Iowa State University

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Dealing with Drift   
Bob Hartzler

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June 28, 2004 Herbicides are an integral component of todayís agricultural system, and are also widely used by homeowners to manage weeds in urban landscapes.  When used properly these chemicals provide a safe and efficient means of controlling weeds.  Unfortunately, some herbicide applications are made under conditions that favor off-target movement, resulting in injury to sensitive plants in adjacent areas.  This article will focus on the implications of drift onto landscape plants and home gardens.

Drift problems usually are first noticed when plants contacted by the herbicide begin to display injury symptoms.  While drift can occur with any herbicide, a relatively small number of products are capable of damaging plants at the low concentrations typically associated with drift.  The growth regulator herbicides (2,4-D, dicamba, etc.) are responsible for the majority of off-target injury reports.  The abundance of problems with this class of chemistry is due to their ability to cause easily noticeable symptoms at fractions of the labeled rate.  Glyphosate (Roundup), the bleaching herbicides (Command, Balance, Callisto) and a few other products also can induce noticeable symptoms at low rates.  It is important to realize that managing drift is important with all herbicides, not just those that may injure plants on adjacent properties.

There are two primary issues when herbicide drift causes damage on residential properties.  The first involves the impact of the herbicide on the health and long-term vigor of trees, ornamentals and other perennial plants.  The second is what should be done with produce from home gardens contacted by herbicide drift.

Diagnosing long-term impacts of herbicide drift on landscape plants is difficult because it is impossible to know how much herbicide contacted the plant and because plants vary widely in their sensitivity to different herbicides.  It is my experience that most situations involving drift onto woody plants result in cosmetic injury, and injured plants fully recover from the damage.  Obviously there are exceptions to this rule, but if affected plants only show foliar malformations or discoloration, rather than leaf necrosis or loss, it is likely that the long-term health of contacted plants should not be compromised.  This assumes the plant was in good condition prior to the drift event and that other stresses are prevented.

The more difficult scenario occurs when drift contacts fruit trees or home garden plants.  The EPA is responsible for regulating pesticide residues in the food system.  Before any pesticide can be registered for use on a crop, tolerance levels are established that specify the amount of that pesticide that is allowable in the harvested crop.  When a herbicide is used according to label recommendations (rate, timing, etc.), pesticide residues should remain below the established tolerance levels.  The problem with herbicide drift is that it usually involves products not registered for use on garden crops, thus there are no tolerance levels established to determine whether the produce is safe for consumption.  Thus, the simple answer is to simply recommend that all produce from affected plants be destroyed.

The problem with this simple answer is that due to the widespread use of herbicides in Iowa, it is unlikely that any plant in the state completely escapes contact with low levels of some herbicide.  Thus, it may be appropriate to involve subjectivity in determining whether to harvest produce from affected plants.  These guidelines should be used for produce from home gardens, rather than commercial production.  Factors that should be considered in accessing risks associated with pesticide drift include:

1)  the amount of herbicide contacting the plants,
2)  the time elapsed between drift exposure and harvest, and
3)  the personal biases of the homeowner.

Most cases of drift involve a very small amount of pesticide, therefore risks associated with the drift are relatively low.  However, it is difficult to determine the amount of pesticide that has contacted garden plants.  The severity of symptoms on plants may provide evidence of the level of pesticide trespass, especially in cases involving 2,4-D and other growth regulator herbicides.  If symptoms on tomatoes and other sensitive plants (grapes, potatoes) are limited to minor distortion of leaves, it is likely that only a small amount of herbicide contacted the garden.  In this situation it is unlikely that harvested produce would have significant amounts of herbicide present.  On the other hand, if sensitive plants are severely injured by the herbicide, it probably would be wise not to harvest produce from any plants in the garden, even those plants not displaying significant injury symptoms.

The length of time between the drift event and harvest and consumption of produce will influence the amount of herbicide in garden produce.  Most plants have the ability to metabolize herbicides, thus the concentration of pesticide in the plant will decline over time.  Plants rarely completely eliminate the pesticide, so low concentrations may remain in the plant later in the season.  However, the amount of residue generally declines relatively quickly in the first few weeks after exposure.  Produce harvested six or more weeks after the occurrence of drift will likely have much less pesticide present than produce harvested within a week of the drift event.

Another important factor to consider is a personís views regarding the safety of pesticides and other synthetic chemicals.  The comfort level with pesticides varies widely among persons.  If someone has any reservation about the safety of pesticides in food, the recommendation should be not to harvest any produce from an affected garden.

Herbicide drift is manageable, but unfortunately poor judgment results in drift problems each year.  In the majority of cases the amount of herbicide contacting plants on adjacent property is relatively low and should not pose a long-term threat to plant health.  Determining whether to harvest produce from home gardens displaying herbicide injury symptoms is a difficult decision, and when in doubt all garden plants should be destroyed.  However, there may be situations when it is apparent that only a small amount of herbicide contacted the plants and the interval between exposure and harvest minimizes the risk of unacceptable residues being present in the produce.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
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Common chemical and trade names are used in this publication. The use of trade names is for clarity by the reader. Inclusion of a trade name does not imply endorsement of that particular brand of herbicide and exclusion does not imply nonapproval.