Control ‘summer weeds’ like fleabane in winter and spring

The Northern Grower Alliance (NGA), led by Richard Daniel, conducted important trial work many years ago that demonstrated a dramatic improvement in control when growers stop targeting large plants in the summer fallow as the primary management timing for these weeds.

“Fleabane plants growing in the summer fallow are generally more difficult to kill as they are often large, moisture-stressed and frequently resistant to glyphosate,” he said. “Targetting fleabane populations pre-plant, in-crop or immediately post-harvest in the winter cropping phase is usually far more effective.”

Crop competition, and particularly avoiding wide row cropping, considerably suppresses fleabane growth. If wide row cropping is required then an emphasis must be placed on effective residual herbicides.

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

“It is important to realise that fleabane can still survive at very small growth stages under a competitive crop and is easily overlooked,” said Richard. “Once the crop is removed, these plants develop very quickly and the opportunity for effective control can be missed if the in-crop escapes go un-noticed.”

Cultivation may be the most economic and effective strategy to kill large, stressed plants growing in fallow, however growers must also prepare and implement strategies to manage the next flush of fleabane emergence.

Richard said there are several knock-down and residual herbicides that growers can use to target small and actively growing fleabane before, during and immediately after the winter cropping program or in a winter fallow. The following crop’s sensitivity to residual herbicide must be taken into account when making product choices.

Another weed that has been very difficult to manage in a summer fallow is feathertop Rhodes grass (FTR). In paddocks with a feathertop Rhodes grass problem, sorghum is generally not the best crop choice because there are no effective post-emergent control options and available residual chemistry will not provide season-long suppression. Crops that allow selective in-crop grass control, such as mungbeans or even sunflowers, can provide more management flexibility.

“Like the fleabane situation, there are also opportunities to commence feathertop Rhodes grass management late in winter or during spring,” he says. “Intensive patch management is also an effective strategy as feathertop Rhodes grass is generally found in well-defined patches in the first few years of colonisation.”

For small patches or new incursions, chipping, pulling, spot spraying or cultivation can be used. Burning, followed by cultivation and or the use of residual herbicides, appears to be a salvage strategy for removing mature plants. Time spent eliminating small patches of feathertop Rhodes grass will be cost-effective in the long run.

The in-crop options for feathertop Rhodes grass in winter crops are not extensive and, like fleabane, feathertop Rhodes grass is generally one of the key weeds that survives in fallows where glyphosate is the dominant herbicide.

A 2020 GRDC Update paper by Richard Daniel, Hanwen Wo and Mark Congreve concluded with the following strategy:

Where there has been a ‘blow-out’ and a paddock has become infested with feathertop Rhodes grass, it is likely that a management strategy will look something like the following:

• Cultivation of established plants (before seed has been shed)
• Residual herbicide applied if there is still the possibility of further germination before winter
• Aggressive crop competition (and pre-emergent herbicides) in the winter crop
• Decide on your strategy for summer before the first spring rainfall event, and potential weed germination
  • If fallowing over summer, apply a residual herbicide before spring rainfall. Monitor frequently for any escapes or breakdown of the residual herbicide. Access to an optical (camera) sprayer can be beneficial in cost effectively treating isolated escapes. Once the residual treatment has broken down, a double knock application will be required, with consideration given to including another residual application with the second knock
  • Where soil moisture is adequate, consider a summer broadleaf crop (or cotton where suitable). Use a pre-emergent herbicide effective on grass weeds; keep row spacing narrow and plant population high to increase the benefit of crop competition; and utilise a selective Group 1 ‘fop‘ herbicide in-crop to control any escapes. Inter-row tillage is also a valid strategy in wide row summer crops.
• Do not plant sorghum or maize into paddocks with a high feathertop Rhodes grass seed bank population. Pre-emergent herbicide options such as Dual Gold are unlikely to provide full season residual control, even at the highest application rate – especially in wet seasons. Late-season germinations can establish after the pre-emergent herbicide has broken down
• If feathertop Rhodes grass is not allowed to set seed, then seed bank viability should be low the following year. Continue vigilant management for another season to ensure depletion of the seed bank.

Unfortunately, the best management strategies for this difficult to manage weed place great reliance on herbicides; and therefore, selection of resistant individuals. Glyphosate is ineffective and Group 1 herbicides are known to be of significant risk of rapid selection for resistance.

While there is currently negligible resistance in the northern grains region to many of the pre-emergent herbicides with efficacy on this weed, one thing we have learnt from history is that if we over-rely on a particular herbicide or herbicide group and don’t stop weed seed set of survivors, then we will break it.

First published September, 2015. Republished May 2026

Fleabane thrives where there is no crop competition and small plants can easily go unnoticed late in the winter crop. (Photo: Anthony Mitchell, NGA)