Herbicide application for declared plants

Page last updated: Thursday, 3 August 2017 - 9:44am

Please note: This content may be out of date and is currently under review.

There are a number of herbicide application methods available for declared plants.  Recommendations for application methods in crops can be used in many cases.  However, when the weeds are sparsely populated, in rugged terrain, firebreaks or on the roadside other application methods may need to be utilised.

Herbicide application methods for declared plants

Spot spraying

Spot spraying consists of treating each weed individually. It can be carried out either by a knapsack sprayer or a hand-lead from power equipment. Knapsacks are used to spray small isolated weed infestations.

Application rates are usually expressed as the quantity of herbicide in millilitres (mL) or grams (g) to add per litre (L) of dilutent (carrier: water or distillate). The output varies between 1000-1500L/ha depending on the speed of spraying.

Hand-lead equipment is used to spot-spray more extensive areas of weeds that are too sparse to warrant boomspraying, or where the terrain is too rugged to allow the use of a boom spray.

Application rates are usually expressed as the amount (L) of carrier fluid (water or diesel fuel) with which 1L or 1kg of herbicide should be mixed. Hand-lead equipment usually applies between 1000-1500L/ha. The pressure used depends on the type of hand-lead, the weed to be sprayed and the herbicide chosen. Avoid using pressures greater than about 500kPa because this may produce small droplets that are likely to drift.

When spot spraying, it is important to wet the foliage thoroughly. Do not waste chemical by spraying after the mixture starts to run off the leaves. Spray to the point of run-off and no more.

Hand-wands and spray guns

It is just as necessary to accurately calibrate hand wands and spray guns as it is to calibrate boomsprays and misters.

The following factors determine how much spray material is applied:

  • the skill and speed of the operator
  • pressures used
  • type of growth
  • nozzle size used.

Invariably, the greatest cause of failure is application of too much or too little chemical. Do not use a fixed ratio, for example, 1:100 mix. It is important to calibrate each time.

It is essential to ensure that the amount of chemical being applied is as close to the recommended rate as possible. Both the operator and the equipment need to be calibrated. This is important as spray operators have different styles and one would not treat an area in the same time as another.

For best results with hand wands:

  • apply the recommended amount of active ingredient/ha, and
  • apply in a minimum quantity of water to obtain complete foliage cover without excessive run-off. That is, spray to the point of run-off not beyond.


Misting consists of pumping spray solution into a high speed stream of air. This air blast breaks the solution up into small droplets.

Large areas of infested land, particularly on difficult terrain, can be covered relatively quickly without needing large volumes of dilutent (carrier). However, the large number of small droplets produced by misters can cause problems if the spray droplets drift onto nearby susceptible plants.

Check both the direction and speed of the wind before spraying. It is important to check the output and effective coverage of the mister under the conditions prevailing on the day of operation as wind speed is the factor most likely to affect the swath width (mister calibration). Output may also vary, depending on the type of material being used.

Avoid using ULV (ultra-low-volume) misters close to susceptible crops.

Power mister do's and don'ts

  • DO set the discharge orifice to point 45° upwards
  • DO always operate from downwind to upwind of the crop. Make the next run further towards the direction the wind is coming from.
  • DO stop operations if the wind falls to less than 5km/h or rises to more than 13km/h
  • DO ensure proper agitation of the mixture in the tank
  • DO NOT spray at more than 8-12km/h due to the effect of the vehicles slipstream on air movement
  • DO NOT apply a swath width greater than 10m until experienced with the machine
  • DO NOT use misters to apply diquat, paraquat, glyphosate or wettable powders.

Wide angle/side delivery - single or multiple head nozzles

These are mainly used for roadside and firebreaks. They operate at high pressure and volume of application and produce a swath of 3-5m depending on nozzle arrangement. Spraying through fence lines is one benefit of this type of nozzle.

Controlled droplet applicator sprayer

A Controlled Droplet Applicator (CDA) sprayer works by having the pesticide fall onto a spinning disc that has a row of fine teeth around the perimeter. The spinning motion of the disc forces the liquid out to the edge of the disc and the fine teeth help to break up the liquid into droplets of a very even size.

CDA sprayers may be used for continuous or spot spraying of contact, residual or selective herbicides. At a walking speed of 3km/h the volume discharged varies between 10-20L/ha depending on the nozzle size selected.

When the spray head is held 200mm above the target plants the swath width is 1.2m.

The droplet size is controlled to about 250µm. Select a nozzles that maintains a flow rate of 1mL/s through the nozzle. The flow rate will vary depending on the viscosity of the formulation.

Do not spray when the wind speed is less than 5km/h or more than 25km/h. Always spray from the upwind side of the target.

There have been many versions of equipment using the principle of the CDA but they have not been readily accepted in broad area agriculture in WA because of the high cost of the sprayers, compared to a conventional boom spray, and the fragile nature of the CDA units.

Further information

Further information on controlling declared plants can be found through the Declared plant control handbook link.

Other topics that may interest you include:

Contact information

Andrew Reeves
+61 (0)8 9780 6224