Minimize herbicide drift with good management

March 23, 2004

Aerial spray application of field.PENDLETON - Spray planes swoop over agricultural fields. Homeowners clear weeds from roadsides and pastures. Spring is in the air, along with seasonal applications of pesticides and fertilizers.

Mary Corp, OSU Extension weed specialist in Umatilla County, warns that the arrival of spring brings increased concern about drifting herbicides and the potential damage they can cause.

"Suburban neighborhoods are encroaching into rural lands at the same time that herbicide-tolerant crops are becoming more common in agricultural fields," said Corp. "That means there is increasing chance that problems can occur if herbicides drift off course."

In addition, spring weather patterns with strong inversions and windy days make it hard to find enough time to make successful herbicide applications, according to Corp.

Damage to crops and ornamental plants can inadvertently occur when some kinds of herbicides drift off target, either by shifting air currents or by spraying at high pressure. Even small amounts of drifting spray can affect new growth of sensitive crops, according to Corp.

"There are many herbicide-sensitive crops across the state," said Corp. "Grapes in particular are sensitive to phenoxy-type herbicides throughout their growing season."

You can minimize spray drift by selecting the proper equipment and using good application techniques, according to Corp. Management instructions and warnings are found on the herbicide label and must be followed carefully. In addition, Corp recommends the following practices to reduce the potential for drifting spray.

Select herbicides that are less likely to injure sensitive crops. All herbicides can drift as spray droplets, but some herbicides vaporize and drift farther as fumes. For example, the esters of 2,4-D or MCPA can produce damaging vapors, while the amines of 2,4-D or MCPA are less volatile and usually drift as heavier droplets or dry particles.

Use nozzles that produce large spray droplets. Small droplets take longer to fall to the ground, and so they drift farther and vaporize more quickly. Switching from standard flat-fan nozzles to venturi nozzles increases droplet size and can greatly reduce the amount of drift.

Reduce the distance between nozzle and target (boom height). Less distance means less time to travel from nozzle to target and therefore less drift.

Do not spray when humidity is low and temperature is high. Low relative humidity and high temperatures will cause more rapid evaporation of spray droplets between the spray nozzle and the target. Evaporation reduces droplet size, which in turn increases the potential drift of spray droplets.

Do not spray when the wind blows toward sensitive crops. The amount of herbicide lost from the target area and the distance the herbicide moves will increase as wind velocity increases, so greater wind velocity generally will cause more drift. However, severe crop injury from drift can occur with even a light breeze, especially under conditions that result in vertically stable air.

Reduce spray pressure. As the spray solution emerges from the nozzle in a sheet, droplets form at the edge of the sheet. Increased nozzle pressure causes the sheet to be thinner, breaking into smaller droplets. Reduced spray pressure and larger orifice nozzles produce a thicker sheet of spray and larger droplets less likely to drift.

Communicate with your neighbors before you spray. Communication is essential among growers, neighbors and crop consultants, especially as more suburban neighborhoods encroach into agricultural areas and as use of herbicide-tolerant crops is increasing.

NOTE TO EDITORS: A downloadable photo of spray planes over agricultural fields is available at:

Author: Peg Herring
Source: Mary Corp