What is a ‘herbicide halo’ and why is it so effective for weed control?

Invasive weeds have always presented a significant challenge for farmers and land managers. These persistent plants do more than just reduce the performance of pastures and crops in paddocks – they can also impact livestock health, increase costs, and threaten native ecosystems.

Some of the trickiest areas to control are around fence lines and water points, where weeds can establish rapidly and aggressively, requiring lots of manual upkeep.

That’s where the ‘herbicide halo’ comes in. This method uses residual herbicides to create a long-lasting weed barrier in tough-to-manage spots.

In this how-to guide, we’ll explore how residual herbicides work, why they make managing invasive weeds easier, and how to implement the herbicide halo for superior, sustainable land management.

Understanding Australia’s invasive weed problem

Invasive and noxious weeds are a major issue for Australian land managers, farmers, and environmentalists. According to the Australian Department of Agriculture, Fisheries and Forestry (DAFF), in an average year, pest and weed control costs Aussie landholders $5.3 billion, with weed control making up 82% of this (approximately $4.35 billion). 

As well as the financial effects, weeds have a number of far-reaching impacts to consider: 

• Competition with desirable plants: Weeds compete fiercely with crops, pastures, and native plants for limited resources like water, sunlight, and nutrients. Many thrive in the Australian conditions, giving them an advantage over desirable vegetation.
• Reduced pasture productivity: Weeds can – and will – quickly take over grazing land, leaving less forage available for livestock and ultimately impacting productivity.
• Harm to livestock: Some invasive weed species are poisonous or cause health issues for animals.
• Fire risks: Tall, dry weeds can increase the impact of fires and create accessibility problems for firefighters. 
• Environmental damage: Weeds can reduce biodiversity and alter ecosystems as a result of outcompeting native plants.

Which weeds are the most problematic in Australia?

There are hundreds of weed species in Australia, but some of the most troublesome include: 

• Parkinsonia
• Serrated tussock
• Giant rats tail grass
• Lantana
• Prickly acacia
• Blackberry
• Siam weed
• Cactus 
• Parthenium weed
• Fleabane
• Gorse

These particular weeds have a habit of establishing easily along fences, waterways, and roadside areas, making management especially difficult to stay on top of.

What are residual herbicides?

Residual herbicides are chemicals with long-lasting activity in the soil that kill existing weeds and prevent new ones from growing. Unlike non-residual knockdown herbicides (short-lived, immediate action), they create a protective zone that is effective for months to years. They are usually applied prior to the planting of crops or before weeds emerge.

According to a report from the Grains Research & Development Corporation (GRDC), the longevity of a residual herbicide is dependent on the chemical itself, as well as the soil matter and conditions. Some active ingredients will support weed control for up to two years. Always consider these factors and your unique conditions when selecting a herbicide.

How do residual herbicides work?

Once a residual herbicide is applied to the soil, it is absorbed through the plant’s roots or foliage. Here, it begins to interfere with the physiological processes of the plant with varying results that depend on the specific herbicide chosen. After weeks or months, depending on the mode of action, the plant will show signs of sickness and eventually, die. Simultaneously, the chemical continues working in the soil to suppress new weed growth.

Where do residual herbicides work best?

Residual herbicides work best in hard-to-reach or high-traffic areas where weeds pose a constant threat. These environments include: 

• Along fencelines where grasses and vines climb and spread
• Around water points where weeds clog troughs or spread via livestock
• In rugged or remote terrain where regular manual spraying is difficult

What is the ‘herbicide halo’ method?

The idea of a ‘herbicide halo’ is simple. It’s a zone of residual herbicide activity that is used as a chemical barrier to prevent new weeds from germinating and establishing.

By using the herbicide halo method, you will drastically reduce the need for ongoing control efforts and mitigate the further spread of weeds throughout your property. 

Most importantly, it’s effective against the weeds that often accumulate in tough spots like fences, water points, and access tracks.

The herbicide halo in action

A Queensland cattle producer we worked with applied a residual herbicide using the halo method, along a 2km fenceline that had been a nightmare for weeds like lantana and prickly acacia. Eighteen months later, the fenceline remains clean, and annual spraying has been reduced to just two quick spot treatments for upkeep.

How to create a herbicide halo

To leverage the herbicide halo method, always treat the bare soil around the weeds, not just the weeds themselves. Think of it as drawing a chemical barrier that surrounds the problem area.

Then, even if weed seeds land inside this zone, they won’t germinate as they will be inhibited by the residual herbicide. Adopting this approach offers a longer-term solution and reduces the frequency of future control efforts.

