Onion Grass (Romulea rosea.) - Management in Pastures
Note Number: AG1389
Published: December 2009
Updated:
Description
A full description of the plant can be found at: http://vro.dpi.vic.gov.au/dpi/vro/vrosite.nsf/pages/water_sss_onion_grass
Origin and distribution
- Originated from South Africa.
- Widely distributed throughout temperate and mediterranean areas of Victoria, Australian Capital Territory, New South Wales, South Australia, Tasmania and Western Australia.
- Generally found in areas with low competition from other perennial plants due to low fertility, overgrazing, fire or poor seasonal conditions.
The problem
Onion grass can significantly reduce the productivity of crops, pastures and animal systems, making it a costly agricultural weed. Onion grass has no nutritional benefit to animals, remaining undigested in the gut. If large quantities are consumed, fibre balls may form and obstruct the bowel, causing death. In crops and pastures, onion grass often grows ahead of desirable species, utilising valuable moisture and nutrients and restricting production.
Life cycle
Onion grass is a perennial herb with distinctive growth behavior. It looks like a grass, grows like an annual, and can spread rapidly on farm land. The plant not only produces abundant seed, but also corms underground to survive the hot and dry summers in mediterranean environments of temperate Australia. Most onion grass seeds require a temperature of < 16.5ºC to germinate and animals can disperse large numbers of seeds through grazing (>500 viable seeds/sheep/day). Seed can also be dispersed by mowing, slashing and by some insects such as ants. Seed and corm germinate or sprout in autumn to winter and grow over winter and flower from August to November. The flowers initially open close to the ground then the stalks elongate to lift them into the air and then bend downward to take the maturing capsule back to ground level. Top growth dies in summer leaving a perennial corm in the soil.
The corm may require 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 diminishes in 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, grows slowly until 14 weeks when the growth rate increases exponentially. There is a short period (week six to eight after emergence) 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 herbicides.
New corms generally develop on top of the old corms; therefore in theory, corms should get shallower with time. However, contractile roots may pull the corm deeper into the soil as it dries out. Some onion grass plants develop tap roots much bigger and stronger than fibrous roots which may well play a role in dragging the corms deeper. Some plants can develop multiple new corms from one plant, behaving like grass tillers.
Management in existing pastures:
Prevention
The two main causes of onion grass infestation are autumn bare ground and lack of competition from desirable pasture species. Prevention involves managing pastures to maintain above 70 per cent ground cover and maximising growth during autumn and winter. Make sure you have a high proportion of desirable perennial species in the pasture, maintain good soil fertility and use rotational grazing to maximise growth and persistence of perennials.
Response to increasing soil fertility and grazing management
Onion grass does not respond to fertiliser as many other 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 fertiliser were applied (Table 1). However, herbage mass of other species, predominantly native grasses, was significantly increased (>40 per cent) by the medium to high fertiliser treatments.
Table 1. Herbage mass (kg DM/ha) of onion grass and other species (predominantly native grasses) under various fertiliser regimes
| Treatment | Onion grass | Other species |
|---|---|---|
|
No fertiliser |
2080 |
4623 |
|
Medium fertiliser |
2204 |
6406 |
|
High fertiliser |
2204 |
6662 |
|
Significance |
Not significant |
P <0.05 |
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 non-defoliated control (Table 2). Cutting to five centimetres 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.
Table 2. Corm mass (g/m 2) , seed pod density (pods/m 2) and plant density (plants/m 2) of onion grass under various defoliation treatments
| Treatment | Corm weight | Seed pod | Plant density |
|---|---|---|---|
|
Cut to 1 cm |
180 |
0 |
1310 |
|
Cut to 5 cm |
250 |
20 |
2160 |
|
Cut to 1 cm at flowering |
430 |
30 |
3170 |
|
No defoliation |
590 |
310 |
3340 |
|
Significance |
P <0.05 |
P <0.05 |
P <0.1 |
These results suggest that more intensive grazing in the field has the potential to reduce the corms and seed produced by onion grass, and consequently the density of onion grass. Fertiliser application may not decrease onion grass density directly but could have an impact on onion grass through competition of other species that respond favourably to fertiliser in autumn and winter.
Use of herbicides
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 Pesticide and Veterinary Medicine Authority (APVMA) maintains a searchable database of chemicals registered for the control of pests and diseases in Australia – go to www.apvma.gov.au and search using the PUBCRIS search function.
Onion grass can be controlled in established pastures using an appropriately registered metsulfuron-methyl herbicide. It is important that spraying is conducted at 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 permit enough chemical to be absorbed by the new corm to kill it. Spraying at flowering can get rid of flowers and seeds but not corms.
Risk to clover: Note that products containing metsulfuron-methyl are likely to kill clover species for the remainder of the season after spraying and may effect the clover population in subsequent years. Use of a wick-wiper or rotary wiper for application of this chemical to onion grass can help to protect pasture in some situations. Some product labels state ‘DO NOT treat newly sown pastures as severe damage may occur.’ It is a legal requirement to comply with all ‘DO NOT’ statements documented on the product label.
Some products registered for the control of onion grass can be residual in the soil, leading to significant plant back periods. Information on plant back periods is contained on the product label.
Risk to native vegetation: Note that if applied inappropriately, herbicides may affect beneficial native plants other than native grasses. Native ecosystems and their constituent flora and fauna have their own intrinsic value. This has been widely recognised by state and federal governments and legislations require the limitation of clearing of native vegetation to prevent further loss of remnant native vegetation including native grasslands and native grassy woodlands. Ensure that prior to using herbicides 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. You can also seek further advice in areas containing native species before commencing work.
The Environment Protection and Biodiversity Act 1999 protects matters of national significance including threatened species and ecological communities. To find out more about the act and permit applications, visit http://www.environment.gov.au/epbc/publications/farming-epbc.html"
Management in crops and new pastures:
Sow a crop or annual grass pasture the year prior to establishing a perennial.
- Spray with an appropriately registered knockdown herbicide to kill all pasture species in autumn
- Sow crop or annual grass with required fertiliser
- If more onion grass emerges after sowing, spray six weeks after the onion grass germinates with an appropriately registered metsulphuron-methyl herbicide.
For a more thorough clean, a summer crop such as millet or rape could be sown to increase ground cover and competition in mid autumn when onion grass is likely to germinate. Spray with an appropriately registered knockdown herbicide the following autumn prior to sowing a perennial pasture including clover.
Note that controlling onion grass prior to spring sowing is often difficult. Products containing metsulfuron-methyl may have an applicable plant-back period usually ruling out spring pasture establishment after spraying in winter. Spraying with a knockdown herbicide in spring to remove the onion grass prior to sowing may not prevent the onion grass from germinating the following year.
Contact/Services available from DPI
Contact your local DPI Agronomist or Pest Management Officer for further advice on the management of onion grass.
Contact your local DPI Chemical Standards Officer for further information on the use of agricultural and veterinary chemicals.
Call the DPI Customer Service Centre on 136 186.
Visit the DPI website
Weeds Australia (external link)
Further References
HerbiGuide - The Pesticide Expert on a Disk
Acknowledgements
Compiled by Kate Sargeant, Zhongnan Nie and Reto Zollinger based on case studies in north east Victoria and a series of glasshouse and field experiments conducted in south west Victoria.
Photos by Kate Sargeant, Farm Servcies Victoria.
ISSN 1329-8062
Published and Authorised by:
Department of Environment and Primary Industries
1 Spring Street
Melbourne, Victoria
This publication is copyright. No part may be reproduced by any process except in accordance with the provisions of the Copyright Act 1968.
The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication
