It could become a problem in irrigated pastures and market gardens in Western Australia. Ecology Both are normally summer growing annual plants although spiny burr grass is sometimes biennial. Mossman river grass germinates in spring and spiny burr grass from spring to summer. They form seeds from January until April. Up to 30 burrs are formed on each flowering spike and hundreds of burrs may be formed on one plant.
Most plants die off in early autumn.
The primary seed is larger and shows little seed dormancy, while the two secondary seeds are smaller and may remain dormant in the soil for up to three years. Deep buried seeds often germinate to produce new plants. However seeds left on the Surface seldom germinate due to the inhibiting effect of light and drier conditions. They may also be contaminants of hay, grain and chaff. Appearance Mossman river grass forms loose tufts up to 60 cm across, while spiny burr grass is erect.