Last week in this column, I started a conversation about droplet size as one best management spray practice.  Today I want to continue the topic of best management spray practices and cover boom height and weather as factors.  Remember that the goal of a spray application is to provide good target coverage while preventing off target movement of the spray material.

Boom height is important because it affects nozzle overlap and spray coverage as well as the potential for drift. Jason Deveau, a sprayer technology specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs in an interview on sprayer best management practices on the sprayers 101 web site said the following regarding boom height; “Imagine holding out your arm and dropping a feather. It will move a ways downwind before landing. Now climb a ladder and do the same thing — it goes considerably further. It’s exactly the same for water droplets. To add insult to injury, releasing spray from a higher point also prolongs evaporation, making droplets even smaller and exacerbating the problem.  If that weren’t enough incentive to lower booms, the high booms create inconsistent spray coverage, undermining the whole reason for spraying in the first place.”

The correct boom height is determined by the spacing between nozzles on the sprayer boom and by the spray angle of the nozzle.  Most sprayer nozzle companies have catalogs with this information. For example, the TeeJet nozzle catalog provides a boom height chart for various nozzle types.   According to the TeeJet catalog, for nozzles spaced 20 inches apart on the boom, the boom height should be 35 inches for a 65-degree nozzle, 30 inches for an 80-degree nozzle, and 20 inches for a 110-degree nozzle.  This boom height refers to the distance from the bottom of the nozzle to the target.  The target may be the soil surface; it may be the top of a crop plant, or perhaps the middle of the crop canopy.

Another best management spray practice is to spray under the right weather conditions.  From a perspective of preventing off-target movement, wind speed is the weather condition we think about most commonly, but there are other weather considerations as well.  Most pesticide labels have a section on drift management that specify allowable wind speeds.  For example, a common grass control herbicide says the following; “drift potential is lowest between wind speeds of 2 to 10 mph.”  While most of us can identify when it is too windy to spray, we also need to be aware that spraying under conditions of no wind is also risky.  Spray droplets reach their target faster when there is some air mixing that allows vertical movement of droplets.  Often calm conditions can signal a temperature inversion.  With a temperature inversion, air moves horizontally, there is no vertical movement.  Spray droplets from an application made during a temperature inversion have the potential to move significant distances, between a half a mile and up to a mile.

Temperature and humidity also have an impact on spray effectiveness.  Low humidity and high temperatures increase the evaporation rate of spray droplets.  As droplets evaporate, they become smaller, they lose mass and they are more subject to drift, especially with high boom heights.  In addition, some products have an increased potential to volatilize, change from a liquid to gas state and move, with high temperatures.  Always read the product label before spraying to understand the impact and risks of spraying under certain weather conditions.

For more information about temperature inversions and best spray management practices, contact the Wayne County Extension office at 330-264-8722.