Although the use of chemicals is not always essential, herbicides can be an important and effective component of any weed control program.
In some situations herbicides offer the only practical, cost-effective and selective method of managing certain weeds. Because herbicides reduce the need for cultivation, they can prevent soil erosion and water loss, and are widely used in conservation farming.
In some cases, a weed is only susceptible to one specific herbicide and it is important to use the correct product and application rate for control of that particular weed. Common mistakes include incorrect identification of the weed or using inappropriate products.
In most cases, weeds must be actively growing to be vulnerable to herbicide treatments.
Herbicide resistance can also be an issue with some species.
Conditions such as wind speed and direction, the possibility of rain and proximity to waterways should also be taken into account when preparing to use herbicides.
It is extremely important to read and follow the information contained on the herbicide label. This includes:
- the signal heading (indicating the product’s hazard level)
- the trade name
- the claims for use
- the active constituent
- the net contents
- directions for use
- limitations for use
- withholding period
- important notes
- storage instructions
- safety directions and first aid
- dangerous goods notification
- expiry date
- mode of action (type of herbicide)
By law, herbicides can only be used in accordance with the label.
The Australian Pesticides and Veterinary Medicines Authority (APVMA) is the Australian Government authority responsible for the independent assessment and registration of pesticides and veterinary medicines. The APVMA keeps a record of all registered pesticides in Australia, and their approved uses and also reviews older chemicals to make sure that they continue to meet contemporary high standards.
How herbicides work
A herbicide is a chemical that affects plants. Although there are a large variety of herbicides available they function in a limited number of ways, known as modes of action. These modes of action determine how the herbicide controls weeds:
- by speeding up, stopping or changing the plant’s normal growth patterns
- by desiccating (drying out) the leaves or stems
- by defoliating the plant (making it drop its leaves).
The well-known herbicide glyphosate, for example, prevents the target plant making key amino acids.
As well as using different modes of action, herbicides can be classified according to how they are taken up by the plant. The main types are:
- Contact – these kill plant tissue at or near the point of contact with the herbicide (they do not spread around the plant). Therefore, they require even coverage in their application.
- Systemic – these move through the plant tissues via the plant’s circulation system, and can be injected into the plant.
- Residual – these can be applied to the soil in order to kill weeds by root/shoot uptake. They remain active in the ground for a certain length of time, and can control germinating seedlings.
Herbicides also have differing selectivities, and can be categorised as either broad spectrum (working on a wide variety of plants) or selective (working on a specific range of plants).
For example some herbicides are effective on grasses, whereas others are more effective on woody weeds and will leave grasses intact to provide competition against re-establishment of the weeds.How to apply herbicides
There are several techniques that can be used to apply herbicides. Some of the most common are outlined below.
Here, the herbicide is diluted with water or another diluent as specified on the product label, and sprayed over the foliage to point of runoff (until every leaf is wetted, but not dripping).
The method is most suited to shrubs, grasses and dense vines less than 6 m tall so that complete coverage is achieved. Advantages include speed and economy. Disadvantages include the potential for spray drift and off-target damage.
Foliar spraying can be done in a number of ways, depending on the size of the weed plant or the infestation. Blanket spraying, using a boom spray from a tractor or aircraft, can be used to treat areas completely infested with weeds, especially with selective herbicides.
For large infestations that need targeted applications of herbicide, a hose and handgun can be used to spray solution from a herbicide tank and pump carried by a tractor or vehicle. Smaller infestations can be sprayed using a backpack/knapsack spray unit. Spot spraying is used to treat individual weed plants or areas that only have small clumps of weed infestations.
Basal bark spraying
This method involves mixing an oil soluble herbicide with a diluent recommended by the herbicide manufacturer and spraying the full circumference of the trunk or stem of the plant. Basal bark spraying is suitable for thin-barked woody weeds and undesirable trees.
Basal bark spraying is also an effective way to treat saplings, regrowth and multi-stemmed shrubs and trees. This method works by allowing the herbicide to enter underground storage organs and slowly kill the targeted weed.
The whole circumference of the stem or trunk should be sprayed or painted with herbicide solution from ground level to a height of 30 cm. It is important to saturate the full circumference of the trunk, and to treat every stem or trunk arising from the ground.
