How to kill onion grass

Dealing with onion grass in your pastures can be a frustrating challenge for farmers. This stubborn weed not only competes with valuable crops for nutrients but also proves resistant to many common herbicides. In this blog, we’ll dive into the most effective strategies for getting rid of onion grass for good. From understanding its growth patterns to choosing the right control methods, we’ll guide you through the process of reclaiming your fields from this persistent pest.

Onion grass control in many parts of Australia is a tough job, especially in  VIC, SA and WA.

Eradicating Onion Grass in Australian Pastures

Understanding Onion Grass (Romulea rosea)

Onion grass, also known as Romulea rosea, is a persistent perennial weed that affects pastures across Australia. Recognizable by its onion-like smell, this invasive plant competes with valuable pasture species for critical resources making it a significant concern for farmers. Its adaptability makes onion grass a formidable opponent in agricultural settings.

For farmers, effective control of onion grass is essential not just to maintain productive grazing land but also to optimize forage quality, enhance livestock welfare, and ensure long-term pasture health.

The Importance of Controlling Onion Grass

For farmers, controlling onion grass is crucial for several reasons:

• Optimising Forage Production: Onion grass competes aggressively for resources such as water, nutrients, and sunlight. This competition reduces the growth and yield of desirable pasture species, directly affecting the quality and quantity of forage available to livestock. Lower forage production can lead to a decrease in livestock productivity, impacting overall farm profitability.
• Maintaining Pasture Health: As an invasive species, onion grass can alter the biodiversity of pasture ecosystems, reducing soil health and the resilience of the pasture. It can dominate overgrazed and low-fertility areas, especially where soil nitrogen levels are depleted. Improving soil fertility and ensuring proper grazing management can help prevent onion grass from taking hold.
• Enhancing Livestock Welfare: Onion grass is unpalatable to livestock, meaning that high levels of the weed in a pasture can reduce forage intake by animals. This, in turn, affects the health and productivity of livestock. Areas infested with onion grass are often overgrazed as animals avoid the weed, further degrading pasture quality.

How To Identify Onion Grass

Onion weed is a tall, clump-shaped plant that can grow up to 75 cm high—about the height of a chair! It has thick, yellow roots that spread out under the ground, and its hollow stems stand straight and firm. The leaves all grow from the base of the plant and look almost like tiny green tubes when you cut them in half. Onion weed also produces pretty white or pink flowers that grow along the tall stems, which are even longer than the leaves. After the flowers are done blooming, the plant forms a small, wrinkled, round fruit.

The seeds of onion weed can start growing at any time of the year, but they sprout most often in late summer and autumn. In spring and summer, the plant grows flowers and fruits, but when those die back, most of the leaves stay alive through the hot months. In autumn, the onion weed gets a fresh start, with new leaves and stems growing again from the base of the plant.

For a closer look, here’s a picture of onion weed with its tall stems and pink flowers!

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

Growth Cycle of Onion Grass

Onion weed seeds can start growing at any time of the year, but they are most likely to sprout in late summer and autumn. However, the plant usually doesn’t flower until it’s about 18 months old. Flowering typically happens between June and October. Once the flowers have bloomed, the stems that held them die back, but some of the leaves will stay alive through the summer. In the following autumn, new leaves and stems grow from the base of the plant, giving it a fresh start.

Interestingly, onion weed behaves differently depending on the conditions. In some cases, the plant can flower and die within the first year, which makes it an annual. In other cases, it lives longer and becomes a biennial or a short-lived perennial. The seeds that onion weed produces are thought to stay alive in the soil for many years. However, in very dry areas, many seedlings don’t survive their first summer. But the ones that do make it through are quite tough and can handle dry conditions really well.

Onion weed has a shallow, fibrous root system and doesn’t have underground rhizomes or bulbs like some other plants. Instead, it stores energy in the base of its leaves. If the plant has enough water, it can live through the summer and become a perennial. But in drier areas, where water is scarce, onion weed focuses on producing lots of seeds, which helps it survive as an annual. Even though some seeds can live in the soil for at least a year, onion weed doesn’t build up a huge seed bank like some other weeds, such as salvation Jane. This means its population can change more quickly than weeds that store more seeds in the soil.

Effective Control Methods

Onion weed can be controlled through a combination of manual methods and the use of herbicides. The most effective way to manage this persistent weed is to prevent its spread and focus on new infestations as soon as they appear. According to past research, this approach is key to keeping onion weed under control. One important point to remember is that onion weed rarely becomes a major problem in well-managed pastures or cropping areas, where healthy plant competition naturally keeps it at bay.

