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Onion weed (Asphodelus fistulosus)
Onion weed is in the family Asphodelaceae. It not a true onion and it does not have the onion taste or smell, nor does it produce a bulb. The leaves are similar in appearance to the true onion. It is also similar in appearance to another weed, once referred to as branched South African onion weed (Trachyandra divaricata) but more recently named strapweed or dune onion weed. The major difference between the two is that South African onion weed has flat rather than round leaves and a much branched flowering stalk.
Agricultural Impact
Onion weed is an invasive weed of roadsides, disturbed areas and overgrazed pastures of the semi-arid to subhumid warm-temperate regions. It is principally a weed on alkaline soils of low fertility and in areas with an annual rainfall of 250 - 500 mm, usually with a winter dominance.
Onion weed does not invade well-managed pastures, but will establish readily in areas disturbed by cultivation such as roadside grading and cable laying trenches. From these areas the plant will grow vigorously and spread through the production of large quantities of seed. In overgrazed areas it grows thickly and out competes other plant species. Because it is not eaten by stock, the carrying capacity of the land is reduced.
Spread
The plant grows throughout the Mediterranean region to western Asia and northern India. More recently it has spread to North America, New Zealand and Australia, where it is a weed problem. It occurs in all states of Australia, predominantly on roadsides, railway lines and neglected areas and sandy coastal areas.
Seeds can be spread on vehicles, machinery, animals, wool, clothing and by running water. Cultivation will spread plants with seed capsules still attached. Wind will also blow dead plants and drop the seeds as the plant rolls along. The seeds are thought to remain viable for several years.
Identification
Onion weed can be annual, biennial or perennial. Seeds germinate at almost any time of the year but the main flush is late summer and autumn. Plants growing from seed do not usually develop flowers until about 18 months old. After flowering and seeding in spring, the flowering top of the plant dies away, but the leaves remain alive over summer and new leaves are produced from the base of the plant in the following autumn.
Stems are rigid and hollow, reaching up to 75 cm in height, and the hairless leaves that emerge from the base of the plant are about 50 cm long and 4 mm in diameter. The flowers are white or pink, 1.5 - 2.0 cm in diameter and formed alternately along the stem. Each petal has a reddish stripe up the centre. The fruit is an almost round capsule, 4 - 6 mm in diameter, and divided into 3 segments, each containing 1 or 2 seeds. The seed is brown or black, triangular and wrinkled.
Spread
The plant grows throughout the Mediterranean region to western Asia and northern India. More recently it has spread to North America, New Zealand and Australia, where it is a weed problem. It occurs in all states of Australia, predominantly on roadsides, railway lines and neglected areas and sandy coastal areas.
Seeds can be spread on vehicles, machinery, animals, wool, clothing and by running water. Cultivation will spread plants with seed capsules still attached. Wind will also blow dead plants and drop the seeds as the plant rolls along. The seeds are thought to remain viable for several years.
Identification
Onion weed can be annual, biennial or perennial. Seeds germinate at almost any time of the year but the main flush is late summer and autumn. Plants growing from seed do not usually develop flowers until about 18 months old. After flowering and seeding in spring, the flowering top of the plant dies away, but the leaves remain alive over summer and new leaves are produced from the base of the plant in the following autumn.
Stems are rigid and hollow, reaching up to 75 cm in height, and the hairless leaves that emerge from the base of the plant are about 50 cm long and 4 mm in diameter. The flowers are white or pink, 1.5 - 2.0 cm in diameter and formed alternately along the stem. Each petal has a reddish stripe up the centre. The fruit is an almost round capsule, 4 - 6 mm in diameter, and divided into 3 segments, each containing 1 or 2 seeds. The seed is brown or black, triangular and wrinkled.
Control
Onion weed can be controlled by regular cultivation and establishment of competing species, however this is not a viable option in pastoral areas. Chemical control is possible, but only two products are registered in Western Australia. The chemicals - one, a mixture of amitrole and ammonium thiocyanate, and the other a premix containing glyphosate - are too expensive for broadscale use. Research is continuing to test for products that will either control the plant or inhibit seed production to restrict the spread. Before these can be recommended they will have to be approved by the herbicide manufacturers and the National Registration Authority in Canberra who regulate the application of pesticides and veterinary products.
Other relevant information related to this topic:
- Agriculture biosecurity in Western Australia
- Plant biosecurity in Western Australia
- Declared plants in Western Australia
- Western Australia Quarantine and Inspection Service
- Permited and quarantine species list
Page reviewed: February 2007
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