Chemical Control of Weeds

Control and Management

The following options are available to control weeds: chemical, biological, physical or integrated weed management.

Chemical control:

Chemicals such as herbicides work by being absorbed by the plant thus destroying it. In some situations herbicides offer the only practical, cost-effective and selective method of managing certain weeds and by reducing the need for cultivation, they can prevent soil erosion and water loss. In most cases, weeds must be actively growing for herbicide treatments to work. Other factors to be taken into account include wind speed and direction and proximity to waterways to prevent spray drift and any possibility of rain, which will dilute the treatment.

The chemicals within herbicides function in a limited number of ways known as the mode of action, which classes chemicals according to the site or enzyme within a plant, disease, or insect that the active constituent works on eg by speeding up, stopping or changing the plant’s normal growth patterns, by desiccating the leaves or stems or by defoliating the plant. An example is the herbicide Glyphosate, which prevents the target plant making key amino acids. Mode of action is particularly important for resistance management.

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.

When using a herbicide it is extremely important to read and follow the information contained on the label, which includes: the active constituent, directions for use, the withholding period, expiry date, tank mixing, resistance and storage instructions as well as other information. The label itself is a legal document, which under legislation, must be adhered to at all times. 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. Compatibility is very important when you are tank mixing chemicals or adding two or more together to control multiple plants with the one spray application.

Some herbicides have a period of time that must elapse before the next crop can be planted back into the treated area. Residual herbicides can have lengthy plant-back periods for particular crops, which must be adhered to in order to prevent crop damage.

The withholding period is the time that must elapse between treatment of a pesticide, and the harvest, or selling of the treated product, and is on the label to prevent higher than acceptable pesticide residues in food products.

The CAUTION denotes the poison schedule of the pesticide, which eg for Roundup CT is a SCHEDULE 5. This means it is available to the public, but requires caution in its storage, handling and use. The SCHEDULE number also indicates the type of personal protection equipment that should be worn when using this product.

More information is available on the manufacturer’s websites and from http://www.herbiguide.com.au/InformationHerbicides.aspx