Containing pompom weed with registered herbicides is not a short-term undertaking, making eradication impossible
Jeremy Goodall, Ed Witkowski
ARC-PPRI & University of the Witwatersrand
Trials were conducted to test the performance of the registered herbicides picloram (Access 240 SL, Dow AgrSciences) and metsulfuron-methyl (Brush-Off, Du Pont) on the control of Campuloclinium macrocephalum (pompom weed) in hydric and xeric grasslands in Gauteng Province. Both herbicide formulations were sprayed as broadcast foliar applications at rates specified on the herbicide labels for three consecutive years in either summer (February) or autumn (April) and monitored for an additional 3 years after spraying was discontinued. Metsulfuron-methyl and picloram were equally effective according to the timing of applications in both vegetation types. However, both achieved an average pompom weed mortality that was <80% of what is expected of registered herbicides. Uncontrolled factors, that included outbreaks of the rust Puccinia eupatorii, fires and drought, occurred from the second year of the study. Gas chromatography of topsoil did not detect either metsulfuron-methyl or picloram during any of the sampling intervals from 0 to 56 days after treatment. Registered herbicides did not reduce C. macrocephalum successfully after three annual applications and it is estimated that around 5 summer or 7 autumn treatments are required to reduce weed density
to <1 plant per plot (25 m2). More research is required to study rust–herbicide interactions, the role of fire in managing asteraceous weed seed banks and fire as a pre-treatment that could be integrated with chemical control.