How much wind is too much wind for airblast spraying?

Airblast spraying in the spring can be challenging. The crop (e.g. highbush blueberry, grape or cane) only has buds or small leaves at best, so for a foliar application there is very little surface area requiring coverage. However, you need enough spray volume to compensate for the spray lost to wind, which is almost always present when there's no canopy to slow it down. These are contradicting requirements that must be balanced to ensure sufficient coverage while minimizing downwind spray drift.

Two tools will help you make your decisions: The first is water-sensitive paper, and the second is a few 25 cm lengths of flagging tape.

I recently calibrated a 1,890 L (500 US g) Durand Wayland with an 81 cm (32 in) fan and tower. We were spraying in a three-year old high density apple orchard. The grower was applying 477 L/ha (52 US gpa) with the fan set on low. The trees were 2.75 m (9 ft) high with a 0.9 m (3 ft) dripline.

We placed two pieces of water-sensitive paper back-to-back in four locations: the top of the tree, two along the widest part of the plant, and two pieces pinned to the ground with wire flags. This arrangement works in any crop. Then we tied a few 25 cm lengths of flagging tape in the tree to see if the air from the sprayer could compete with the prevailing wind.

Consider that 20% coverage for most insecticide and fungicide applications is sufficient, and see figure 1. Note that the cards facing into the wind (denoted by a 'W') were drenched with excessive coverage (100%). The card with 1% was owing to a "dead zone" in the air handling where there was little or no air to carry the spray into the tree at that height. We diagnosed this issue when we saw that the fan was not pushing the flagging tape into the prevailing wind in this location. We also noted a great deal of spray drifted downwind during this application. We determined this to be far too much spray volume for the job.

Figure 1 - Spraying in a 7 km/h wind with 477 L/ha (52 US gpa). Excessive coverage on windward side (W) and an obvious gap in coverage.

Figure 1 - Spraying in a 7 km/h wind with 477 L/ha (52 US gpa). Excessive coverage on windward side (W) and an obvious gap in coverage.

We reduced the output to 262 L/ha (28 US g/ac), and attempted to compensate for the one empty card by switching to air induction tips in that location (see Figure 2). Air induction tips rely more on pressure than air to propel spray into a crop, but often do so at the cost of slightly reduced coverage.

Figure 2 - Spraying in a 10 km/h wind with 262 L/ha (28 US g/ac). Still excessive coverage on windward side (W) and an obvious gap in coverage.

Figure 2 - Spraying in a 10 km/h wind with 262 L/ha (28 US g/ac). Still excessive coverage on windward side (W) and an obvious gap in coverage.

By now, it was later in the morning and the wind had increased to 10 km/h. The card with 4% shows how the sprayer laboured to spray into the wind, even when the target was right next to the nozzles. Again, the flagging tape bore this out, suggesting we should either stop spraying for the day (a reasonable decision in 10 kmh winds) or increase the air output by shifting the fan into high gear. Given that we were running a trial and only spraying water, we switched to high gear.

By now, the wind had increased to 12 km/h, which I feel is too high for airblast applications. The sprayer may or may not achieve suitable coverage, but far too much spray is lost to drift. Figure 3 shows the resultant coverage with the fan in high gear. The windward side still received excessive spray, but all other targets received ideal coverage.

Figure 3 - Spraying in a 12 km/h wind with 262 L/ha (28 US g/ac). Still excessive coverage on windward side (W) but high fan gear in conjunction with air induction hollow cones improved coverage. A great deal of spray was lost downwind due to high gear and high prevailing winds.

Figure 3 - Spraying in a 12 km/h wind with 262 L/ha (28 US g/ac). Still excessive coverage on windward side (W) but high fan gear in conjunction with air induction hollow cones improved coverage. A great deal of spray was lost downwind due to high gear and high prevailing winds.

In conclusion:

  • Use 25 cm lengths of flagging tape tied into the canopy to indicate whether the sprayer fan is pushing air through the target and outcompeting the prevailing wind. You can compensate by increasing or decreasing fan speed, or increasing or decreasing ground speed, but this may require re-calibration.
  • Use water sensitive paper to confirm coverage. A minimum of 15-20% with even coverage (~85 drops per cm2) are required for most applications.
  • Know when increasing fan speed is required, and when it is time to pack it in and wait for the wind to die down (usually around 10 kmh).

For more information:
Toll Free: 1-877-424-1300
E-mail: ag.info.omafra@ontario.ca