Ground Cover Supplement : GC Supplement - Summer weeds
11 Issue 137 | Nov – Dec 2018 | GRDC GROUNDCOVER SUPPLEMENT: SUMMER WEEDS GROUNDCOVER FLEABANE FLAXLEAF FLEABANE: A YEAR-ROUND CHALLENGE Flaxleaf fleabane is a troublesome weed in the northern region and an emerging weed in southern Australia, with more than 65 populations confirmed as resistant to glyphosate. Effective control will require diligence and the application of multiple herbicide and non- chemical control tactics during both winter and summer KEY POINTS n Control flaxleaf fleabane in all phases of the rotation n Double-knock spraying while plants are less than one month old provides the most effective control in fallow By Dr Michael Widderick n While flaxleaf fleabane (Conyza bonariensis) is the most common species found, tall fleabane (C. sumatrensis) is becoming more common and there are also populations of Canadian fleabane (C. canadensis). A single uncontrolled flaxleaf fleabane plant has the potential to set up to 110,000 seeds, so stopping seed-set is critical to managing this weed. Flaxleaf fleabane has the ability to emerge at temperatures of between 10°C and 30°C with a preference for autumn and spring, but is also able to emerge during mild winters and summers if sufficient moisture is available. It produces very small seeds that only emerge when the seed is in the top centimetre of soil. As a result, this weed is most common in reduced and zero-tillage systems. The majority (90 to 95 per cent) of flaxleaf fleabane seeds lose their viability within 12 to 18 months on the surface soil. However, a small percentage can persist for several years, particularly if seeds are buried two to five centimetres below the surface. CULTURAL CONTROL Flaxleaf fleabane is poorly competitive, so growing a competitive crop such as barley or increasing the competitiveness of wheat using a narrow row spacing and increased crop density can greatly reduce in-crop fleabane growth and seed production (Figure 1). Current GRDC research through the Queensland Department of Agriculture and Fisheries and the University of Sydney is evaluating the impact of a competitive winter wheat crop on the growth of spring-emerging fleabane. Cultivation is very effective at controlling large fleabane plants; however, if the plants have started to set seed, any buried seed can remain viable for up to three years. This means any subsequent cultivation will bring viable seed back to the soil surface layers for emergence. HERBICIDES Flaxleaf fleabane is best controlled with herbicides at a young age, up to 5cm diameter rosette – or less than one month old. Knockdown of large plants, especially in summer fallow, gives variable results and should be avoided. Double-knock is the most reliable tactic for fallow knockdown control of fleabane. Research has consistently shown glyphosate + 2,4-D or picloram/2,4-D products (for example, TordonTM 75-D) as the first application followed by paraquat or paraquat+diquat five to 10 days later provides excellent control (93 to 100 per cent). For low-density populations, consider applying the double-knock using a weed detection unit to minimise herbicide use and cost. Residual herbicides can provide good control of flaxleaf fleabane seedlings for several months in fallow, pre-planting and during the crop phase. These herbicides need to be applied before weeds have emerged, or following (or mixed with) an effective knockdown herbicide or double-knock. Balance® and TordonTM 75-D can provide residual control of flaxleaf fleabane in fallows. For fallow herbicide treatments to be effective, full label rates are needed as well as high water volumes (100 litres per hectare) for paraquat products, particularly for dense populations and high stubble levels. o GRDC Research Codes UQ00062, US00084 More information: Dr Michael Widderick, Queensland DAF, 07 4529 1325, email@example.com; www.grdc.com.au/GRDC-FS-Fleabane PHOTO:QUEENSLANDDAF 25cm 50cm 90 80 70 60 50 40 30 20 10 0 Density: 50 plants/m2 100 plants/m2 Flaxleaf fleabane seeds per m2 Row spacing SOURCE: QUEENSLAND DAF FIGURE 1 Increasing crop competition in wheat greatly reduced the number of fleabane flower heads at crop maturity at Warwick, Queensland. Efficacy of the double-knock (right) on flaxleaf fleabane in fallow. Untreated control on left.
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