Improving waterhemp control by managing spray water pH

Eric Yu, UMN Extension Crops Educator, Ryan Miller, UMN Extension Crops Educator

Most spray solutions are more than 95% water, yet water quality is rarely considered when herbicide performance comes-up short. The pH of that water and the minerals dissolved in it can directly influence how well an herbicide works once it is sprayed. Certain dissolved cations, such as calcium, magnesium, and iron, can tie up herbicides, and alkaline water can reduce how readily some products move into plant leaves. The good news is that these issues are manageable with simple water testing and the use of water conditioners or pH-buffering adjuvants.

Study Location and Treatments

A 2025 field study conducted in Rochester, Minnesota, evaluated how spray water pH adjustment, and the timing of spray solution mixing, affected waterhemp control in a corn herbicide program. All treatments included a tank mix of atrazine and Acuron GT (applied at either full or half rate) with NIS and AMS. Treatments differed by the presence or absence of a pH-lowering and buffering adjuvant (AlpHa Shot) and by the timing of spray solution mixing, mixed either six days prior to application to simulate a rain delay or at the time of spraying. Waterhemp control, density, and biomass were evaluated 24 days after treatment (DAT) and again at 112 DAT just prior to harvest.

Results from the Study

Adding the pH-lowering adjuvant reduced spray water pH from 7.5 to 5.8, and this change in pH had measurable effects on weed control. Inclusion of the pH-adjusting adjuvant improved waterhemp control by 10% at 24 DAT and by 7% at 112 DAT (Figure 1A), demonstrating that the benefit extended beyond early-season injury to season-long suppression. Treatments where the pH adjuster was included, waterhemp density decreased by 64% and biomass was reduced by 55% (Figure 1B), resulting in substantially lower late-season weed pressure and seed production potential. Neither the timing of mixing the pH adjuster nor the rate of Acuron GT affected waterhemp control, density, or biomass. Whether the solution was mixed six days ahead of application or at the time of spraying made no difference, which is important for applicators managing weather delays. The response to pH adjustment was observed at both full and half rates of Acuron GT, indicating that water quality can influence herbicide performance regardless of rate.

Figure 1. Effects of including a pH-adjusting adjuvant on A) waterhemp control and B) waterhemp density and dry biomass.

An important takeaway for applicators is that the improvement in weed control did not depend on when the spray solution was mixed. Mixing the solution several days ahead of application performed similarly to adding the conditioner at the time of spraying. What mattered most was ensuring that the spray water was conditioned so the herbicide could work as intended. These results align with what we know about how many herbicides enter plants. Several common products behave as weak acids and are absorbed more efficiently when applied in slightly acidic water rather than alkaline water. By lowering and buffering spray water pH, more herbicide remains in a form that can readily pass through the leaf surface and into the plant.

If waterhemp control has been inconsistent, especially with programs that should be performing well, spray water chemistry may be part of the issue. Testing water pH and using a water conditioner or buffering agent is a simple step that can protect herbicide performance and improve season-long weed management. When most of the spray tank is water, ensuring that water quality supports rather than hinders herbicide performance can be a simple but impactful step for improving weed control.