	C. annua is an erect winter annual herb, to 60 cm in height (Cooke 2003: Weber 2003). Growth rate probably similar to the same life form.	M	M
Stress tolerance to frost, drought, w/logg, sal. etc?	Cooke (2003) describes frost not killing C. annua plants. ‘No plant mortality was recorded when plants were exposed to frost at -5°C and -2°C. C. annua is commonly described as a common annual desert plant (Gutterman and Shem-Tov 1997), referring to as drought tolerant. Tolerant of frost and drought maybe tolerant of another.	MH	MH
Reproduction
Reproductive system	C. annua is capable of self pollination, although seed set is at least twice as high when cross pollination occurs (Boaz et al. 1990 in Cooke 2003).	ML	H
Number of propagules produced?	Carrichtera annua is capable of producing 3,000-30,000m² seeds annually (Cooke 2003), the plant is able to grow to 60 cm in height (Weber 2003) and a diameter of up to 30 cm (Halevy 1989). Therefore, C. annua has the capability of producing above 2000 propagules annually.	H	MH
Propagule longevity?	In drought periods seeds of Carrichtera annua can remain on the parent plant for more than one year (Gutterman and Shem-Tov 1997). It is unclear to what extent C. annua forms a long lasting seed bank, but it is suggested to be very limited and barely sufficient to sustain one recruitment event and that seeds have very short persistence when buried (Meissner and Facelli 1999).	ML	MH
Reproductive period?	C. annua is a winter growing annual plant, following plant senescence the dead plant remains upright over the summer months (Cooke 2003). The mature plant produces viable propagules for only one year.	L	MH
Time to reproductive maturity?	Flowering may begin as early as 40 days after seed germination (Cooke 2003). The first seeds mature about two months after germination (Halevy 1989).	H	H
Dispersal
Number of mechanisms?	Dominant mechanism is by rain, although may also spreads through vehicle and stock movements due to its ability to adhere to a substrate (Cook 2003). Small proportions of ingested seeds can also survive herbivory of native animals and stock, dispersal vectors may include vertebrate herbivores including goats, macropods, emus, sheep, cattle and rabbits (Cooke 2003). The seed of C. annua contains a mucilaginous layer which aids its floating capabilities so that seeds can be carried in runoff water along runnels in the desert (Gutterman and Shem-Tov 1997). C. annua seeds are described as a possible food source for emus and ingested seed can survive herbivory, therefore dispersal by birds and highly mobile animals may occur.	H	MH
How far do they disperse?	When it rains, seeds can be flicked up to 25 cm from the parent plant (Loria and Noy-Mier 1979/80). Although, this is insignificant to account for rate at which C. annua spread in Australia. C. annua has the ability to spread through vehicle and stock movements including railways, demonstrating the ability to be transported long distances, presumably by seed (Cooke 2003). C. annua seeds are also described as a possible food source for emus and when ingested, seed can survive herbivory (Cook 2003). Spennemann and Allen (2000) describe that emus are able to disperse seeds greater than 50 km. It is very likely that at least one propagule will disperse greater than one kilometre.	H	MH

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