Once only found in localised areas in Western Australia's eastern and south-eastern wheatbelt, matricaria (Oncosiphon suffruticosum and Oncosiphon piluliferum) is a weed that is fast becoming a problem in other parts of WA's grain growing region.

Well-known for its distinctive and unpleasant odour, particularly during the flowering stage, matricaria is an invasive weed that requires only limited moisture to germinate, producing up to 100,000 seeds per plant.

Newly germinated seedlings also have a strong smell, which is one way to identify the weed in new areas when no flowers are present.

Previously only found in the eastern wheatbelt and a small part of the south-eastern region, matricaria has now been found in the northern wheatbelt and has spread west into some parts of the central wheatbelt.

With co-investment from GRDC, research into the biology, germination triggers and management options of matricaria by the Department of Primary Industries and Regional Development (DPIRD) forms part of the five-year national Integrated Weed Management project.

This research, specifically relating to matricaria populations on pasture paddocks, has revealed seed can remain viable in the ground for up to five years.

Germination

DPIRD research officer Alex Douglas says trials in Merredin show the tiny seed can remain dormant in soil when buried between two and 10 centimetres, and will only germinate when exposed to sunlight and a small amount of moisture.

"This means that when any type of soil inversion or ploughing occurs, and seed is bought up to the surface, it just needs a small rainfall event and it will germinate, particularly in standard WA autumn temperatures," Ms Douglas says.

"As an example, one of the sites that I was monitoring received no summer rain, but the seed was able to germinate after receiving just 15 millimetres in autumn."

Ms Douglas says even though paddocks may be left to pasture or fallow in a mixed cropping and livestock enterprise, control of the weed is critical to reduce the seed bank for future grain crops.

She says many fallow paddocks are left alone for the season, meaning weeds are not controlled and seed banks can increase greatly.

While not toxic to livestock, the weed is relatively unpalatable, and sheep will eat all other weeds before matricaria. Ms Douglas says her research has shown early herbicide applications on small plants is the best way to manage the weed. However, later season spray topping just prior to flowering and seed set is also a way to reduce seed populations.

Trials testing herbicide applications at three different stages from early bud formation to full flowering demonstrated that the earlier the application, the more effective the control of seed set.

"However, the research also showed that using a non-selective herbicide in that latest time bracket, during flowering, can reduce seed viability even if the seed has been set.

"Meaning if you have a matricaria population that you haven't been able to control during the winter period, there is still a salvage option later in the season," she says.

While some growers have previously thought matricaria was a summer weed, Ms Douglas says it is more likely to germinate through the autumn, winter and spring months.

"Any spring rain can see the plant germinate late and hang on into the summer, which then poses a problem for further seed set," she says.

Ms Douglas says halting the spread of the weed must now be a key priority for the industry.

"This is such an invasive weed that needs to be controlled urgently to reduce its impact on future farm profitability," she says.

"With many growers owning or managing properties in different locations, our concern is that the weed could be transported on machinery and moved across the grain growing areas, spreading the problem into non-traditional matricaria growing zones."

More information: Alex Douglas, alex.douglas@dpird.wa.gov.au