Summer weeds

Page last updated: Tuesday, 4 September 2018 - 11:29am

Fleabane

In Western Australia, there are three species of fleabane present, flaxleaf fleabane (Conyza bonariensis), tall fleabane (C. sumatrensis) and Canadian fleabane (C. canadensis).

Two species of fleabane (flaxleaf fleabane (L) and tall fleabane (R) at Woogenellup, South Coast Western Australia
Canadian fleabane at Esperance
Flaxleaf fleabane at Esperance

All species are very difficult to kill in summer, when they become large and hairy, have a decent tap-root and are under moisture stress. It is recommended to try and control them directly after harvest and not wait until mid-summer.

  • Flaxleaf fleabane is the most common, prevalent along the south coast but is found to be spreading across much of the wheatbelt. It is a weed of both pasture and cropped paddocks but appears to be worse in areas that have recently been cropped.
  • Both tall fleabane and Canadian fleabane are more prevalent in the wetter areas. There has been some indication of the species hybridising in the field although this is as yet unconfirmed.

Fleabane seeds have greatest germination at 20°C (although some will germinate at temperatures ranging from 4.2-35°C). Therefore, in WA, germination is reduced in late autumn and early winter, and greatest germination occurs in spring and early summer. Following rainfall (in optimal temperature conditions), there is an initial rapid germination of seed after rain followed by a slow but steady decline in seedbank numbers over time. However, the seed bank can last for over three years.

Research in southern Queensland suggests that most fleabane seeds germinate from the soil surface with very few seeds germinating from below 1cm. This suggests that the recent fleabane problem in WA is a result of the switch from conventional to minimum tillage systems. These low disturbance tillage systems are less likely to bury seeds below 1cm depth and provide moist conditions for better emergence of seeds that germinate on the soil surface.

In the WA wheat belt, fleabane often germinates in spring and early summer prior to harvest. Once the crop is removed, the fleabane has no competition for light or moisture and can grow rapidly, especially with further summer rain. By the time there is a window for control, the fleabane are often mature, with a large root system, a reduced leaf area and a high tolerance to most herbicides. Large infestations of summer weeds have been implicated in reductions in available soil moisture for the following crop resulting in yield losses.

Control

  • Fleabane can most effectively be controlled when they are emerging, mainly in early spring while they are still small. Small fleabane plants are relatively easy to kill and a late post-emergent application of some Group 1 (phenoxy) herbicides during spring can control them in cereals.
  • Mature fleabane can be difficult and expensive to kill, especially in mid to late summer.
  • Trials conducted by Department of Primary Industries and Regional Development researcher Sally Peltzer in 2009 indicated that the best control of large fleabane in stubble used a ‘double-knock’ approach with a range of primary herbicides followed by paraquat 7-10 days later.
  • The most effective treatment was an application of glyphosate (540g/L) at 2L/ha or a mixture of glyphosate (540g/L) at 2L/ha and 2,4-D amine (625g/L) at 2L/ha with a follow up spray of paraquat at 2L/ha seven days after the initial treatment

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