Herbicides kill plants by disrupting physiological processes that are essential for plant growth. In the case of the Metsulfuron Methyl (Brushoff, Ally etc.), the active ingredient stops cell division. Glyphosate (Roundup etc.) inhibits the production of certain enzymes within the plant disrupting plant growth and ultimately killing the plant.

A variety of factors can affect the efficacy of herbicides and can be the difference between a good “kill” and plants that look almost dead but ultimately recover to grow on. Some of the factors that need to be considered are listed below:

Application temperature/frost

Plants respond to climatic conditions the same as any other living thing. When conditions are outside of the comfortable temperature range and become either too hot or too cold, the efficacy of herbicide application can be compromised. As a general rule plants will close down reducing the amount of active ingredient translocated into the plants tissue when the ambient air temperature gets above around the 33°c mark. This is a survival mechanism where plants close the stomata (small pores in the surface of leaves and stems) to save moisture loss. Once plants respond in this way the amount of herbicide absorbed by a plant applied as a foliar spray is drastically reduced.
The same is true for low temperatures that result in a plants physiology slowing down the colder it gets. This will therefore have a marked result on the efficacy of herbicides, slowing the action of the active ingredient to the point where it will take longer for the chemical to take effect and possibly reduced efficacy due to chemical breaking down within the plant over a longer time period. Plants exposed to heavy frosts can become less receptive to foliar herbicide application for up to a day later. This is however highly variable for different species and is also site dependent with areas receiving heavier frost more affected than a hill top for example.

Soil type

Herbicides, particularly those that are translocated into plants through the roots can have varied results dependant on soil type. This will vary for different herbicides but is usually dependant on the time the active ingredient takes to breakdown within the soil. Binding of the active ingredient by soil particles (particularly clay particles) can also make less chemical available for absorption by the target plant. Special considerations are usually listed in the registered chemical label supplied with chemical products.

Soil moisture and rainfall

The amount of soil moisture at the time of spraying and for a period thereafter can determine how effective and for how long the chemical will remain active. This is particularly important for chemicals with a residual action in the soil with increasing rainfall and soil moisture generally decreasing the time the chemical remains active. Moisture in the soil not only assists the chemical to be translocated from the soil into plant roots but also assists degradation of chemical by soil microbes and chemical processes. Increased rainfall can also be responsible for leeching active ingredients deeper into the soil profile away from the root zone making it less available to plants.

Plant stress

In the same way plants respond to excessive high and low temperature by shutting down; dry conditions can have the same result. This can result in plants going into survival mode over the hotter months not producing any new growth. This will negate the effect of a lot of herbicides that depend on the plant to be actively growing to take effect. Some species can be successfully sprayed all year round if seasonal conditions are favourable. If plants look to be dropping leaves and are looking in poor condition there is a good chance chemicals that depend on the plant actively growing will be in ineffective.

Water quality

Water is the main mode of transport for an active chemical ingredient to be applied and translocated into plants. The pH, suspended particles (clay and organic matter) and mineral content can all have an effect on the efficacy of herbicides. A general rule is to use the best quality water that would be fit for animal and human consumption to use in the spray mix.


This is particularly important for broad acre boom spraying application of herbicides that are dependent of a certain rate of herbicide being applied to a given area of land as opposed to spot spraying where the amount of herbicide can be applied at a given rate to “run off” stage. Boom spraying equipment needs to be set up and calibrated accurately with the correct ground speed, droplet size and mixing rate to reflect the correct application rate required for a given species of weed.

MethodsReminder – If chemical control is necessary near drainage lines or creeks and water bodies, use extreme caution as the effects on aquatic life can be damaging even in minute amounts. Always seek expert advice and consider alternative methods of control.


An appropriate herbicide is applied as fine droplets to the surface of foliage using a knapsack or spray unit. For woody weeds and those weeds with tough or waxy leaves (e.g. Spiny Rush) a penetrant is necessary to get the active ingredient of the herbicide into the plants tissue. Tough weeds like Broom, Gorse and Spiny Rush can therefore be killed with less harmful (to animal life at least) chemicals like Glyphosate. There are generally two application methods for foliar spraying: spot spraying and boom spraying.
Spot Spaying is generally with a hand held spray nozzle from a motorized spray unit or a hand operated back pack. Used for targeting individual plants or smaller areas of blanket coverage of a weed.
Boom Spraying is generally from a motorized pressure unit with a boom assembly with multiple spray nozzles to blanket spray larger areas. Can be on a trailer or tray of a vehicle and can also be mounted on fixed wing aircraft and helicopters.


The preferred method for small shrubs to large trees where foliar spraying is either too difficult due to the target plants size or more expensive due to the amount of herbicide that is required to spray the entire plant. Cut and paint is as simple as that; the target plant is cut off close to the ground and the stump is painted with undiluted herbicide. Glyphosate is generally the herbicide of choice for this job and needs to be applied immediately after the cut is completed. On multi-stemmed plants this will require painting of each stem after it has been cut; don’t wait as plants immediately start to form a barrier against the herbicide penetrating the roots.
Herbicide can also be applied to vines and larger woody weeds by scraping the bark, making cuts in the bark or drilling holes in the stem or stump and then applying neat herbicide. Refer to the chemical label for more detailed information.

Legal responsibilitiesIn Victoria there are a number of legal requirements for anyone using agricultural chemicals. Some of these include possessing an Agricultural Chemical Users Permit (ACUP) to purchase and use certain chemicals, weather and equipment considerations, storage and transport requirements and record keeping. Please refer to the below link for a detailed and up-to-date explanation.

Withholding periodsA withholding period can be defined as the minimum period of time that must elapse between the last application of an agricultural chemical, and the ‘use’ of the agricultural produce to which the chemical was applied. The use of the agricultural product includes the harvest, sale, grazing, slaughter, cut for stock food, feed to animals. Withholding periods are listed in the text of each registered chemical product label and are usually listed in number of days / weeks. Consult the following links for more information.


Off Label use of herbicidesOff label use is the use of a chemical on a species not listed in the accompanying registered product label that does not exceed the maximum rate on the label which is permitted in Victoria.  The exception to this are restricted chemicals (e.g  Grazon) which are not permitted to be used off-label. Off label permits for restricted chemicals can be applied for at the Department of Environment and Primary Industries (DEPI) or Australian Pesticide and Veterinary Medicines Authority.


Did you know…?

Onion Grass Romulea rosea is a broad leaf herb and not a true grass. Onion Grass is a very widespread and dominant species of pasture paddocks in the upper catchment thriving in areas of low soil fertility. Due to its ability to thrive in soils with low nutrient value it can out-compete preferred pasture species and eventually dominating entire paddocks. This results in a dramatic loss of productivity for grazing animal production due to the very low nutritional value of Onion Grass. The good part of this story is that a paddock can be turned around easily with the right herbicide at the right time paired with the addition of fertilizer and possibly reseeding with preferred pasture species if necessary. Take a look at the following link for more information.