Pest shrubs and trees invade forests, riversides, high country, wetlands, and coastlines. They alter natural ecosystems by shading the ground, changing the soil conditions, and displacing native species.
Several methods of control can be used. The one you choose depends on the size of the infestation, and how persistent the species is. Be aware that most pest shrubs and trees will resprout if you simply cut them down.
Non-herbicide control
Ring barkingHand pulling
Many seedlings can be pulled out by hand. Try not to disturb the soil more than is necessary or new weed seeds will germinate.
Ring-barking
Has limited effectiveness as many trees can re-sprout from the base, or heal the wound in their bark (e.g. sycamore). Ring-barking can be useful for controlling pines.
If you do want to try it, use a sharp chisel, axe or saw to make two parallel deep cuts right around the base of the tree. Make the cuts into the sapwood at least 5cm apart and remove all bark between the cuts.
Herbicide control
Cut & paint stump treatment
Darwin’s barberry cutThis is the best technique to use for most situations, particularly with smaller trees and shrubs and those that are likely to re-sprout from the base (e.g., wilding pines, elderberry, Darwin’s barberry)
First, cut the trunk of the plant close to the ground with a straight flat cut. The cut must be flat so that the herbicide will sit on the cut area.
Then, paint the stump with herbicide within 30 seconds of cutting to get uptake of the herbicide before the sap stops flowing. Use a squeeze bottle or paintbrush to just wet the surface, avoiding excess run-off.
Herbicide gel formulations are useful for this and can be purchased from most garden supply stores. Follow the manufacturer’s instructions.
Spray
Spraying young plants less than a metre high can be effective. Spray all parts of the plant using a knapsack sprayer or hand gun.
For bigger woody plants, use the cut & paint stump method outlined above. It minimises the release of herbicide into the environment while still being effective.
6 top tips for spraying
- Check the best time of year to apply the spray. Plants usually absorb the most lethal dose of chemical during growth spurts prior to flowering or fruiting.
- Check you have the correct herbicide for the weed you want to kill.
- Follow the manufacturer’s instructions regarding mixing and application. Do not dip used containers into a water supply, and if you’re spraying over or near water, ensure you’re using the recommended spray.
- Add a sticking agent (surfactant) to improve the effectiveness of the spray. You could also consider using marker dye to see where you’ve been, or a foaming agent to minimise spray drift.
- Choose fine, calm weather. A very light breeze can help you control the direction of spray drift.
- Cover new plantings when spraying nearby.
Drill (or slash) & inject
Useful for large trees (e.g., crack willow) and shrubs in places where felling would damage the surrounding vegetation. You can use a sheep drench pack and gun, or a plastic squeeze bottle with a long nozzle.
First: Drill holes sloping down into the sapwood at regular intervals around the base of the tree. Alternatively, use a sharp chisel or axe to make deep cuts into the sapwood at regular intervals around the base of the tree. Don’t completely ring-bark the tree as that will reduce the uptake of herbicide.
Then: Place the correct dose of herbicide (see manufacturer’s instructions) into each hole or cut as soon as possible.
Note: this method involves leaving dead trees standing, so check for potential danger to people using the area.