How do I stop gazanias invading my crops?

Gazanias are a popular and robust perennial groundcover plant in gardens, with pretty flowers, dense foliage and multiple modes of propagation. These same traits give this species opportunities to invade and establish in agricultural settings as a weed.

Dr Ali Bajwa, a weed scientist at La Trobe University, says that gazanias have become more common on road verges during the last decade and, more recently, have started to invade cropping paddocks in several low rainfall regions of South Australia and Victoria.

“Our first step in the GRDC’s National Grower Network project to investigate management options for gazania was to conduct a survey to determine the extent of gazania spread through the Southern grain production region,” he says. “There are a two main species of gazania present in Australia, and while we are seeing some distinction between the species that prefers coastal environments compared to the one that prefers drier, inland areas – it is likely that the species are interbreeding, which makes management more difficult.”

“We are also investigating the immediate actions that growers can take to restrict the invasion of gazania into their cropping areas.”

There are no herbicides registered for the control of gazania in Australia, and growers have generally observed that knockdown herbicides applied to control other weeds, particularly in summer fallows, have little effect on established gazania plants.

“The leaves of mature gazania plants are thick and waxy, and the plants shut down their metabolism when conditions are dry,” says Ali. “Once plants have more than 10 or 12 leaves, knockdown herbicides are fairly ineffective. Mechanical control is the only option, but this also comes with risks of spreading the weed’s vegetative propagation structures, commonly known as rhizomes.”

Ali’s research on gazania will extend to weed ecology studies that will potentially point toward a combination of tactics to control this invasive species. He expects to identify how the WeedSmart Big 6 weed management strategies can be adapted to suppress the impact of this weed and restrict its spread.

What have you observed during the survey and preliminary weed ecology studies?

The surveys along major road networks in the Southern region identified major hotspots where gazania infestations have established in cropping paddocks in the Mallee region and on South Australia’s Yorke and Eyre peninsulas. The research team collected data at over 800 sites, mainly from roadsides adjacent to cropping paddocks.

The survey also identified considerable variability in gazania plant growth stages, leaf shape and overall plant architecture. This variability adds another layer of complexity when devising management strategies for this invasive species.

In a separate project, a PhD student working with Dr. Bajwa, and supported by a La Trobe University scholarship, is investigating the seed ecology of gazania populations. Gazania plants are prolific seed producers, and seed germination rates are high (up to 95%), even after fresh collection. Seed from four sites were germinated under three day/night temperature ranges, and it was clear that gazania seeds have a consistently high germination rate (over 80%) potential throughout the year.     

Seeds responded to soil moisture for germination, and being a coastal plant, the seed can tolerate high soil salinity. Seeds buried below 2 cm of soil did not germinate, although some seed did emerge from a depth of 4 cm of sand.

Although the occurrence of gazania is currently sporadic in the southern broadacre cropping regions, modelling by Weed Futures shows that vast tracts extending from New South Wales to Western Australia provide suitable climatic and environmental conditions that would allow this weed to spread widely. The highest invasion risk being in South Australia, where gazania is a declared weed (and it is on the advisory list in Victoria). Stricter legislation and enforcement are required to restrict the sale of this species to gardeners.

Are there any effective herbicide options growers can deploy?

Gazanias have unpalatable waxy leaves that protect the plant from grazing, salt air and moisture loss during drought.

In addition to their competitive traits, such as being drought-hardy, prolific seeders, surface germinators, vegetative reproduction via underground rhizomes and likely exuding allelopathic compounds into the soil, gazania leaves also resist herbicide uptake.

A glasshouse experiment on chemical control at La Trobe has included four major growth stages of gazania. In the first plant stage (0–18 days from germination) the plants grown in the glasshouse had just three leaves, and by stage 2 (18–35 days) the plants had 12 leaves and were 19 cm high. At maturity (stage 4, 60–80 days), the plants had 106 leaves, were 30 cm high, had a canopy diameter of 51 cm, and had produced 13 flowers.  

Screening of 20 herbicide treatments in the glasshouse showed that 77 per cent of plants sprayed at stage 1 (3 leaves) were killed, but this dropped to just 50 per cent when plants were sprayed at stage 2 (12 leaves). Plants sprayed at stages 2 to 4 were also more likely to recover after sustaining herbicide injury.

The most promising herbicide tactics are currently being tested at field trials at Loxton and Arno Bay in South Australia.

What other strategies can growers implement?

While herbicides are being tested and potentially identified for registration or emergency use permits, growers can utilise high label rates of knockdown herbicides through optical spot sprayers during summer fallow to suppress weed spread.

Cultivation may have a place to bury surface seed and induce germination from both seed and rhizome buds. This would then be followed with herbicide application when the gazania plants were at growth stage 1 (3 leaves or less). However, further research and development work is required to devise and test management strategies to minimise the risk of spreading gazania through cultivation.

Once established, gazania can compete strongly with legume crops and can also survive well within dense cereal crops. The hope is that an effective pre-emergent herbicide will be identified to give growers another tactic to stop the spread of this weed.

GRDC project code: ULA2402-002RTX