Pre-emergent herbicides in Canada are an essential tool for many farmers to control weeds early in the season and to provide cropswith a head start on the season without having to compete with weeds for light, water and nutrients. Even the best products will not perform up to expectations when applied under the wrong conditions. While many growers are focused on product selection, timing and placement, there is one element that can often be overlooked, and that is the water – the volume of water used in the spray tank and the quality of that water.
The importance of water
Many pre-emergent herbicides in Canada are applied with a sprayer and work when they in soil water solution, available for uptake by germinating weeds. This means they need to reach the soil and be applied with enough water to form a blanket near the soil surface once activated by spring snowmelt or rain. If the water volume is too low, the product may not be delivered in a uniform manner, and gaps in coverage may occur in the field. This lack of coverage can create an opportunity for weeds to escape and establish.
If applying in the spring, it’s best apply shortly before a small rainfall event. Depending on the product ½ inch to one inch of moisture is usually required to activate many pre-emergent herbicides. If too much water is present, causing with pooling and runoff shortly after application, efficacy can be compromised.
Water quality can also be a factor in pre-emergent herbicide performance. The pH of water, as well as water hardness, will influence how well different herbicides dissolve or stay in solution.
Water with a high pH or “hard” water that is high in calcium and magnesium may cause some herbicides to precipitate out of the water or bind to minerals in the soil, making the product unavailable to control weeds. The result may be a pre-emergent herbicide application made at the perfect time that still does not perform up to expectations because the water chemistry was off.
Determining the right spray tank volume
The water volume that is required to make an application will vary by product and also with the soil type and field conditions. A general rule of thumb is to use no less than 10 gallons of water per acre.
Water volumes that are recommended for use will be on the herbicide label, and growers must follow those instructions to maximize their chance of success. Sprayers also need to be calibrated to deliver the appropriate rate over the entire field. Variations in spray patterns, nozzle clogging or speed can change the actual rate delivered in the field, so regular equipment maintenance and calibration checks are key to ensuring that the right volume is being applied.
Treating water quality: pH and hardness
The pH of the water and the hardness are two factors that are not obvious but can contribute to poor performance of pre-emergent herbicides. If the water is alkaline (high pH), it can break down certain herbicides before they ever get to the soil. In the case of hard water, which has elevated levels of calcium and magnesium, certain pre-emergent herbicides can bind with the minerals or even form a precipitate (solid material that settles out of the water). These water quality factors are not always obvious to growers and can be remedied by conditioning the water before use with the herbicide.
Buffering agents can be added to change the pH, while water conditioners chelate (lock up) the minerals in the water. It is critical to follow product recommendations when it comes to using water additives, as over-application can affect crop safety or cause issues with the spray equipment. The easiest way to avoid potential water issues in the field is to test water sources in advance of the spray season using a jar test.
Activation by rain or irrigation
Rainfall or irrigation water is often the final step required to make a pre-emergent herbicide perform. It moves the herbicide into soil water solution and available for uptake by germinating weeds. Typically ½ inch to 1 inch of moisture is required to active pre-emergent herbicides,
Practical tips for Canadian growers
Follow herbicide labels for recommended water volumes and calibrate sprayers accordingly
Test water for pH and hardness before spraying
Use water conditioners when required to adjust water pH and chelate minerals
Check nozzles, spray pressure and sprayer speed to maintain even coverage in the field
Time irrigation or forecasted rainfall to activate the herbicide properly
These may seem like small details, but they can have a large impact on pre-emergent crop protection solution performance. Water volume and quality play a key role in the product reaching the soil surface in a uniform manner and being available for uptake to control weeds as they germinate. Water may be one of the most overlooked inputs when it comes to crop protection. Using adequate volumes and managing pH and hardness can have a significant impact on herbicide performance.
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