Summer weed control needs to set up next season

Image of fleabane

A fleabane rosette at 5 centimetres diameter. Once larger than this, fleabane becomes difficult to control.

PHOTO: Michael Widderick

Staying on top of weeds will be critical over the 2016-17 summer to preserve stored soil moisture from the above-average winter rainfall and any rainfall over late spring and summer.

In noting this, University of Adelaide weeds researcher Dr Gurjeet Gill says GRDC-funded research in New South Wales found that for every dollar invested in herbicides during the summer fallow period, the return on investment ranged from $3 per hectare to $8/ha.

This return on investment comes from more nitrogen in the soil in wet years and an increase of up to 86 millimetres more plant-available water at sowing when summer weeds are controlled.

However, the list of weeds for growers to control over summer is expanding and improved tactics need to be adopted to control these weeds while also avoiding glyphosate resistance.


Dr Gill says fleabane has been one of the most problematic broadleaf summer weeds in the southern region over the past 10 years.

Image of Dr Gurjeet Gill

The University of Adelaide’s Dr Gurjeet Gill.

PHOTO: Sharon Watt, Porter Novelli

According to the GRDC’s Impact of weeds on Australian grain production study, fleabane is the third-ranked summer fallow weed nationally by area, yield loss and revenue loss.

The University of Adelaide team, led by Dr Gill and Dr Christopher Preston, has noticed fleabane plants starting to germinate in trial plots during late winter and early spring.

“Fleabane can be found throughout the southern region, usually on roadsides, but given the opportunity with rainfall in spring, it can start to establish in paddocks because of wind dispersal,” Dr Gill says. “Fleabane plants at pinhead stage are there waiting for the right conditions and warming of temperatures. They will then start to grow and often only become large getting close to harvest.

“Once a crop has taken off and the canopy opens up, fleabane will start to grow quite quickly, which is when growers notice it.”

Dr Gill says trials in South Australia, Western Australia and NSW have found a double-knock approach to be the most effective way to kill fleabane. This involves using glyphosate mixed with phenoxy herbicides as the first spray, followed by a second spray of paraquat a week to 10 days later.

It is important that the first knock is as effective as possible to get high levels of control with the double-knock approach.

New kids on the block

The encroachment of feathertop Rhodes grass (FTR) and windmill grass into the southern region from northern cropping areas is of particular concern for Dr Gill.

As part of a GRDC-funded project, Dr Gill has been looking into the biology and management of these weeds.

“FTR is an interesting one because its base temperature for germination – about 4°C – is fairly low by summer weed standards,” he says. “That is much lower than ‘normal’ summer weeds, which germinate at about 10°C.

“FTR appears to be a weed that is likely to be able to germinate under August or September temperatures and get a foothold earlier in the season than something like windmill grass, which belongs to the same genus.

“The other worrying thing about FTR is that seeds can spread up to 30 metres from the original plant, which confirms grower observations as to how easily it can spread.”

Windmill grass does need warmer temperatures to germinate, however Dr Gill says it does not rely on seeds for establishment.

“It can behave like a perennial plant, producing new leaves and regenerating from the crown,” he says. “Shorter periods of wet soil over summer may not be conducive for its seeds to germinate but because windmill grass can grow from the crown it can establish from that.”

Dr Gill’s research into FTR has found that a single panicle or head can have up to 1000 seeds, making it is easy to see how it has managed to spread so far. Furthermore, both FTR and windmill grass are hard to kill with glyphosate and therefore a double-knock is also the best approach to controlling these weeds.

“These weeds – both FTR and windmill grass – are inherently tolerant to glyphosate,” Dr Gill says. “What makes it worse is that we have identified two populations of FTR in SA which are glyphosate resistant.

“Again, we need to be using a double-knock. Farmers may be using glyphosate but they need to add a Group A herbicide to it as a first knock and then follow up with paraquat a week or 10 days later. Glyphosate alone will not solve the problem.”

Established foes

Another summer weed for growers to look out for across the southern region is heliotrope, or potato weed – the number-one-ranked summer weed in the southern region by area, yield loss and revenue loss.

Heliotrope (blue weed) behaves similarly to fleabane but can be killed with straight glyphosate in normal situations, Dr Gill says.

Sowthistle, traditionally regarded as a winter weed, has been found to be germinating later in the season and causing problems for growers through summer. The same goes for other weeds such as prickly lettuce and marshmallow weed.

“Their management, much like fleabane, works best with a double-knock strategy,” Dr Gill says.

“This year summer weeds could do particularly well. They are inherently hard to kill with glyphosate because of their genetic make-up but also because hotter, drier conditions are not conducive for glyphosate to work. This is why growers need to be looking at double-knock strategies.

“We often see broadleaf weeds such as sowthistle and prickly lettuce coming out of paddocks that have weak competitors such as lentils, field peas or pastures, allowing these weeds to continue into spring and summer and become a bigger problem.

“Cereals, on the other hand, tend to be more competitive against these weeds,” he says.

More information:

Dr Gurjeet Gill, University of Adelaide,

Impact of weeds on Australian grain production
Summer fallow weed management


Gas-tight a must for grain storage


Cost-effective weed control in the Mallee

GRDC Project Code UA00149

Region South