WeedSmart Big 6 focus weed: Fleabane

Fleabane is estimated to cost growers $56 million in yield loss and control costs. It is also the most common weed in cotton fallows. More… Download the Impact of weeds on Australian grain and cotton production publication

The yield loss is mainly associated with soil moisture and nitrogen losses over summer. A weed-free summer fallow conserves soil moisture and nitrogen, doubling the yield potential for the following winter crops in Central NSW. In fact, complete control of summer weeds is the main driver of grain farm profitability. This research also demonstrates the economic benefit of prioritising weed control early in the fallow. More… Winter crop profits hinge on effective summer weed management

South Australian research found that controlling fleabane in summer conserved 45 mm and 71 mm of soil moisture at the Bute and Pinnaroo sites, respectively, as measured in April. More… Fleabane control in the south and west

There are three main species of fleabane (Conyza spp) in Australia:

• Flaxleaf fleabane (Conyza bonariensis) is the most common and widespread of the three Conyza spp. It is found in all cropping regions of Australia. Its leaves are blueish green, and the plants are highly branched.
• Tall fleabane (Conyza sumatrensis) is less common than flaxleaf fleabane. Its leaves are darker green and larger than those of flaxleaf fleabane. Its flowerheads are pyramid-shaped.
• Canadian fleabane (Conyza canadensis) is rare in Australia. It has lighter green leaves compared to flaxleaf fleabane.

Most Australian research has focused on flaxleaf fleabane. Tall fleabane has different characteristics and may require alternative management strategies, particularly in herbicide treatments. Correct identification is essential.

Key attributes of fleabane ecology (broadly applies to both flaxleaf and tall fleabane):

• Individual plants produce up to 110,000 wind-dispersed seeds.
• Wind dispersal of seed occurs from 3 to 35 meters from the parent plant, depending on seed release height, wind speed and humidity.
• Seeds are 80% viable and can germinate immediately upon landing on the soil surface. Establishment requires consecutive wet days (e.g. 20 mm over 4 days).
• Seed on the surface (0–0.5 cm depth) will germinate within 20 days of suitable moisture and temperature conditions being met (around 25 per cent of the seed present will germinate). The remaining seed does not persist.
• Seed can persist for 3-4 years when buried below 2 cm (deeper burial = longer persistence).
• Germination occurs at temperatures ranging from 5°C to 30°C (can germinate in winter crops, usually in gaps or late in the season as the crop matures).
• Very susceptible to crop competition.
• Naturally tolerant to glyphosate. After long-term exposure to glyphosate, most populations are now resistant to glyphosate once elongation has begun.
• Glyphosate application on mature plants reduces seed viability.
• Resistance to paraquat is becoming increasingly common.
• No resistance to 2,4-D amine or glufosinate has been detected to date.

Resistance status of flaxleaf fleabane

As part of the 2020 national random weed survey, an additional survey was conducted in northern NSW and Queensland just prior to the harvest of summer crops in 2021 to determine the resistance status of weed species, including flaxleaf fleabane. All of the nine fleabane samples collected in the survey were resistant to glyphosate. These samples were all susceptible to the other herbicides screened (2,4-D amine and paraquat+diquat). More… GRDC update paper: Herbicide resistance survey update (Feb 2026) and Have we lost some key herbicides in the Northern region

In 2018, random herbicide resistance surveys conducted by Mechelle Owens at AHRI found that 11 of the 94 fleabane populations collected in Western Australia were glyphosate-resistant.

In the 2023-24 summer, seeds were collected from flaxleaf fleabane populations at four locations in Western Australia’s grain belt – Northam, Badgingarra, Scaddan and Albany. Seedlings from these populations were screened at the rosette stage for susceptibility to glyphosate and 2,4-D ester.

The populations were tested against the standard rate of glyphosate registered for the control of common annual weeds in fallow (1x). The four populations varied from highly tolerant to glyphosate (10% injury) to highly sensitive (93% injury) at the standard rate.

All populations were sensitive to 2,4-D ester, but the level of injury caused by the standard rate varied from 63 to 98 per cent. This variability highlights the value of herbicide susceptibility testing before investing in chemical control strategies. More… Effective fleabane double-knocks tested in Western Australia

Big 6 management tactics to manage fleabane

The WeedSmart Big 6 program helps to tailor an integrated weed management strategy to tackle fleabane throughout the year.

1. Rotate crops and pastures

Fleabane is usually targeted as a fallow weed, often when it has matured somewhat and is difficult to control with knockdown herbicides. Targeting fleabane populations pre-plant, in-crop or immediately post-harvest in the winter cropping phase will reduce the number of escapes that form the first cohort of this difficult weed in the summer fallow. More… Control summer weeds like fleabane in winter and spring

There are opportunities to use fallows, summer and winter crops, as well as cover crops in a diverse farming system that provides competition and opportunities to use a range of herbicide and non-herbicide tactics to keep weed numbers low.

