Managing onion grass and broadleaf weeds

Onion grass is a perennial herb with distinctive growth behaviour. It looks like a grass, grows like an annual and can spread rapidly on farmland. The plant not only produces abundant seed, but also underground corms that survive the hot and dry summers in Mediterranean environments of temperate Australia.

Most onion grass seeds require a temperature of less than 16.5ºC to germinate, and animals can disperse large numbers of seeds through grazing (greater than 500 viable seeds/sheep/day). Seed of onion grass germinates in autumn to winter, grows over winter and flowers from August to November.

The corm may require a slightly different temperature to grow but generally sprouts in autumn as well. Initially, the corm supplies nutrients for the plant to sprout and the nutrients in the corm diminish approximately eight weeks after the plant emerges (Figure 1). The residual of the corm remains unchanged in weight for about 11 weeks and then diminishes from week 19 onwards. The new corm starts to develop six weeks after emergence, then grows slowly until 14 weeks when the growth rate increases exponentially. There is a short period (week six to eight after emergence) of increased vulnerability when the old corm is exhausted, and the new corm starts to develop. This is a window of opportunity for the most effective control with selective herbicides.

Figure 1 shows the mean dry weight of old and new corms after emergence of onion grass plants. The left vertical axis shows the old corm weight in grams per corm, while the right vertical axis shows the new corm weight in grams per corm. The horizontal axis indicates the weeks after emergence from week 1 to week 26. The old corm weight is gradually decreasing from 0.13 grams at week 1 to just over 0.05 grams by week 8 indicating old corm exhaustion. From week 9 to 19 the residual of the old corm stays the same in weight before diminishing by week 26. In contrast the new corm starts to develop from week 6, slowly increasing in weight to week 11 then exponentially increasing from week 14, reaching well over 0.35 grams per corm by week 26.

Response to soil fertility and defoliation

Onion grass does not respond to fertiliser as many pasture plants do. A glasshouse experiment conducted in south west Victoria revealed that the herbage yield of onion grass did not differ significantly when various rates of phosphorus were applied. However, herbage mass of other species, including native grasses, was significantly increased (greater than 40 per cent) by the medium to high levels of phosphorus treatments.

Onion grass is highly sensitive to close defoliation. Cutting to one centimetre above ground at three- to five-week intervals reduced onion grass corm mass by 70 per cent, seed pod density by 100 per cent and plant density by 60 per cent compared with the control without defoliation. Cutting to 5 cm above ground also reduced onion grass corm mass by 58 per cent, seed pod density by 94 per cent and plant density by 35 per cent. Cutting at flowering only considerably reduced seed pod numbers (90 per cent) and corm mass to a lesser degree (27 per cent) but did not affect onion grass plant density.

Lifting soil fertility or grazing heavily will help to control onion grass by either enhancing competition from companion pasture species or weakening the growth of onion grass. Fertiliser and close defoliation will not kill onion grass and if used excessively will reduce the persistence of perennial native grasses. Consider other options to control onion grass if a significant reduction of the weed in pasture is expected to occur in a growing season.

Control with herbicides

Onion grass can be controlled effectively in established pastures using an appropriately registered metsulfuron-methyl herbicide. It is important that spraying, wick wiping or carpeted drum wiping is conducted at, or as close as possible to, the point that the old corm is exhausted and the new corm is developing (Figure 1), approximately six to eight weeks after onion grass has emerged. This will enable enough chemical to be absorbed by the young plant to kill it. Weed control at flowering can get rid of flowers and seeds but not corms. If broadleaf weeds such as capeweed are of concern, mixing metsulfuron-methyl with a compatible broadleaf herbicide will control both types of weeds.

Before using any agricultural chemical, users must ensure they read and understand the entire product label. There are a variety of selective and non-selective herbicides registered for the control of onion grass in a number of different situations. The Australian Pesticides and Veterinary Medicines Authority (APVMA) maintains a searchable database of chemicals registered for the control of pests and diseases in Australia called the Public Chemical Registration Information Search System.

Risk to native vegetation

If applied inappropriately, herbicides may affect beneficial native plants such as native herbs. Native ecosystems and their supporting flora and fauna have their own fundamental value. This has been widely recognised by state and federal governments and legislation that limits the clearing of native vegetation to prevent further loss of remnant native vegetation, including native grasslands and grassy woodlands. You should seek further advice from relevant authorities before starting work in areas containing native vegetation. Prior to using herbicides ensure that you read and understand all sections of the product label, including the ‘protection of crops, native and other non-target species’ section that specifically addresses these issues.

Risk to clover

Products containing metsulfuron-methyl are likely to kill clover species for the remainder of the season after spraying and may affect the clover population in subsequent years, depending on the soil seed reserve of clovers. Use of a wick wiper or carpeted drum wiper for application of this chemical to onion grass can help protect clovers and non-target species in some situations. Sub-clover has been found to recover well in soils with good clover seed reserves in the second year after spraying.