Be careful when spraying herbicides

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By Dennis Morgeson

I have gotten several calls about plants doing weird things and having weird diseases only to get samples and home visits to determine that the plants were actually suffering from herbicide spray drift.  What gardeners need to realize is that not all herbicides stay where they are sprayed and that some plants are very sensitive to some of these chemicals.  Also, if you spray herbicide and it damages someone else’s plants you are liable.

There are three basic types of herbicides used by home gardeners, post-emergent broad leaf herbicides, grass or lawn herbicides, and total vegetation herbicides.

Post-emergence broadleaf herbicides are used to selectively kill broad leaf plants while they are actively growing and include chemicals such as  2, 4-D and dicamba.

Grass herbicides are used to control undesirable grasses. Some are used before the grasses sprout (pre-emergent) and are unlikely to cause drift problems. Some are used after the grasses germinate (post-emergence) and may drift.

Total vegetation control herbicides are designed to kill whatever plant they are applied to.

The formulation, or form, of active ingredient of a herbicide will determine how it is applied and the likelihood of unintentional spread. For example, in 2, 4-D, ester formulations can vaporize and be carried by the wind; an amine formulation is less likely to vaporize. In general, granular formulations are rarely carried far from the intended site.

Fine spray droplets have a greater potential for drifting from the site of application than do large droplets.  Temperatures greater than 85 degrees F during or immediately after application may cause some herbicides to vaporize and drift to areas outside the site of application.  Herbicides are not to be applied on windy days; even on seemingly calm days, small gusts can move droplets of spray away from the intended site.  2,4-D can drift several miles and can affect leaves on tomatoes and grapes in very low concentrations.  Some grape growers even joke that if you say 2,4-D in a vineyard the grape vines will twist.  Personally and from personal experience if you have tomatoes and grapes in your yard don’t spray anything with 2,4-D in it.  Your neighbors may appreciate it as well.

The possibility of root uptake of soil applied herbicides depends on the herbicide applied, the type of soil, and its moisture content. Some herbicides, like dicamba, are relatively mobile and will move ready in porous soils, especially after a rain or irrigation.

Some symptoms which may resemble herbicide injury may be due to a variety of other factors such as insects, disease, adverse weather, soil compaction, drought stress, root stress, improper soil pH, etc.

 In order to accurately diagnose injury due to herbicides, one must have knowledge of the symptoms produced by a particular herbicide on a specific plant. This information is not readily available for most ornamentals. So what do you look for?

First is timing. Injury from herbicides usually appears within one to two days after exposure, but symptoms may develop up to several weeks after exposure. Injury may show up after longer periods, for example when tree roots grow in heavily treated sites.

If suspected herbicide damage is observed, it is likely that adjacent plants will show symptoms with the same time frame. Look for injury on two or more different species. It is extremely unlikely that herbicide drift injury will occur on just one tree or shrub in a landscape.

Determine where the injured plants are in relation to the area where herbicides were applied. Vaporization of growth regulator herbicides can result in exposure of plants away from the site of application, but again, surrounding plants must be examined.

Whether or not a plant will recover from non-target herbicide injury depends on the overall vigor of the affected plant, the amount of herbicide it received, and the type of herbicide used. Healthy woody plants which receive a low dose of a growth regulator type herbicide will most likely recover. However, if a greater dose is absorbed, the chemical may persist within woody plants and symptoms may appear for the next two or three seasons. Plants which have absorbed a total vegetation type or soil sterilant may not recover.

The time when exposure occurred will also affect recovery. Plants that receive an accidental herbicide exposure late in the year when they are preparing to enter dormancy will not be injured as much as plants exposed early in the growing season.

For more information, please contact the Washington County Extension Service, 336-7741. Educational programs of the Cooperative Extension Service serve all people regardless of race, color, age, sex, religion, disability or national origin.