The use of herbicides to control weeds or other undesirable plants is a common practice in many integrated pest management (IPM) strategies. However, improper application methods and drift of herbicides to desirable plants can cause similar damage as seen on the targeted weeds. Off-target movement of common herbicides, such as glyphosate, 2,4-D, metsulfuron, atrazine, and others may damage desirable landscape plants with some plants being more susceptible than others. The toxic effect of pesticides, such as herbicides, on a plant is known as phytotoxicity. Diagnosing the source of damage can prove difficult because herbicidal effects may appear similar to symptoms associated with other biotic and abiotic issues, such as wilt, disease, and insect damage. What makes it more difficult is that the severity of symptoms can be related to the amount of herbicide exposure and the method of absorption, such as by direct surface contact or root absorption. Additionally, symptoms may not appear for days to weeks after the application.
Phytotoxic Symptoms, or Plant Damage, as a Result of Commonly Applied Herbicides Include:
|Glyphosate||Delayed yellowing of leaves that turn to brown
Stunted or ‘witches broom’ appearance of new growth
|2,4-D/dicamba||Twisted and bent petioles
Scorched appearance to leaves
Sudden reaction in oaks
|Diquat||Rapid wilting and desiccation followed by necrosis (browning) of selective areas
One half of leaf may show symptoms while other half appears healthy
|Atrazine||Yellowing between leaf veins and edge followed by necrosis
Older leaves show greater damage than new growth
Avoiding improper application of herbicides is crucial to prevent damage to nearby landscape plants. Thoroughly read the herbicide label and check for susceptibility of nearby desirable plants to the herbicide. Avoid spray treatments on days of excessive heat (greater than 90 F) or high winds speeds (greater than 10 mph), so as to limit volatilization (liquid transforming to gas form) and drift of herbicides, respectively. Also, be sure of the target application area and rate of application.
Other methods of reducing the potential for drift include adjusting the droplet size of your spray to create larger droplets for all herbicides. To do so, adjust/change the sprayer nozzle and sprayer pressure (pounds per square inch). Larger spray droplets are heavier and less affected by wind than finer droplets would be. Avoid spraying weed plants excessively, as spraying to runoff may cause an herbicide to drip from the plant onto the soil and be absorbed by roots near the soil surface. The use of a nozzle hood attached to the end of a sprayer wand lessens wind drift. Avoid spraying woody plant suckers, surface roots of desirable plants, or the trunks of thin-barked species of woody plants, such as maples and cherries, to lessen the possibility of herbicide absorption and damage.
The use of wiping techniques can also be considered. This is done by putting the herbicide on a sponge or paint brush and applying it to the targeted weed. Be sure to apply the herbicide to green leaves and stems and onto freshly made cuts for more vine-like plants.
These alternative methods do not lessen the need to wear proper safety attire when mixing or applying herbicides. Applicators should still wear protective gloves and eye protection in addition to other specified personal protection equipment, as recommended by the herbicide’s label.
If herbicide damage is suspected, closely examine adjacent plants for similar symptoms, as herbicide overspray or drift rarely affects just one plant. Light herbicide damage to trees and other healthy, well-established landscape plants may not cause plant death. If symptoms of herbicide damage are already seen, it is too late to remove an herbicide; however, root damage by an absorbed herbicide may be limited by thoroughly irrigating the area to reduce plant moisture stress, as well as to flush loosely soil-bound herbicides through the soil. Rinse exposed foliage of desirable plant with clean water immediately after application to aid in removal of spray residue and limit absorption of the herbicide. However, be aware that roots may still take up some herbicides.
- Fraedrich, Bruce R. Diagnosing and Preventing Herbicide Injury to Trees. Bartlett Tree Research Laboratories Technical Report. TR-43.
- McCarty, L.B. 2018. 2018 Pest Control Guidelines for Professional Turfgrass Managers. Clemson University. EC 699.