Onehunga weed has finely divided leaves, which may get confused with several other weeds found in the lawn. One of these is yarrow, but yarrow is a perennial weed which has a creeping rhizome system underground, whereas Onehunga weed has fairly shallow fibrous root system. Yarrow has many more leaflets per leaf than Onehunga weed. The other main weed which looks like Onehunga weed is soldier’s button (Cotula australis), but this tends to be found more in gardens and paths than in the lawn. Soldier’s button is also an annual, which quite quickly produces noticeable seed-heads in the form of little brown buttons on the end of stalks, unlike the inconspicuous fruits of Onehunga weed.
Even though Onehunga weed is only a small annual weed, phenoxy herbicides such as MCPA, 2,4-D and mecoprop do not control this weed very well by themselves. By adding dicamba to any of these herbicides, better control is obtained, eg 2,4-D/dicamba (Banvine), MCPA/mecoprop/dicamba (several trade names).
Probably the most favoured way of commercially controlling this weed in New Zealand turf though is through use of clopyralid (eg Versatill) or a picloram/triclopyr mixture (eg Victory Gold). Note however that we have recently located a strain of Onehunga weed in several locations north of Auckland that is resistant to both clopyralid and picloram, and this is now being found in other parts of New Zealand as well, such as Mount Maunganui. This is a warning not to rely too heavily on these two herbicides year after year to control Onehunga weed.
The resistant strain looks different to the normal Onehunga weed (see picture above-right). If you find you have Onehunga weed which doesn't die after spraying with one of these products, and it has leaves like those shown in the picture, try spraying it with an alternative herbicide before it gets a chance to seed and thus multiply. An MCPA/mecoprop/dicamba mixture has been found to be effective at killing this resistant strain.
Other herbicides that can be used include a mixture of mecoprop, ioxynil and bromoxynil (eg Image) and bentazone (eg Basagran). These generally work best under warm conditions but before the plants get too mature. In golf courses, a very low rate of metsulfuron is often used in New Zealand for turf weed control, though this is not a registered use for this herbicide, and it would not be a good idea in ryegrass sportsfields. Metsulfuron would presumably do a good job of controlling resistant Onehunga weed plants.
Whatever herbicide is used on the Onehunga weed, obviously it is best to kill the weed before the fruits begin forming, otherwise prickles will still end up in the feet of people walking across the dead plants. But for the long-term management of the weed, it is also useful to stop new seeds being added to the soil each spring so that eventually there will be less seeds from which new plants can establish in autumn.