This is the most popular chemical wilding control technique, with the main determinant of success being the degree of foliar coverage/penetration achieved. Hence, kills are not difficult to achieve on scattered smaller trees readily accessed by ‘nozzle’ pressure sprayers ( ground machine or helicopter mounted). Mixes of metsulfuron (1g/10 l) and glyphosate (1%), plus a surfactant such as Pulse (1%), have been consistently successful. However, this mix can cause considerable collateral damage to non-target species. In this context, Reglone (10%) plus a surfactant (1%) can be used with far less collateral damage, but results on wildings have been more variable.
Boom spraying of conifers from the air has proven much more of a problem, with results characterised by considerable variability. One exception is larch, which has not been hard to kill, but it is a different story for other conifers. Hence, a series of new trials have recently been installed by DOC, Scion and ECan – employing a range of chemical mixes on contorta pine, dwarf mountain pine (P. mugo), Corsican pine and Douglas-fir. The results of some of these look promising with the prospects of good conifer kills (particularly on Douglas-fir), but it is too early to be able to give recommendations confidently. The most likely chemicals are also being tested for helicopter ‘nozzle’ spraying of lone trees. Impacts on non-conifer species, especially native plants, have yet to be quantified.
In summary, it will probably be a year to 18 months before more reliable recommendations are available for the cost-effective spraying of conifers from the air. Refer to the Manual (for download click here) for extra present-day detail.
Finally, a mention must be made of the growing international concern about the use of herbicides, particularly when applied extensively from the air - to the extent that aerial application is banned in places. It is absolutely inevitable that this concern will increase in NZ, particularly as we depend on exports for our wealth – and they rely considerably on our maintenance of a ‘clean, green’ image.