2. Crop competition

Fleabane was once considered a weed of summer fallow, particularly in the northern grains region. This was partly due to the effectiveness of crop competition in winter crops, which suppresses germination. Narrow row spacing of both summer and winter crops is an effective strategy against fleabane. Narrowing wheat rows from 50 cm to 25 cm spacing has a marked effect on fleabane seedhead production, with an additive advantage if the crop density is also increased from 50 plants per m² to 100 plants per m². More… Non-herbicide weed control in the Northern region

In summer crops, narrow row spacing (e.g. 25 cm) is the main driver to restrict weed growth, provided the crop is kept weed-free for at least the first three weeks (six weeks is better!) after planting. More… Creating stiff competition against summer weeds

Crop competition can also be achieved through summer cover cropping to drive down the fleabane seed bank. For example, the best summer cover crop options identified for Kingaroy were white French millet, Japanese millet, forage sorghum and buckwheat. Early-season biomass and ground cover were key to suppressing both grass and broadleaf weed mimics by up to 95 per cent when compared to a weeds-only fallow. More… Cover cropping for fallow weed control

In other research, cereal-dominated cover crops generated higher cash crop yields, system profitability, and improved soil health by generating additional crop residue, recruiting free-living soil nematodes, and improving soil aggregation. More… Cover crops suppress fallow weeds and support profitable grain crops

For maximum farming systems benefit, cover crops should be terminated mechanically or with herbicide at peak biomass and before seed set.

3. Mix and rotate herbicide MOA

Mixing, rotating and double-knocking herbicide strategies are important for fleabane control. For these to be effective, each component must be contributing to the overall control. For example, South Australian research suggests that the first knock in a double knock should provide at least 60 per cent control in its own right. More… Fleabane control in the south and west

Although many fleabane populations are resistant to glyphosate, fleabane is sensitive to a wide range of residual chemistry. Applying residuals pre-plant, combined with crop competition, will minimise in-crop establishment and reduce the initial summer fallow weed pressure. Be aware of the potential impact of residual herbicides on future crop choices.

For most (northern region) growers, glyphosate alone does not provide an effective knockdown treatment for fallow weeds, and where growers have moved to a double-paraquat use pattern, weed regrowth is becoming more common, including known paraquat resistance in tall fleabane.

Recent research and experience in the northern region suggest that incorporating both residual and knock-down herbicides into the summer fallow double-knock program is the best way to manage tough broadleaf weeds like fleabane. More… Boosting knockdown efficacy against hard-to-kill weeds

In Western Australia, three alternative double-knock strategies were tested on mature fleabane plants in summer fallow, with two of the treatments providing high-level control. These herbicide treatments also reduced the viability of the seed on the mature plants to very low levels. More… Effective fleabane double-knocks tested in Western Australia NSW DPIRD research showed that applying herbicide at early budding results in the greatest level of seed sterilisation in fleabane. More… Can herbicide affect weed seed dormancy and viability

4. Spray efficacy

Flaxleaf plants have a natural tolerance to some herbicides, including glyphosate, which is why very few glyphosate labels claim control of fleabane.

The leaves of fleabane plants are thickly covered with fine leaf hairs (trichomes). These fine hairs protect the leaf cuticle and interfere with herbicide uptake, particularly with water-soluble herbicides. An effective surfactant system is required to achieve adequate herbicide contact on the leaf cuticle to enable uptake.

The requirements for low-drift applications of 2,4-D also present some challenges. There is no simple formula for an ideal application set-up or surfactant system for fleabane. The GRDC’s Integrated Weed Management of Flaxleaf Fleabane publication provides helpful guidelines on pages 11–13.

Trichomes on small rosette-stage plants (< 5 cm), less than a month old, provide less protection for the cuticle, making young plants more susceptible to glyphosate. Once stem elongation begins, a much higher dose is likely to be required to achieve similar results. More… Fleabane control in the south and west

5. Stop seed set

It is important to realise that fleabane can still survive at very small growth stages under a competitive crop. Once the crop is removed, these plants mature very quickly, and the opportunity for effective control can be missed if in-crop escapes go unnoticed.

Crop topping in canola and pulses can reduce post-harvest spray program pressure before the main summer fallow period. This will potentially control small plants and reduce the viability of seed buds on more mature fleabane plants in the crop. The early fallow period is a pivotal point in any fleabane management program.

Weed detection and spot-spraying technology is particularly effective for targeting individual plants or small patches. More… Expanded options for spot sprayers

In some situations, mechanical control is warranted. Strategic tillage of weedy patches or paddocks can destroy mature plants and bury the weed seed bank. Keep in mind that fleabane seed buried below 2 cm can persist for 3–4 years, so avoid disturbing the treated area again for several years. Cultivation will typically trigger a germination flush of weeds that may require a follow-up double-knock herbicide application for small weeds.

6. Harvest weed seed control

Due to its light, fluffy seed, fleabane is not a strong contender for harvest weed seed control. Where a harvest weed seed control tool is used to manage other weed species, there will be some benefit for fleabane management through the destruction of immature seed heads.

Fleabane seed retention at harvest varies widely, ranging from less than 20 per cent to over 90 per cent. Consequently, the impact of HWSC on the seed bank is not a sure thing, but in some years could be very effective. More… Is weed seed control an option for managing Northern region weeds

Key resource: the GRDC’s Integrated Weed Management of Flaxleaf Fleabane publication