The latest on chemical use – by Pete Raal (Oct 3, 2011)

1.         Basal bark
All DOC field operatives (and some private persons) have been trained in using this methodology and are expected to go fully operational this summer. The method (Fig 1) is achieving great results both in terms of kill rates and financial savings. In a Twizel experiment, 250 hectares were treated using the basal bark methodology at a cost of $30/ha, compared to $72/ha using traditional methods. The work took 2 days to complete as opposed to what normally would have been >10 days work. This equates to a saving of >$10,000 for this job alone.



Figure 1 Field operative applying a basal bark application to wildings.


2.       Aerial wand
All DOC field operatives (and some private persons) have been trained in using this methodology and are expected to go fully operational this summer.


3.       Cut stump
Where the aesthetics of dead standing trees are an issue, the cut stump method can be used (Fig. 2a). Instead of having to remove all green needles as was done in the past, the remaining stump is sprayed with the basal bark herbicide mixture applied from a nozzle attached to the cutting boom (Fig 2b & c), which results in the death of the trees (Fig. 2d). Similar to basal bark, this new methodology is resulting in significant cost savings because of the quick application of the treatment compared with cutting, as in the past.

Figure 2 a) operator demonstrating the cutting technique, b) shows the herbicide lance along the boom, c) a close-up of the herbicide application to the cut stem, and d) dying foliage of a cut and sprayed stump.


4.       Frilling
Frilling of large trees (stem diameter >20cm) with the basal bark herbicide mixture (Fig 3.) is showing great promise. It is still too early to know whether the trees will die, as trials were only installed in late February 2011 and still have some time to run, but all indications are good. If this method works then field operatives have a system to treat trees of all sizes using one herbicide mixture, which solves the problem of site re-visits (again a significant cost saving).

Figure 3 Bark frilling technique with herbicide application is shown.


5.       Boom spraying
Although they are looking really promising, none of the boom spray experiments for wilding conifers have reached their final assessment.

The pilot trials installed in 2009 failed to kill all of the trees but provided the best indication of which herbicides would potentially kill wildings. These herbicides were re-applied in 2010 with higher application rates. The application rate was increased from 150 litres per hectare to 400 for improved coverage. One year after treatment we got a 98% kill of the 5-10m high contorta within one of the replicates which had a stocking of approximately 10 000 stems per hectare.


The baseline herbicide for our mixtures is now 30 litres of Grazon with 20 litres of Kwickin oil ( applied in 400 litres of water. However, one replicate is not significant in scientific terms and the experiment using this herbicide mixture was repeated this January 2011 (Fig. 4). Three replicates each have been installed in Twizel, Wanaka, Queenstown and Mid Dome on contorta (6 replicates), mugo, Douglas fir, Scot's and Corsican pines (3 replicates each). Three other herbicide mixtures (with Grazon as the baseline ingredient) have also been installed in an attempt to get a 100% kill rate. The results of these experiments will only be known in 18 months time.

Figure 4. Twizel trial with 12 replicates of 3 different herbicide treatments applied to a number of different wilding species.


I will assess these again in November-December 2011 and if the experiments indicate that they are going to be successful (kill >95% of trees) this summer then I will make available the recipes to DOC operational staff to field test the herbicide mixtures on dense infestations of trees in different localities.