There are many hard to kill weeds, including flaxleaf fleabane, however one approach delivers consistent success – spray small weeds and use the double knock technique.
At the end of grain harvest in the southern and western regions flaxleaf fleabane will be establishing and growing strong root systems ready to take up any soil moisture available over summer. Flaxleaf fleabane plants feature hairy leaf surfaces, thick cuticle and few stomata, a combination that affords the weed a natural tolerance to herbicide.
Fleabane seedlings germinate throughout the spring and summer, making the timing of control very difficult.
In South Australia fleabane seeds start germinating in late winter to late spring, but initial seedling growth rate is very slow. With suitable spray conditions being few and far between over summer, fleabane is able to establish and take advantage of any small falls of rain to produce up to 120 thousand viable light fluffy seeds per plant that disperse on the wind and in runoff water over summer and autumn.
Weeds researcher Ben Fleet from the University of Adelaide says the timing and staggered germination of fleabane, coupled with the need to treat plants when they are small, is a combination that makes the weed very difficult to control with herbicides.
“All flaxleaf plants have a natural tolerance to herbicides but they are much more susceptible to herbicide control when the plants are young, less than a month old,” he said. “While rosette stage plants can be easily killed with lower rates of glyphosate, once stem elongation begins a far greater dose is likely to be required to achieve similar results.”
“In NSW and Queensland, glyphosate resistance has been identified in flaxleaf fleabane populations, indicating that while glyphosate has proven an effective tool on fleabane, increasing resistance will mean this herbicide will be less effective in the future.”
In summer fallow, herbicide control trials at Bute and Pinnaroo in South Australia, robust rates of glyphosate provided the greatest level of control. Use of paraquat in a double knock herbicide strategy helped to achieve high levels of control, but only when the first herbicide application was capable of providing at least 60 per cent control in its own right.
“Controlling fleabane in summer conserved 45 mm and 71 mm of soil moisture at the Bute and Pinnaroo sites respectively, as measured in April,” said Mr Fleet.
Flaxleaf fleabane, melons, sow thistle, windmill grass and feathertop Rhodes grass have all been associated more with the northern region but in fact they are all weeds that perform well in zero and minimum tillage systems. As these practices are becoming more common in the south and west, so the associated weeds are becoming more of a problem, particularly in years with mild, wet spring and autumn conditions.
“The mix of weeds present in a summer fallow varies dramatically between seasons in the southern region as these weeds respond to the prevailing seasonal conditions,” he said. “A few years ago, the combination of wet springs and good summer rainfall led to serious infestations of fleabane on many farms in South Australia. Then the subsequent run of dry spring and summers led to a decline in fleabane populations on farms to the point that researchers had difficulty in finding suitable trial sites.”
Mr Fleet emphasised that the efficacy of glyphosate on fleabane varied considerably in different seasons. “For example, glyphosate alone applied at 2 L/ha provided a modest 55% weed kill in 2012 but gave 97% control in 2014. In all seasons a double-knock of paraquat after glyphosate treatment ensured a higher weed kill,” he said.
Aside from herbicide controls, don’t underestimate the value of strong crop competition. Fleabane thrives along crop borders and in gaps that may appear within the crop.
Mr Fleet said that while herbicide control can be effective, particularly when plants are treated at the seedling stage (rosette), it is also important not to underestimate the value of crop competition in the winter cropping phase.
“Fleabane seedlings are highly sensitive to crop competition and any bare patches in a paddock provide an ideal environment for fleabane establishment,” he said. “Under moisture stress conditions in spring there tends to be extremely high seedling mortality.”