New research to understand the spread of gazania (African daisy) in grain paddocks and identify potential chemical control options is showing promise for growers plagued by this aggressive weed.
The research is supported by the Grains Research and Development Corporation (GRDC) after growers raised concerns about the weed via the National Grower Network.
Gazania is a member of the daisy family and was introduced to Australia from South Africa as a hardy garden flower. It is well suited to fragile, sandy soils and has proved to be drought hardy and highly invasive.
However, it has since spread rapidly – usually via poor, unmanaged soils such as those along roadsides and fence lines.
It is now infesting cropping paddocks on South Australia's Eyre and Yorke Peninsulas, as well as the SA and Victorian Mallee. It is encroaching grain crops in the New South Wales Riverina and parts of Western Australia.
South Australian grower Robin Schaefer, Loxton, has seen gazanias overtake parts of the SA's northern Mallee region in less than a decade, after spreading from just one or two domestic front gardens.
"They would spread along the roadsides and we'd see the odd plant pop up in our paddocks," he said.
"We didn't really take a lot of notice, but they were slipping through our weed control program."
Mr Schaefer said more gazanias started to appear around those first individuals six or seven years ago. In the past four years, large, late-season rain events have helped gazanias germinate more successfully in the early fallow period, when there is little crop competition after harvest.
Dr Ali Bajwa shows the substantial seed load produced by a single gazania plant each season. The seeds are easily dispersed by wind, water and animals, and germinate readily under most environmental conditions. Photo: Dr Ali Bajwa, La Trobe University
"Some aspects of our rotation system may have helped the gazanias to spread and, because they're so robust and hardy, conventional weed control strategies just don't work," he said.
"Plenty of local growers have seen the same pattern on their properties.
"I know of at least one farmer who hasn't been able to grow a crop on several of his paddocks for two or three years because gazanias have completely taken over and he's got no way to control them."
Gazania plants are perennial and grow vigorously, forming thick monocultures that quickly outcompete other vegetation including native plants and crops. They store energy and reproduce via underground rhizomes, while also producing thousands of highly mobile wind-borne seeds each year. These seeds readily germinate across a wide range of environmental and soil conditions.
The rhizomes mean physical removal is often unsuccessful, while the waxy leaves reduce herbicide absorption, resulting in poor control.
Over the past two years, La Trobe University Senior Lecturer in Weed Science & Agronomy Dr Ali Bajwa and his team have undertaken extensive surveys to map gazania infestation and invasion trends across the southern region as part of the GRDC investment 'Gazanias incidence and impact on grain cropping in the southern region'.
They have also trialled several herbicide options against a range of gazania growth stages in pot trials at La Trobe, as well as in field trial sites on Mr Schaefer's Loxton property and near Arno Bay on the Eyre Peninsula.
Their research shows certain knockdown herbicides can be highly effective while gazania plants are very young (2-4 leaf stage). However, the efficacy of most products dropped significantly once plants were established with 10-12 leaves, while controlling plants with 20-plus leaves was almost impossible with most chemicals.
Unfortunately, these plants are naturally tolerant to our typical nonselective knockdown herbicides such as glyphosate and paraquat, Dr Bajwa said.
However, we are seeing some promising results from products that have residual activity – with data from extensive glasshouse and field trials suggesting certain group 4 and group 14 herbicides in different combinations may provide effective control.
No herbicide products are registered specifically for gazanias, and therefore any research findings cannot be recommended to growers yet.
Further field-based validation of various chemical control options will be needed to identify effective herbicide use patterns and support potential registration.
In the meantime, Dr Bajwa recommends growers act early to eradicate fenceline gazanias and any new incursion into paddocks. Digging out and deep burial or burning of single gazanias is the best approach, while spot application of suitable nonselective herbicides is recommended for small, sporadic infestations.
Dr Bajwa's team has observed that lentils and other legume rotations can be a weak link in gazania management, so it is important to strengthen weed control in and around those crops. Good farm hygiene when moving machinery or stock from infested paddocks can also help reduce the spread of weed seeds.
Gazanias need national action, as they are already present next to grain production paddocks in the southern, northern and western regions, Dr Bajwa said.
While we will have more to share on effective chemical control through this GRDC investment, grain growers are going to need a complete integrated weed management strategy to keep gazanias out of their paddocks.
Gazanias were raised as an issue during GRDC National Grower Network Forums in SA in 2023 following the wet season of 2022.
Shortly afterwards, WeedSmart Southern Extension Agronomist Chris Davey provided GRDC with results of an informal social media survey that mapped roadside gazania populations near grain production paddocks across WA, SA and Victoria.
GRDC Crop Protection Manager – South Ruth Peek said the NGN proved to be an invaluable channel for growers to communicate their concerns.
"Once we were made aware of the issue, GRDC partnered with Dr Bajwa and team at LaTrobe University to further research gazanias and investigate possible control options for growers," she said.
"Gazanias have fast become a significant concern for growers in several regions and Dr Bajwa's research has put us on the front foot when it comes to evaluating control strategies.
"His findings will help inform future investments in gazania management and GRDC will continue to monitor the issue in partnership with growers through the NGN."