Benefits of the herbicide halo

Using residual herbicides to create a herbicide halo offers several additional benefits, including:

• Reduced spraying: By employing this method, you’ll need to spray less along fences and water points in particular. 
• Cleaner infrastructure: Use a herbicide halo to prevent weeds from climbing fences, clogging water troughs, blocking waterways, and restricting access to property points.
• Time and cost savings: This application method reduces the need for manual labour and lessens chemical use over time, which can result in significant savings.
• Environmental benefits: Targeted control reduces weed competition, protecting pastures, crops, native plants, and soil health as a result. 

Choosing the right residual herbicide for your needs

Selecting an herbicide that doesn’t address your needs can waste money or damage your land – so it’s critical to get it right.

Which residual herbicides are the most effective?

At Specialist Sales, we have over 20 years of experience advising landowners and farmers. There are a handful of systemic residual herbicides we think are worth considering when dealing with stubborn, hard-to-kill weeds in the Australian context: 

• Tebuthiuron granules: Ideal for long-term woody weed control in non-crop areas, tebuthiuron eradicates woody weeds while leaving pasture grasses unharmed.

• Hexazinone: This is an excellent choice for controlling regrowth in mixed woody weed infestations. 

• Aminopyralid, picloram and triclopyr mix: These three actives offer the best combination for knockdown and residual control for over 50 weed species, particularly when used alongside a good penetrant or wetting agent.

• Flupropanate (granular or liquid): Flupropanate provides ideal long-term treatment for noxious grasses like giant rats tail grass, serrated tussock, and African lovegrass. 

• Metsulfuron: This chemical is used to selectively control broadleaf and woody weeds. 

How to apply residual herbicides

Selecting the right residual herbicide formulation and following application instructions to the letter is critical for comprehensive weed control.

There are two main formulations available on the Australian market – granular and liquid. Let’s compare the two: 

Granular residual herbicides (e.g., flupropanate or tebuthiuron)

• Applied directly to the soil
• Activated by rainfall
• Best for large or remote areas
• Long-lasting (up to several years’ control) 

Liquid residual herbicides (e.g., aminopyralid, triclopyr, and/or picloram, flupropanate)

• Applied using a sprayer
• Absorbed quickly by the plant
• Suitable for targeted applications
• Requires careful application to avoid drift

Select which formula works best for you depending on your unique weed control needs and setting. Both granular and liquid herbicides are effective when employing the herbicide halo method.

Top tips for applying residual herbicides

Before you get stuck into applying residual herbicide on your property, consider these best practice application tips:

• Always calibrate your sprayer to avoid over- or underapplication.
• Avoid steep slopes near waterways to ensure there is no leaching or runoff risk. 
• Apply to moist soil for the best possible product uptake. 
• Combine with a knockdown herbicide (if appropriate) for immediate residual control.

Timing your residual herbicide application

Residual herbicides work best before weeds are actively growing, and when soil moisture is adequate. In Australia, the best windows are late autumn, early winter, and late spring. 

Integrating residual herbicides into your weed management plan

Residual herbicides are powerful tools, but they work best as part of an integrated management plan. Here’s what else to consider for best results: 

• Grazing management: By strategically grazing livestock, you can support the suppression of weed growth.
• Mechanical methods: Slashing areas can reduce seed production, as long as the weeds have not yet set seed. If they have, slashing is counterproductive and will only spread the weeds more.
• Improving pastures: Establishing dense, competitive pastures can crowd out weeds and limit further growth.
• Regular monitoring: Early detection and treatment of weeds enables more effective treatment and less spread.

Frequently asked questions about weed control with residual herbicides

Can residual herbicides damage pasture grasses?

It depends on the type of chemical you choose. Some, such as tebuthiuron, are selective and won’t affect pastures, however, flupropanate will harm native grass species. Always check the product label to ensure your chosen herbicide does not have any undesirable effects for your specific context. 

How long does flupropanate stay in the soil?

Depending on soil type and rainfall, the common residual herbicide flupropanate can remain active in the soil for between 12 and 24 months. 

Are residual herbicides safe near water points?

Some residual herbicides can be used near water points as long as label restrictions are followed, but generally, due to the ongoing control nature of these chemicals, it is not recommended. Always avoid direct contamination of water sources and create buffer zones.

Why not just spray the weeds you see (instead of using a herbicide halo)? 

Only spraying residual herbicides on existing weeds is a common mistake. While this can help to treat the weeds you see, it doesn’t stop new weeds from germinating from seeds that blow in or remain dormant in the soil. Spraying the surrounding areas and creating a protective zone is much more effective for ongoing weed control.

Invasive weeds are a relentless challenge, but by using a quality residual herbicide to create a herbicide halo, landowners can establish strong, long-lasting barriers to protect infrastructure and reduce spread. Both existing weeds and new invasions will be kept under control, saving money and time and supporting sustainable land management.

Always follow the product label directions and check the Safety Data Sheet for your chosen herbicide. If you need help selecting the right residual herbicide for your needs, get in touch with the team of experts here at Specialist Sales.