Oct 19, 2012Editorial: Reduce spray drift
In the August 2012 issue of VGN, I wrote a story that discussed pesticide drift and volatilization and the damage they can do to specialty crops like fruits and vegetables (“Herbicide-tolerant crops spark concerns,” front page).
Exploring the issue of pesticide drift a little bit more, I ran across Ohio State University Bulletin 816, “Reducing Spray Drift.” The bulletin discusses “how weather-related factors influence pesticide drift and how growers can minimize their effects.”
Droplet size, downward velocity, air turbulence and spray-boom height all influence the distance a droplet travels before it deposits on an object. But of all the meteorological conditions that affect drift, wind velocity is the most critical, according to the bulletin.
“The greater the wind speed, the farther off-target a droplet of a given size will be carried.”
If wind speeds are excessively high, stop spraying. I don’t need to quote the bulletin to give that piece of advice.
The bulletin also states, however, that when it comes to reducing drift damage, wind direction can be as important as wind velocity.
“The presence of sensitive vegetation near the spray site, particularly downwind, is one of the first things that should be evaluated,” according to the bulletin. “If there are sensitive crops downwind, leave a buffer strip of at least 100 feet, or whatever the label recommends. Spray the buffer strip later, when the wind has shifted away from sensitive crops.”
Relative humidity and temperature also affect spray drift, though usually not as critically as wind velocity. The bulletin described it like this: “As a particle falls through the air, surface molecules of water evaporate into the atmosphere. This evaporation reduces the size and mass of the particle, enabling it to remain airborne longer and, under the right conditions, to drift farther from the application site. The rate at which water evaporates from the spray particles depends primarily on the ambient air temperature and relative humidity.”
Atmospheric stability is another factor that can influence drift. The bulletin mentioned atmospheric inversion, when a “warm air layer at some distance overhead may become a blanket, holding down cooler air underneath.”
If this happens, spray particles suspended in the cool layer can only move laterally. Eventually, they might encounter a downdraft, which would force them back to earth and possibly deposit them over a sensitive crop, according to the bulletin.
“Inversions are part of a daily atmospheric cycle, occurring in the early morning hours when the ground cools the air layer immediately above it,” according to the bulletin. “Inversions tend to dissipate during the middle of the day, when wind currents mix the air layers.”
Therefore, it’s best to wait until late afternoon or early evening to spray, when there is less chance of the atmosphere being inverted, according to the bulletin.