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John Fulton

John Fulton
Former County Extension Director

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In The Backyard

Horticulture columns and tips done on a timely basis

Herbicide Injury

Posted by John Fulton -

As if trees didn’t have enough leaf problems with the diseases, herbicide drift has shown up in a big way this past week or so. Particularly noticeable is drift damage on tomatoes and grapes. All cases I have seen, the herbicides involved have been members of the growth regulator group. This group includes products such as 2,4-D and dicamba (Banvel.) Both products are used in agricultural production, right-of-way maintenance, and in home lawn care. Just check the label on your favorite broadleaf weed control product, and if you can get by the technical chemical name, it will usually end with salt of dicamba as one of the ingredients in the three-way combination products.

Leaf symptoms usually appear as some sort of abnormal growth. This can include twisting, cupping, elongation, and rolling. Since these chemicals are systemic growth regulators, they move throughout the trees (or shrubs or flowers) and then show the most damage on the newest growing points. Think of what a dandelion looks like after it has been treated with 2,4-D and you get the general idea.

Where the products come from on your trees and gardens is generally a big mystery. They can drift during the actual spraying process (called spray drift), or they can come back up off the ground as a vapor and move with winds (called vapor drift). The difficulty with vapor drift is that it can occur for up to one and one-half weeks after the application, and then can drift for up to a mile and a half. This vapor drift problem is more common with esther formulations of the chemical (basically oil based) as compared to the amine formulation (basically water based).

Different species of plants are more susceptible than others, and the full-size leaves are less likely to show symptoms. Red buds, oaks, and lilacs are among the most susceptible trees. Grapes and tomatoes are among the most susceptible garden plants. The chemicals concentrate in the newest growing tissues such as the buds, tips, and newest leaves.

If you do have damage from herbicide drift, the end results can vary. Generally, on established perennials, the damage is ugly leaves for at least part of this growing season. You can also have some “wave” to the ends of branches, and possibly the loss of some small branch ends. On younger stock, transplanted in the last year or so, the damage may be fatal. It usually takes several weeks to get an indication of the amount of damage done, but a year is even better.

As for treatment, water plants when the weather stays dry. Don’t fertilize at this time. Remember that growth regulator herbicides make things “grow themselves to death.” You have to walk a fine line between keeping the plant healthy and making matters worse.

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