Controlling privets
There are many ways of killing weed trees. Each has advantages that suit particular sites and situations. Here, two rapid-action herbicide methods are contrasted with a slower one that uses no herbicide. They all work well for us.
Cut & paint method
Cut or saw the privet as near to the ground as practicable. Immediately apply glyphosate herbicide (360 g/l, dyed with food red) using only as much herbicide as the wood will absorb without overflow. The red dye shows which ones you have treated. A speedy application is needed, because the newly-exposed sapwood actually sucks in the herbicide. If you delay, it simply sucks in air.
Make a compact pile of the privet branches - resist the urge to burn them! All kinds of small creatures use such piles for shelter and food. In any case, burning gets rid of the nitrogen nutrient in the branches and litter, nitrogen that otherwise would encourage the growth of native trees.
Stem injection
This method is used to kill large privets and camphor laurels without cutting them down. You need a chisel of about 1 cm end - the one in the photo has a red ribbon so that it doesn't get lost in the bush! Hammer the chisel through the bark and downwards into the sapwood. Three holes should be sufficient for this tree - more for bigger trees. You must hammer downwards so as to get a good hole to receive about 0.2 ml of herbicide. This will be quickly absorbed into the sapwood. An alternative is to make holes with a battery drill and a 10-mm drill bit.
This method kills within a few weeks, and most of the leaves should fall off sooner than that. Birds will perch in the killed tree and poop seeds of rainforest species such as cheese tree and lillypilly. They are attracted by the insects that invade the dead wood.
Different physical methods of getting the herbicide into the tree can be equally effective - the point is to get the glyphosate into the sapwood, which has a negative pressure and therefore sucks the herbicide up. Depending on what tools you have, chiselling, drilling or frilling the base of the tree with a hatchet will work as long as you apply the herbicide straight after making the cut into the sapwood. Thin stems can simply be bent over and a drop of herbicide applied to the fractured stem at the bend. This is useful when you are faced with a privet stump that was cut down a year or two back and has responded by producing a thicket of shoots. However, bear in mind that the dead shoots may interfere with subsequent weeding.
Ringbarking (no herbicide needed)
You may not want to use herbicide. Also, rainforest seedlings sheltering under a privet canopy can be frizzled by the sun if the canopy opens up too suddenly. This is where ringbarking is particularly useful, as the privet dies slowly. This allows rainforest seedlings that had been sheltered by the privet time to adapt and harden to exposure. However, ringbarking does demand a little more care and follow up than the herbicide method.
Scrape about 10 cm of bark down to the wood. Do that at waist height rather than at ground level. The blunt side of small bush saw makes a convenient scraper. Do not cut into the wood – but do scrape the wood free of the soft tissue under the bark.
This technique removes the vessels called phloem – they transport the sugar that is made in the leaves to the roots. The roots require a constant supply of sugar to grow. When your ringbark stops the supply of sugar to the roots, they slowly starve.
Bear in mind that ringbarking takes time to kill. In addition, there is some aftercare that you must attend to -
A few soft shoots will appear in the months following the treatment, generally just below the ringbark, as in the bottom photo. Just brush these off with your hand. If you don’t, they will harden up and grow into a new privet tree!
Also in the photo, you can see a corky layer growing down from the top end of the ringbark. If you didn't scrape the ringbark well enough, a green layer of new tissue can grow over the ringbark, turn into cork and bridge the gap.
Ringbarking may stimulate the tree to flower – it gives one big try to reproduce before it dies. But the burst of flowering doesn’t usually produce much fruit. And, once the tree dies, it leaves a skeleton through which birds eagerly search for insects. It's also interesting to see how fungi tend to grow above the ringbark on the dead privet. This is presumably because they are feeding the carbohydrate that was unable to pass the ringbarked section.
A ringbarked privet - note the wood has been scraped clean but not cut into. Depending on how well you did the scraping, the ringbark may need cleaning up after about a week or so. This is because living tissue left on the wood tends to regenerate, first forming a soft green layer, then cork under which phloem will re-form.
The ringbark after a couple of months. The soft shoot that has emerged just below the ringbark is easily brushed off.
The photo on the right shows the well-designed herbicide dispenser that Council supplies to its Landcare groups. The nozzle can be adjusted so that a precise drop of herbicide can be applied to the exact place where it is needed.
Note: glyphosate herbicide (in our case Roundup Biactive) is a Schedule 5 poison, that is, it:
has a low toxicity
has a low to moderate hazard (exposure has been associated with some increase in cancer incidence)
can cause only minor adverse effects to human beings in normal use
requires caution in handling, storage or use.
Additional Information & Notes
In its native countries of China, Vietnam and Laos, some 170 natural enemies of privet have been recorded, so there may well be potential for biological control of this pest in the future. Reference to the insects that feed on privet in its native range:
Yan-Zhuo Zhang, James L. Hanula, Jiang-hua Sun (2008) The Florida Entomologist, 91, Survey for potential insect control agents of Ligustrum sinense (Scrophulariales:Oleaceae) in China http://journals.fcla.edu/flaent/article/view/75822
Note: if you choose to dilute it, only prepare sufficient herbicide to use immediately or within a couple of days. Glyphosate is quickly broken down by microorganisms and, within a few days of dilution, it is likely to have been inactivated. Undiluted herbicide doesn't break down so readily.
Stumps left projecting from ground level are DANGEROUS - they can impale you if you fall on them, so make sure to cut them off near ground level.
Sap in the trunk is under negative pressure (it is being sucked up by transpiration from the leaves). This is why the herbicide is so quickly absorbed, as long as you apply the herbicide immediately after cutting.
Weeds are a valuable source of nutrients. Use them as a mulch where you want rainforest trees to grow faster. Remember, rainforest plants require soils richer in nutrients than plants adapted to sandstone areas.
Stem injection of herbicide doesn't disturb the roots - an advantage when you are weeding erosion-prone areas.