Basal bark spraying is a very effective control method and is a good way to tackle inaccessible areas such as steep banks. This method will usually kill difficult-to-kill weeds at any time of the year, as long as the bark is not wet or too thick for the solution to penetrate. The work is often best performed by contractors.
Stem injection involves drilling or cutting through the bark into the sapwood tissue in the trunks of woody weeds and trees. Herbicide is immediately placed into the hole or cut. The aim is to reach the sapwood layer just under the bark (the cambium growth layer), which will transport the chemical throughout the plant.
It is essential to apply the herbicide immediately (within 15 seconds of drilling the hole or cutting the trunk), as stem injection relies on the active uptake and growth of the plant to move the chemical through its tissue.
Stem injection methods kill the tree or shrub where it stands, and only trees and shrubs that can be safely left to die and rot should be treated this way. If the tree or shrub is to be felled, allow it to die completely before felling. The use of chainsaws, particularly in the felling of trees, is a dangerous activity that should only be undertaken by an appropriately trained person.
One method is the ‘drill and fill method’ also referred to as tree injection, and is used for trees and woody weeds with stems or trunks greater than 5 cm in circumference. A battery-powered drill is used to drill downward-angled holes into the sapwood about 5 cm apart. The placement of herbicide into the hole is usually made using a backpack reservoir and syringe that can deliver measured doses of herbicide solution.
Another method is the ‘axe cut method’ which involves cutting through the bark into the sapwood tissue in the trunk, and immediately placing herbicide into the cut. This method can be used for trees and woody weeds with stems or trunks greater than 5 cm in circumference. Using an axe or tomahawk, cuts are made into the sapwood around the circumference of the trunk at waist height. While still in the cut, the axe or tomahawk is leaned out to make a downward angled pocket which will allow herbicide to pool. The herbicide is then immediately injected into the pocket. Cuts should be made no further than 3 cm apart. This method of using an axe to make the cut is often referred to as frilling or chipping. A hammer and chisel can be used to make the pocket cuts, or a spear to make cuts in the trunk closer to ground. It is important not to entirely ringbark the trunk, as this will decrease the uptake of the herbicide into the plant.
Cut stump application
Here, the plant is cut off completely at its base (no higher than 15 cm from the ground) using a chainsaw, axe, brush cutter or machete (depending on the thickness of the stem/trunk). A herbicide solution is then sprayed or painted onto the exposed surface of the cut stump emerging from the ground, with the objective of killing the stump and the root system.
It is imperative that the herbicide solutions are applied as soon as the trunk or stem is cut. Refer to the product label instructions for information on timing, as delayed application will give poor results.
Two operators working as a team can use this method effectively. The herbicide can be applied from a knapsack, or with a paint brush, drench gun or a hand-spray bottle. It is a good idea to use a brightly coloured dye in the solution to mark the stumps that have been treated.
For trees with large circumferences, it is only necessary to place the solution around the edge of the stump (as the objective is again to target the cambium layer inside the bark). The stump circumference should be bruised with the back of an axe and each successive blow treated with herbicide.
This method has the appeal of removing the weed immediately, and is used mainly for trees and woody weeds. This method is also referred to as cut and spray or cut and paint.
Cut and swab
This method is similar to the cut stump method, but is suited to vines and multi-stemmed shrubs. Here, the plant stems are cut through completely, close to the ground. Herbicide is then applied immediately to the cut surface emerging from the ground, via spray or brush application.
In the case of Madeira Vine and some other vines with aerial tubers, both ends of the cut stems must be treated with herbicide. An effective way of doing this is to hold both ‘bunches’ of cut stems in a container of herbicide for 15 seconds after cutting, so that maximum translocation occurs to both ground and aerial tubers. Extra care should be taken when doing this to ensure spillages do not occur.
Stem scraping is used for plants and vines with aerial tubers. A sharp knife is used to scrape a very thin layer of bark from a 10 cm section of stem. Herbicide is then immediately applied to the exposed soft underlying green tissue.
This method is also called bark stripping or stem painting. Some woody weeds can have their bark surface peeled away and the exposed wood painted or sprayed with herbicide.
This method of applying a herbicide consists of a wick or rope soaked in herbicide from a reservoir attached to a handle or assisted with 12 volt pump equipment. The wetted wick is used to wipe or brush herbicide over the weed.