The topic of herbicide strength has been touched on before. However, incorrect dilution of herbicides is a very common mistake so, this article will help to explain why concentration is important.
When talking with gardeners or people carrying out weed control on rural blocks, I often hear that herbicides are not being diluted at all. Many people have a container of Glyphosate (common brand name Round Up) in the shed and paint or spray it at full strength. They aren’t aware of the concentration and the necessary dilution ratio. Glyphosate comes in different concentrations, most commonly 360 g/L, but also 450g/L, 540g/L and premixed.
The premixed is the only one that doesn’t need diluting with water.
When using the 360g/L concentration, for most weeds the dilution ratio is 50/50 for cut and paint or dab on stems. If spraying, the dilution ratio is 10ml/L, so 100ml of Glyphosate in a 10L Knapsack sprayer full of water.
For the 450g/L concentration the dapper ratio is 40/60 and the spray ratio 8ml/L.
Why does it matter?
The risk to the environment, especially aquatic ecosystems, is frightening if chemicals are used incorrectly. Always read the label and only used as directed. The misconception with applying herbicides at a stronger than directed rate, is that it will obtain a better kill or work faster. The opposite can be true. Some plants have the ability to shut down after being treated with a high dose of chemical. This allows them to survive the spraying and recover. A slow kill is a better kill.
Some plants are developing a resistance to herbicides. Fleabane is no longer responding to Glyphosate. Overuse and stronger than necessary strengths will only hasten the resistance.
The last reason to use the correct rate is cost. Herbicides are expensive and using a higher than necessary rate is a waste of money.
To find the recommended rate have a look on the bottle. Some come with a fold out booklet that contains a lot of information. ‘Weeds of Southern Queensland’ is a useful publication with weed information and herbicide options and rates. Thanks to Noosa council, I have some of these publications. If you would like one, send me a text on 0412074103 with your name and address.
Michelle Newall
