Pest plants

Pest plant control techniques

There is a huge range of control methods for plants. Most methods are listed below, in generally descending order of environmental selectivity i.e least disturbance. For recommendations on control for individual species use our Plant search.

Drill and fill

Generally best for big trees, uses less herbicide than most other methods and uses minimal amounts of water. Can generally be used all year round. Aim is to access cambium layer (under bark) so deep holes not necessary. Only suitable for use in areas where eventual tree fall will not pose a risk to people or property.

Cut and squirt

Good for soft trees (quicker to do than drilling). Cut a notch in a downward angle in the trunk and squirt concentrated herbicide in notch. Use drench gun and pack. Generally use all year round.

Frill

Usually inferior to drilling as it uses more herbicide and requires complete ring around trunk to be made. Herbicide can run off. It is superior to stump painting for large trees, some hardwoods and self-sprouting species (e.g. willows, coral tree). Semi-frilling (feathering) can hold herbicide better.

Vial treatment

Use to give selective control of rhizomatous or layering creepers (jasmine, convolvulus, ivy, etc.). Individual flower vials are ideal for this, and are available from garden centres.

• Cut vines 5-10 m apart, place end of vine in bottom of bottle containing concentrated herbicide. Vine sucks up herbicide and kills 2-to-many metres away.
• Follow up monthly on missed spots.

Stump paint

Application of herbicide to cut stump surface. Solution is usually 10% herbicide in water. We do not recommend using diesel as advised on some labels.

• The cambium layer (the slippery zone under the bark) must be coated, as this is where most growth sprouts from.
• ‘Feathering' of bark of hardwood species and big trees is generally recommended. This means peeling back the bark so that the cambium layer is well coated and more herbicide mix can be applied. 
• ‘Painting' can be done by paint brush, drench gun, knapsack or pistol-grip sprayer. Pistol-grip sprayers are especially recommended, as they don't spill if tipped over and require only one hand to operate.
• Stump painting should never be used for resprouting stem species (e.g. willows, phoenix palm, coral tree) because felled stems resprout constantly, only adding to the problem.

Weed wipe

This is particularly useful for grasses, rushes and soft herbs. Non-selective herbicides (eg glyphosate) can be made to act in a selective manner using a hockey stick type wiper. Likewise a residual herbicide (e.g. Metsulfuron, Amitrole, 2,4-D) can be applied by wiper to minimise or even eliminate residues, as the herbicide is contained within the plant rather than drifting or dripping onto the ground. Most or all of the herbicide is broken down within the weed. Look for a weed wiper that has a narrow or controlled release reservoir.

Hand dig

Usually only suitable if entire root system can be dug out. Soil disturbance can lead to more weeds. Not recommended for resprouting species (e.g. tradescantia), as any fragments left will regrow.

Foliar spray

Can be used to apply non-selective herbicides semi-selectively.
To ensure minimum drift of spray :

• Do not spray if there is any wind.
• Use a medium sized nozzle or adjust nozzle so that droplet size is such that the spray doesn't ‘float' away.
• Ideally use a marker dye with the spray mix to avoid missing areas or spraying areas twice.
• Always test the sprayer with water before starting and clean & adjust nozzles if necessary.
• Avoid spraying plants above shoulder height. It is better to cut plants to a more manageable height and minimise spray damage to desirable species.

Weed mat

Offers good control of many submerged aquatic weeds, especially non-seeding species. However it is expensive and time consuming to install.
Aquatic situations:

• Can be laid in small sections, beginning at upstream-most point of infestation, and reused several times. Also has limited terrestrial applications.
  Terrestial situations:
• No disposal needed, but mat can have undesirable effects on soil (lowers water and gas exchange, kills microbes) and can affect adjacent desirable plants by interfering with their roots.

Machine dig

Occasionally useful to remove hard-to-kill individual plants (eg Arundo, bamboo) or to clear aquatic plants where control can be achieved by digging and the removed plants are guaranteed to perish on dry land. Obviously machine digging is an extreme measure, causing a host of environmental effects, and should only be used where the weed has high potential and no other control method exists.