Chemical

To successfully control onion grass in your fields, it’s important to use a mix of techniques. These can include manual removal, applying herbicides, and utilising specialised equipment like the Redback Weedwiper, which is designed to apply herbicide more precisely, making it cost-effective. There are a few herbicides that are registered specifically for onion weed control, but spraying large areas can be expensive if you’re not using controlled application methods like a weed wiper. Ongoing research is testing different chemical options to find products that can either stop onion weed from spreading or prevent it from producing seeds. 

Non Chemical

In addition to chemical treatments, there are non-chemical options available for controlling onion weed. One common method is hand-pulling, which can be effective, especially in smaller infestations. However, larger plants can sometimes re-root after being dislodged, making them harder to get rid of. Mechanical control, such as cultivation, can also be helpful, but it’s important to be cautious—dislodging big plants might cause them to re-root, and using equipment in areas heavily infested with onion weed could result in damage to your machinery.

Another way to tackle onion weed is through competition. Onion weed tends to thrive in areas where the pasture is overgrazed or the soil is low in fertility, particularly in nitrogen. Healthy, well-managed pastures with strong plant growth can crowd out onion weed and prevent it from spreading. Ensuring that your soil has adequate nutrients, especially nitrogen, can go a long way in keeping onion weed from becoming a problem.

Currently, we are not aware of any known biological control agents that have proven to be effective against onion weed. If you know of any biological controls that have worked, we encourage you to share your experiences in the comments!

Using a variety of strategies, both chemical and non-chemical, can help manage onion weed in your pastures. Keeping your land healthy and taking quick action against new infestations are essential steps in preventing this weed from taking over. 

Effective Herbicides

According to Agriculture Victoria, onion grass can be effectively controlled in established pastures using a registered metsulfuron-methyl herbicide. They say that it is crucial to apply the herbicide when the old corm is nearly exhausted and the new corm is forming, which is typically about six to eight weeks after the onion grass has emerged. This timing ensures that the young plant absorbs enough chemical to kill it. In addition, while treating during flowering can eliminate flowers and seeds, it won’t affect the corms. If broadleaf weeds like capeweed are also a concern, mixing metsulfuron-methyl with a compatible broadleaf herbicide will help manage both types of weeds.

Before applying any agricultural chemicals, it’s essential to read and understand the entire product label. There are various selective and non-selective herbicides registered for controlling onion grass in different situations. The Australian Pesticides and Veterinary Medicines Authority (APVMA) offers a searchable database called the Public Chemical Registration Information Search System, where users can find information on chemicals registered for pest and disease control in Australia. Please see the APVA for further chemical information www.apvma.gov.au 

Legal Status Of Onion Weeds

Onion grass was first introduced to Australia in 1857 in the Botanic Gardens of Melbourne and Adelaide as an ornamental plant. However, it soon escaped cultivation and became established as a weed. The first recorded instance of onion grass as a weed in the wild occurred in South Australia in 1881, with subsequent reports from Victoria, Western Australia, and New South Wales following soon after.

Today, onion grass is found across all Australian states and territories, though it is not classified as a Weed of National Significance (WONS). It is a declared weed in the Northern Territory and Tasmania but is not included on the National Alert List for Environmental Weeds or the Agricultural Sleeper List. While the weed poses significant challenges for farmers, it is not subject to national eradication programs.

We are preparing a separate blog to cover the legal status of onion grass in more detail, including state-by-state declarations and links to relevant resources.

Killing weeds with the Redback Weedwiper

The Redback Weedwiper is an innovative tool designed for the efficient application of herbicides, As a result, it allows farmers to target onion grass while minimizing the impact on desirable pasture plants. Here’s how to use it effectively for onion grass control:

1. Select the Appropriate Herbicide
Choose a systemic herbicide effective against onion grass, such as those containing glyphosate. Always follow our guide instructions for application rates and safety precautions.

2. Prepare the Redback Weedwiper
Ensure the weedwiper is clean and in good working condition. Proper maintenance ensures even herbicide application, which is crucial for effective weed control.

3. Adjust the Wiping Height
Set the height of the Redback Weedwiper to target the onion grass without harming desirable pasture species. The adjustable design allows for precise application, ensuring that only the weeds are affected.

4. Apply the Herbicide
As you move through the pasture, the Redback Weedwiper will apply the herbicide directly to the leaves of the onion grass. This targeted approach ensures the herbicide is absorbed and translocated to the roots, effectively killing the weed.

5. Monitor for Regrowth
After treatment, keep an eye on the affected areas for any signs of regrowth. This weed can be persistent, and follow-up applications may be necessary to ensure complete eradication.

6. Implement Integrated Weed Management
Combine the use of the Redback Weedwiper with other management practices, such as rotational grazing, competitive planting, or mulching, to prevent onion grass from re-establishing in your pastures.

REDBACK Weedwiper

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