How to spray a lawn for weeds

The following information is a comprehensive overview of How to spray a lawn for weeds.


Weeds come in all shapes and sizes. From the little, fleshy sprout growing in your back lawn to large, towering trees overhanging your house. A weed, by definition, is an unwanted plant growing in the wrong place.

The first herbicides employed by farmers were salt and oil-based chemicals. Sulphates and nitrates of copper and iron were used, and sulphuric acid proved even more effective. Sodium arsenite was also very popular as a spray and as a soil sterilant.

The first main herbicide to be used was an organic herbicide called Sinox, which was developed in France in 1896. Nothing changed much over the years until after World War II and within 20 years, over 100 new chemicals were synthesised, developed, and put into use. Chemicals evolved very quickly and the introduction of 2,4-D (2,4-dichlorophenoxyacetic acid), 2,4,5-T (2,4,5-trichlorophenoxyacetic acid), and IPC (isopropyl-N-phenylcarbamate) really made their mark- the first two selective as foliar sprays against broad-leaved weeds, the third selective against grass species when applied through the soil.

Glyphosate was invented by a Monsanto chemist named John E. Franz in 1970 and put on the market for agricultural use in 1974. It was first used in Australia in 1976, in Moama in southern NSW.


There are 5 types of herbicides:

  • broad spectrum- these work on a wide variety of weeds and are, basically, knockdown herbicides
  • selective- these work on a narrow range of weeds
  • contact- these destroy plant tissue at or near the point of contact and do not travel throughout the plant. They require even coverage in their application
  • systemic- these move through the plant’s circulation system, and can be ingested into the plant by foliar application, injection or root uptake
  • residual- these can be applied to the soil and destroy by root uptake. They remain active in the ground for a certain length of time, and can control germinating seedlings.


There are a number of reasons why plants build up a resistance to chemicals-

  • Using the wrong chemical
  • Not following the correct application rates
  • Using the same chemical over and over again.
  • Not applying the chemical correctly
  • Spraying at the wrong time

We’ll look at these factors and find precautions to follow, to help you

  • Use the right chemicals
  • At the right application rates
  • On the right plant
  • At the right time


A Mode of Action (M.O.A.) causes a structural change in the metabolism of a living organism at a cellular level when that living organism has been exposed to a substance. In spraying terms, it’s the path taken by a herbicide to try to kill a plant.
When herbicides first appeared, they really only had one M.O.A.. Over time, and for various reasons, plants started to build up a resistance to various chemicals resulting in a poor kill-rate. During that time, chemicals, too, evolved with different M.O.A’s. Then the suggestion was made to rotate the chemicals being used so that one M.O.A. was used for one application and another M.O.A. was used on the next. Resistance, using this method, was slowed. However, resistance still occurred. The latest theory is Tank-Mixing- using two or more M.O.A. in a single application and hitting the weed from various paths; not just one.


Surface tensions occurs when water molecules are drawn towards each other and bind together.
When a chemical is applied from a sprayer, tiny particles of water are dispersed over a wide area. As they fall upon the leaves, little droplets/ beads form. These, in turn, have a tendency to join together and once the droplet becomes large enough, and heavy enough, it rolls off the leaf and down to the ground. This is a problem if you need the chemical to penetrate the surface of the leaf. This is what is called ‘run-off’.
To remedy the problem of run-off and break down the surface tension of water, a detergent is required. These can also be called wetting agents, surfactants and adjuvants, although there are slight differences in their meaning. Once mixed with water, they break down the binding effect of the water molecules, allowing the water to spread and cover a greater area.


Spray nozzles come in a very wide range of types and sizes. As a layman or ‘Amenity Operator’, all you need to know are two things-

  • Fine nozzles are for foliar spraying; and
  • Course nozzles are for soil spraying.

Foliar Spraying- This is where you want the chemical to land and stay on the foliage of the plant. You might be applying a fertiliser, herbicide or a fungicide. Ultimately, the chemical is needed to stay on the leaf. (e.g.- broad-leaf herbicides/ foliar fungicides)
To help you do this, a nozzle with a small aperture is required so that a finer spray can be applied to the leaf. The finer the spray, the smaller the droplet. The smaller the droplet, the lighter the droplet.

Soil Spraying- This is where you want the chemical to land on the soil. Again, you might be applying a fertiliser, insecticide, herbicide or a fungicide. However, in this case, the chemical is such that it will activate and do its job in the soil. (e.g.- pre-emergent herbicides/ residual insecticides)
To help you do this, a nozzle with a large aperture is required so that a courser spray can be applied. The courser the spray, the larger the droplet. The larger the droplet, the heavier the droplet. The droplet will land and be absorbed into the soil where the chemical will activate and do its job.


When applying a chemical to a weed, it is necessary to spray for good coverage. It’s no use spraying half the weed as, if you’re using a contact herbicide like ‘Slasher’, there’s a great chance the weed won’t die if it hasn’t been sprayed properly. A ‘contact herbicide’ will kill anything it comes into contact with, unlike a systemic, like glyphosate, will migrate through the plant and take out the roots, as well. When applying chemicals, give the target weed a good cover of that chemical. Spray to the point of run-off. When I say the ‘point of run-off’, I mean DO NOT SPRAY TO SATURATION POINT where it runs off the leaf.

When a weed doesn’t die after an application of chemicals has been sprayed it can lead to the seedbank of that weed building up a resistance to the chemicals you’re using.


This element is probably the single-most important factor when spraying. The Delta T is a cross reference between dry temperature and relative humidity and indicates the optimum conditions for spraying chemicals.

When the value of the Delta T is too high, the spray droplet will dry too quickly, resulting in the plant not taking up enough chemical. When the value of the Delta T is too low, the spray droplet will dry too slowly, resulting in the possibility of the chemical droplet running off the leaf.

The ideal Delta T when spraying is between 2 & 8 however, if knapsack spraying, avoid spraying in temperatures above 28 degrees and spray when wind is ideally between 0 – 3 km/h (this will lessen the chance of drift onto other plants, like hedges and border plants like annuals and grasses.


In Australia, you must be able to read, understand and be in a position to comply with the instructions written on the label BEFORE using a herbicide. The label is a legal document which will give you instructions to ensure that the herbicide is effective, safe to use, will not harm the environment and will not pose a risk to human life.
The information provided on the label includes:

  • Trade name
  • signal heading (indicating the product’s hazard level)
  • active constituent
  • net contents
  • directions and limitations
  • withholding periods
  • storage instructions
  • safety directions and first aid
  • dangerous goods notification
  • expiry date
  • mode of action (type of herbicide)
  • important notes.


  • Read, understand and comply with the directions on the label
  • Think of the consequences before using an herbicide and not following its’ directions

When spraying-

  • Always tank-mix using two or more Modes of Action
  • Use a wetting agent (surfactant, adjuvant, detergent)
  • Spray to the point of run-off (do not saturate)
  • Use a fine spray- foliar/ course spray- soil
  • ALWAYS check the Delta T before spraying…


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