Conyza bonariensis (L.) Cronquist, flaxleaf fleabane, originating from South America, is a

major emerging weed threat for dry-land cropping systems in Australia. Conyza bonariensis

is particularly increasing in importance within the northern cropping region of Australia, is

one of the most difficult-to-control weeds in minimum tillage systems, and is tolerant to

important herbicides. Conyza bonariensis is common in fallows, thereby depleting the soil

stored moisture, and has caused a doubling of control costs in certain areas of the northern

cropping region. Control costs are likely to further increase due to the weed’s rapid

development of herbicide resistance. Control of C. bonariensis is greatly dependant on

herbicides, thereby increasing the risk of herbicide resistance.

In this study, ecological aspects of the key life stages of C. bonariensis were investigated. All

findings were compared with a congeneric species, C. sumatrensis (Retz.) E. Walker (tall

fleabane), which is currently not problematic in cropping systems in Australia, despite being

present in the region within other ruderal sites (e.g. roadsides), as a way of determining

what ecological characteristics in C. bonariensis may be responsible for its increase in the

northern region cropping system.

Germination was limited by temperature, moisture and light. Seeds of C. bonariensis

germinated between 10 and 30oC, with optimal germination at 25oC. Conyza bonariensis

seeds were able to germinate under moisture stress down to -0.8 MPa. Light was essential

for germination of C. bonariensis. In a 90% shade environment, C. bonariensis germination

was reduced by 80% compared with a full light environment. With adequate temperature,

light and moisture, C. bonariensis seeds can germinate within 2 to 3 days. Soil type and

stubble levels affected C. bonariensis emergence. Emergence was reduced in heavy black

vertosol soil compared with lighter soils. There was no significant difference in emergence

with 1.8 t ha-1 of stubble compared with no stubble.

There are differences in development and fecundity between C. bonariensis emergence

cohorts. More than 85 000 seeds were produced per plant in the overwintering cohort,

which was 40% higher than the spring emerged plants. The root:shoot ratio at the time of

stem elongation in overwintered C. bonariensis plants was 60% higher than spring emerged

plants. This ecological feature makes the late-autumn cohort more difficult to control.

There was a short period of six weeks between stem elongation and seed production in

C. bonariensis, and with a slow response to herbicide, this adds to the success of this weed.

Conyza bonariensis seed settling velocity and pappus geometry was affected by humidity.

Settling velocity was 0.28 m s-1 at 30% humidity and increased to 0.33 m s-1 at 90% humidity.

Conyza bonariensis seeds were not able to emerge from burial depths of 0.5 cm or greater,

although the length of time that the seed remained viable increased with burial depth. Seed

longevity at 1 cm burial depth was 37 months and at 10 cm depth, this increased to 80

months. Seeds which enter a minimum tillage system typically remain on or near the soil

surface, the preferred germination site for C. bonariensis, therefore adding to the weed’s

success in these systems.

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In comparison with C. sumatrensis, C. bonariensis produced more seed, had a higher relative

reproductive effort, developed more rapidly, could germinate in milder temperatures only

and had a longer lived seed bank. These ecological findings are likely to account for the

greater success of C. bonariensis in minimum tillage cropping systems. The effective longterm

management of C. bonariensis requires an integrated approach to weed management, in

which herbicide use is complemented with non-chemical control measures. Through

limiting soil disturbance, there will be a reduction in the burial of seed and thereby a more

rapid depletion of the seed bank, assuming there is no further addition of seed to the soil

seed bank from elsewhere. Where appropriate, cultivation could be used to bury the seeds

of C. bonariensis and prevent germination or to perhaps kill overwintering plants with large

taproots. Agricultural practices should also aim to maximise competition, including shade,

against C. bonariensis. Diligent control is required to prevent seed set, especially for the

overwintered plants which are more difficult to control and have a higher seed production.