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How to minimize herbicide drift — and damage

Tree Talk: Growers of non-target plants such as trees are nervous heading into the 2019 season. Here’s how to mitigate possible damage.

Spring may be right around the corner, but as I write this following record-breaking low temperatures and wind chills, spring seems a long way off. However, with spring and the field season approaching, it’s important to review key points regarding herbicide application and how it may affect non-target plants.

What are we talking about exactly? We’re referring to growth regulator herbicides, such as dicamba and 2,4-D products, and their propensity to volatilize and move off target; the symptoms associated with damage to non-target plants; and better awareness on how to minimize plant damage. We still have plenty to learn about these new growth regulator products, and please understand the intent of this article is not to point fingers at anyone. We are all in this together and will need to work together to increase our awareness, be more responsible in how we use these products, understand their benefits and liabilities, and recognize the potential damage they can cause if we do not apply them properly.

The products
Herbicides are categorized based on the kinds of plants they kill, when they are applied and their mode of action. Postemergence herbicides selectively kill broadleaf weeds and commonly include growth regulators with active ingredients containing 2,4-D; 2,4-DP; MCPA; MCPP and dicamba. They are labeled for use around homes, farms and industry, and are prone to drift and volatilization.

Products containing 2,4-D have been around since the 1940s and have been used to control a variety of perennial weeds. The newer dicamba herbicides, while they may have lower volatility, may still volatilize, particularly when applied under higher temperatures in May, June and July, and can damage plants even at very low rates.

Many plant species are extremely sensitive to dicamba drift. Distinguishing between dicamba and 2,4-D damage may not be easy. In soybeans, it may take a higher amount of 2,4-D to cause damage symptoms compared to dicamba. Fruit trees, ornamental trees and shrubs, vegetables, annual and perennial commercial flowering plants, and grapes can be injured by use rates of dicamba at 0.1, 0.01 or 0.001 the normal use rate.

Non-target plant injury and symptoms caused by growth regulators include twisted and downward cupping of leaves and narrow strap-like leaves on the new growth. This is because dicamba is translocated to the growing points of the plant, affecting the youngest growth. Root uptake of these products can be even more damaging to plants but is poorly understood.

Product formulation must be considered. Growth regulator products are formulated as amines or esters. Ester formulations typically have higher vapor pressures than amines, which accounts for their greater volatility. Generally, amine salts are less volatile than esters. As the temperature increases, the vapor pressure increases, leading to higher volatility.

So, should you use esters or amines? Unfortunately, this is not a cut-and-dried question. Ester formulations are generally more active on weeds than amines, mainly due to the fact that esters are more soluble when they contact a plant’s waxy surface, and are usually safer to use during the cooler spring months. As a result, plant leaves may absorb esters more quickly than amines. In warmer months, a shift to amines might be appropriate to reduce the potential for drift. Make sure to consult the label regarding temperature ranges for using the product.

Final steps
As with any pest management program, attacking the organism (i.e., insect, pathogen or weed) at its most vulnerable stage is critical in achieving effective control. Always read the label prior to and during application of any pesticide. Consider the time of year, location of application, weed pressure and application methods. Be aware of non-target plants adjoining the application site, and be considerate of your neighbors. In this way, you can minimize or avoid non-target plant damage and still get effective weed control.

For more details on proper herbicide application and products, consult your local Extension office or crop consultant.

Miller is a horticulture professor at Joliet Junior College in Joliet, Ill., and a senior research scientist in entomology at The Morton Arboretum in Lisle, Ill. Email your tree questions to him at fmiller@jjc.edu.

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