Good Spray Coverage: Watch the Air

Written by: Franz Niederholzer, UCCE Farm Advisor, Colusa and Sutter/Yuba Counties and Luke Milliron, UCCE Horticulture Intern

Good spray coverage is vital to effective pest control.  Nowhere is this truer than for NOW control at hull split.  Excessive sprayer speed kills spray coverage, especially in the tree tops.  But is there a simple way to figure out how fast to drive a sprayer to deliver good coverage in the tree tops while getting across the orchard in a timely manner?  Yes, watch the air.  Since airblast sprayers use forced air from the sprayer fan to carry pesticides throughout the tree, if the air from the sprayer fan(s) reaches the top of the canopy, the pesticide spray should get there, too.

The following is a simple way to evaluate sprayer air movement in the canopy tops at different ground speeds.  Free hanging surveyors tape makes a great air movement indicator.  Tie 12-24” of surveyors tape to the top of a length of PVC pipe threaded through the branches in the middle of the tree row into the tree tops.  [Another option is to use a pruning tower to get up in the tree tops and tie several lengths of tape onto the highest shoots.]  Fill the sprayer half full of water, turn on the pump, close the spray booms, and run the sprayer down the row at a set speed with the fan on at operating tractor RPMs.  Have someone record a video of the movement of the tape(s) in the tree tops with a smart phone or iPad.  Review the video after the spray moves past the pole.  Did the tape move at all?  If no, then the sprayer air didn’t reach the tape, and neither will pesticide.  The sprayer needs to drive slower, or you need a sprayer with a larger fan in that orchard.  Did the tape stand straight up?  You might consider driving faster, as air moved past the tree tops at good speed, likely moving pesticide above the tree and creating spray drift.  If the length of tape just rustled and moved 45-90 degrees from vertical, you are probably at the ground speed you want.  Double check the coverage at that ground speed (the speed where the tape just rustles at 45-90 degrees) with spray tracer such as water sensitive cards or Surround clay.  Details on how to use tracers to assess spray coverage will be in a future post to this blog.

As an example of the use of flagging to show air movement in almonds, here are links to three videos of the same sprayer (standard airblast PTO sprayer with a 36” axial fan) driven in three very different orchard settings based on rootstock or time of year:

Video 1.  Click HERE to see the sprayer output at 3 MPH in a vigorous Nonpareil on Lovell rootstock row at hull split.  The lower branches thrash around and the iPad is buffeted, but at 20’ up in the tree row, the flagging barely moves.  The sprayer must slow down to provide better coverage.

Video 2.  Click HERE to see a video of the same sprayer at 3 MPH in a low vigor Butte on Marianna plum rootstock on the same day as Video 1.  The flagging at 20’ off the ground is lifted almost vertical by air from the sprayer’s fan.  Here, 3 MPH or faster should work to get pesticide into the tree tops.

Video 3.  Click HERE to see a video of the same sprayer at 3 MPH in a moderate vigor Nonpareil on Lovell planting at petal fall in late February.  This speed looks to deliver good air movement in the tree tops of this orchard at that stage of development (at hull split, this sprayer is run at 2 MPH in this orchard.)

Speed doesn’t kill, but time is money.  Match the sprayer speed to the orchard, and time of year, for the good spray coverage with fastest ground speed — combination that is vital to effective, efficient pest control.


Thanks to Kim Blagborne, President and CEO of Slimline Manufacturing, Penticton, B.C., Canada for the idea of using flagging to trace sprayer air movement and Stan Cutter, farm manager, Nickels Soil Lab, Arbuckle, CA for shooting Video 3. 

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2 thoughts on “Good Spray Coverage: Watch the Air

    1. Hi Sara:

      Great question. The short answer is speed is more important than volume in getting better coverage higher up.

      Long answer: More spray volume (gallons per acre) improves coverage where the spray reaches (the volume needed to do the job differs based on the target pest), but doesn’t change the area of the tree reached by the spray. Where the spray reaches is mostly or completely due to sprayer (fan) air. With an airblast sprayer, the slower the sprayer moves, the further the spray is pushed. Imagine using an oscillating fan to blow out a candle across a room, the slower the fan moves (oscillates) the more air reaches the distant candle.

      “Bad coverage is possible at any spray volume.” Tim Smith, WSU Extension Hort Agent, Wenatchee, WA


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