Onion grass (Romulea rosea) is a persistent perennial weed that poses a significant challenge for farmers across Australia, particularly in Victoria, South Australia, and Western Australia [1, 2]. Recognizable by its onion-like smell, this invasive plant competes with valuable pasture species for critical resources such as water, nutrients, and sunlight. Effective control of onion grass is essential to maintain productive grazing land, optimize forage quality, enhance livestock welfare, and ensure long-term pasture health. 

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Identifying Onion Grass

Onion grass, also known as Romulea species, are small perennial herbs that grow from underground stems called corms. These plants have annual leaves and small pink, white, purple, or yellow flowers. R. rosea var. australis and R. rosea var. communis are the most widespread species in Australia.

Key identifying features include:

  • Tall, clump-shaped plants that can grow up to 75 cm high.
  • Thick, yellow roots that spread underground.
  • Hollow stems that stand straight.
  • Leaves that grow from the base and look like tiny green tubes when cut in half.
  • White or pink flowers that grow along the stems.
  • Small, wrinkled, round fruit that form after flowering.
  • It’s worth noting that other species are also called ‘Onion Weed’, including Allium triquetrum, which has triangular stems and smells of onion when crushed, and Asphodelus fistulosus, which has cylindrical hollow stems and does not grow from bulbs .

How to kill onion grass

Understanding the Growth Cycle

Onion grass seeds can germinate at any time of the year, but they most commonly sprout in late summer and autumn. The plant typically doesn’t flower until it’s about 18 months old, with flowering occurring between June and October . After flowering, the stems die back, but some leaves remain alive through the summer. New leaves and stems grow from the base in the following autumn .

Onion grass can behave differently depending on the conditions [11]:

  • In some cases, the plant can flower and die within the first year, making it an annual.
  • In other cases, it lives longer and becomes a biennial or a short-lived perennial.
  • The seeds can stay alive in the soil for many years.
  • In dry areas, many seedlings don’t survive their first summer but those that do are drought-hardy.
  • Unlike some other plants, onion grass has a shallow, fibrous root system and stores energy in the base of its leaves, not in rhizomes or bulbs [12].

Photo of Onion Grass In Australia

Photo of Onion Grass In Australia – Source https://weeds.org.au/

Why is Onion Grass a Problem?

Onion grass is a significant weed for several reasons:

Competition: It competes with desirable pasture species for resources, reducing forage production and impacting livestock productivity.
Pasture Health: It can alter the biodiversity of pasture ecosystems, reducing soil health and resilience [3]. It often dominates overgrazed and low-fertility areas.
Reduced Palatability: Livestock find onion grass unpalatable, which can reduce forage intake and affect animal health and productivity. Animals tend to overgraze areas with onion grass, further degrading pasture quality [3].
Blunted Mower Blades: The tough leaves of Romulea species can blunt mower blades.
Fibre Balls in Livestock: Fibre balls can form in the stomachs of livestock that eat it.

Effective Control Methods

A combination of manual, chemical, and preventative methods is essential for effective onion grass control  The most effective approach is to prevent its spread and focus on new infestations.

Chemical Control

Herbicides: Metsulfuron-methyl herbicide is effective in controlling onion grass in established pastures.
Timing: Apply herbicides six to eight weeks after onion grass emerges, when the old corm is exhausted and the new corm is developing. Treating at flowering can get rid of flowers and seeds but not corms..
Application: Use spraying, wick wiping, or carpeted drum wiping for application 
Broadleaf Weeds: If broadleaf weeds like capeweed are also a concern, mix metsulfuron-methyl with a compatible broadleaf herbicide.
Product Labels: Always read and understand the entire product label before using any agricultural chemical. The Australian Pesticides and Veterinary Medicines Authority (APVMA) maintains a database of registered chemicals..

Non-Chemical Control

Manual Removal: Hand-pulling can be effective for small infestations, but larger plants may re-root.
Mechanical Control: Cultivation can be helpful, but dislodged plants may re-root, and equipment can be damaged.
Competition: Maintain healthy, well-managed pastures to crowd out onion grass, particularly by ensuring adequate soil nutrients, especially nitrogen.
Defoliation: Close defoliation, such as cutting to 1 cm above ground at three to five-week intervals, can reduce onion grass corm mass and plant density. Cutting to 5 cm above ground is also effective. Cutting at flowering considerably reduces seed pod numbers.
Soil Fertility: Lifting soil fertility can help to control onion grass by enhancing competition from companion pasture species.

The Redback Weedwiper Advantage

The Redback Weedwiper is an innovative tool designed for the efficient and targeted application of herbicides. It allows farmers to apply herbicides directly to the leaves of onion grass while minimizing the impact on desirable pasture plants, making it a cost-effective solution 

Here’s how to use the Redback Weedwiper effectively

Select the Appropriate Herbicide: Choose a systemic herbicide effective against onion grass, such as those containing glyphosate  Always follow instructions for application rates and safety precautions.
Prepare the Redback Weedwiper: Ensure the weedwiper is clean and in good working condition for even herbicide application 
Adjust the Wiping Height: Set the height to target the onion grass without harming desirable pasture species 
Apply the Herbicide: As you move through the pasture, the Redback Weedwiper will apply the herbicide directly to the leaves, ensuring it is absorbed and translocated to the roots.
Monitor for Regrowth: Keep an eye on treated areas for any signs of regrowth, as follow-up applications may be necessary
Implement Integrated Weed Management: Combine the weedwiper with other weed control practices, such as rotational grazing and competitive planting 
The Redback Weedwiper offers a precise, targeted approach that can significantly improve the efficiency of your weed management efforts 

When to Kill Onion Grass

Timing of Herbicide Application: Apply herbicides six to eight weeks after onion grass emerges, when the old corm is exhausted and the new corm is developing [16-18].
Defoliation: Cut or graze heavily to weaken the plant.

Legal Status

Onion grass was first introduced to Australia in 1857 as an ornamental plant [33]. While it is now found across all Australian states and territories, it is not classified as a Weed of National Significance (WONS) [33]. It is a declared weed in the Northern Territory and Tasmania .

Summary

Onion grass is a significant challenge for farmers, but with the right tools and strategies, effective control is achievable. The Redback Weedwiper, combined with appropriate chemical and non-chemical methods, provides an efficient solution for protecting your pasture health, optimizing forage production, and enhancing livestock welfare.

For more information or to share your experiences, please feel free to reach out. Your insights are valuable and can help others facing similar challenges.

Conclusion

Onion grass is a significant challenge for farmers managing pastures in Australia, but effective control is achievable with the right tools and strategies. The Redback Weedwiper provides an efficient, targeted solution for eradicating weeds while preserving the integrity of your pasture.

By integrating mechanical and chemical control methods, you can protect your pasture health, optimize forage production, and enhance livestock welfare. If weeds are a concern in your pastures, consider investing in a Redback Weedwiper to streamline your weed management efforts and secure a thriving agricultural operation.

We’d love to hear your thoughts! If you have any feedback, comments, or questions about managing Onion Grass or using the Redback Weedwiper, please feel free to reach out. Your insights and experiences are valuable to us and can help others facing similar challenges!

References: 

https://weeds.org.au/profiles/onion-weed-asphodel/#prosection6
https://weeds.org.au/overview/lists-strategies/
DPI NSW (2019); Western Australian Herbarium (1998–).
https://nre.tas.gov.au/invasive-species/weeds/weeds-index/declared-weeds-index/onion-weed/onion-weed-herbicides-for-control
https://nre.tas.gov.au/Documents/herbicide_guidelinesFINAL2012.pdf