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VEGETATION TION MANMANAGEMENT Photo courtesy of Dow AgroSciences LLC S pray drift is one of the most common causes of off-target injury. This occurs when the herbicide being applied moves or drifts to areas that are not targeted by the applicator, usually resulting from the physical movement of very small droplets or fines from the target area at the time of application.When this happens, even in small amounts,it can lead to off-target damage to desirable vegetation or sensitive crops,or unintended environmental and financial consequences. Understanding its causes is essential to minimizing its affects,according to Travis Rogers and Rich Hendler, both IVM specialists with Dow AgroSciences. "Applicator training on the causes and effects of spray drift is critical," said Rogers."This is especially true since each spray season can bring with it significant numbers of new applicators." Understanding equipment factors There is a variety of spray equipment used to apply herbicides, and being familiar with and choosing the right equipment and treatment method can help minimize spray drift.Applicators should always start by referring to the herbicide's product label for application guidelines.For example,Milestone specialty herbicide includes a "Precautions for Avoiding Spray Drift and Spray Drift Advisory"on page three of its label. "Also,whether using a backpack sprayer or a high-volume spray gun, selecting the correct nozzle is crucial,"said Hendler."Nozzle types vary, and control critical variables such as spray volume per minute, droplet size, spray angles and patterns.Always refer to nozzle manufacturing guidelines for information." Wind speed and direction Wind speed is one of the most critical factors affecting drift,which is the leading cause of off-target damage.As wind speeds increase, spray droplets are carried further.In very calm conditions, spray droplets can remain suspended in the air and move with wind gusts. Before spray- ing, determine wind direction relative to desirable vegetation, especially 22 Arbor Age / May/June 2012 sensitive crops,or bodies of water. "It's also important to remember that wind direction can change during an application,"said Rogers."So it should be checked periodically —not just at the beginning of a project." If current wind speed and direction make drift into sensitive areas likely, applicators should consider a couple of options.First,think about rescheduling spray activities to another date or time.Or, determine a buffer zone at the edge of the spray area — 50 to 100 feet — and spray the strip later when the wind shifts or conditions improve. Otherwise, change application methods from a foliar spray to a more targeted method such as a basal application. Droplet size and nozzle height After wind speed and direction, spray droplet size is the second most important factor affecting drift.Droplet sizes can vary based on a num- ber of things such as pressure,climatic conditions and nozzle size.This is important, because, as shown in the figure below, smaller droplets can drift considerably longer distances when released at the same height. Image provided by Dow AgroSciences LLC www.arborage.com