Basal bark application

This is a new and simple technique with potential for killing wildings still in the ‘soft-bark’ phase of growth and up to medium-size (up to 15cm in diameter at their base).  Trials have been installed over the last 2 years (mostly by DOC), so no guaranteed recommendations can be made yet – although the mix of 20% Grazon in oil/diesel looks promising (P. Raal, pers comm.). Refer to the Manual (download click here) for further detail and also see this (Link) for the most recent news on this work.


Copied from email sent around by Pete Raal (DOC, Dunedin), 2/11/2010

Attached are my latest specifications for the basal bark spraying of willows (and most other woody species including pines, alders, elders, sycamores, hawthorns, cotoneasters, rowans, barberries etc) using a knapsack. All of the treated willows in the experiment (and some by-catch hawthorns and sycamores) have completely died.

A clearer description of the technique is given below.


As discussed at NETS last week, pines, sycamores and willows (and most other woody species) can be poisoned using low volume basal bark applications of Grazon (600g/l triclopyrbutoxy ethyl ester) herbicide. The technique is only really effective on trees with a stem diameter of less than 15cm and which have not yet developed a thick bark. As the trees become bigger, the bark becomes rougher and thicker and the technique becomes less effective (although I would try it on bigger diameter trees to see what happens - just spray higher up the stem for about 2m rather than just 40 - 50 cm). I killed a 20m high sycamore with a very thick trunk by spraying all the way around the trunk from ground level to as high as I could reach.


Basal bark applications of 200 millilitres Grazon in 800 millilitres petroleum-based oil or diesel should be used to poison the trees. For difficult to control species such as hawthorns, use 300 ml Grazon in 700 ml oil.


No water or water-based products such as dye, for example, must be added to the mix. This is because an invert emulsion will be formed which will clog up your knapsack. If you want/need to mark the trees you have treated, spray them with spray paint or mark them in some other way, just don’t use dye in the mix.


The technique is effective year-round for the selective control of unwanted pine trees and other woody species (i.e. it can be applied at any time of the year, including winter months, except when water or snow prevent spraying at the desired height above ground level - see below). Treatment should, however, ideally occur 6 weeks prior to leaf expansion until 2 months after to ensure the control of the root systems of targeted plants.

For best and reliable results, spray to saturate the entire circumference of the bottom 30 - 50 cm (up to 2 m for bigger trees) of trunk, including the root collar area, until just before the point of runoff using a knapsack (one dedicated for oil use only) and a solid cone or flat fan nozzle. Care must be taken to minimise the amount of spray drift and chemical/oil that runs into the soil which could potentially damage adjacent non-target trees. This is only because there is the possibility of injury to plants whose roots may extend into areas treated with the herbicide. Particular care must be taken to ensure that the oil does not get into water in a wetland situation (you may want to apply the solution using a paint brush in these instances).

The spray should not be used if there is "free water" on the trunks which would cause the oil/diesel to emulsify and run down the trunk without penetrating the bark. If this happens, a chemical girdle will not form and the control is unlikely to be complete. For this reason, spraying should not be done during or after recent rain or when there is frost on the trunks.

Triclopyr does not remain active in the soil for long where it is dissipated by microbial action into triclopyr acid.

Although much quicker and efficient than cutting and pasting, frilling or drilling and filling, basal bark treatments are labour-intensive because each and every stem needs to be treated. For this reason it can reasonably be expected that some trees and saplings will be missed during a poisoning operation. Follow-up operations should therefore be planned for missed stems, new saplings and root suckers. Usually one or two follow up spot treatments at 6-month intervals will provide a complete kill if the trees are susceptible. Re-treatment should include any living parts of treated stem(s) and re-sprouted stems.

Spray entire saplings with the brew to kill them.

If you use this method, please let me know how it all goes (practicality of application in your situation, effectiveness of the brews on your woody species etc) so that I can collate all the results/outcomes into a database that I am maintaining (This email address is being protected from spambots. You need JavaScript enabled to view it.). Please also bear in mind that the trees may take up to 1 year to die so don't draw conclusions too soon.