Schmidt: Herbicide Injury to Plants
Herbicides, when used properly, rarely cause problems on non-target plants. However, these products can cause injury when applied inappropriately, when they turn into a gas (a process called volatilization), or when they are blown by the wind away from the targeted area (a process called drift).
Each growing season many calls come into my office and colleagues’ offices across the state from clients describing curling, cupping, and/or twisting leaves in garden and landscape plants and trees. In most cases these signs indicate exposure to some type of herbicide coming from an inappropriately applied broadleaf weed herbicide for lawns. According to Tom Kalb, NDSU Extension Service Horticulturalist, “Now is not a good time to kill dandelions and other broadleaf weeds in the lawn. You will have better success at killing weeds if you wait until late September this is when weeds move nutrients and herbicides down to their roots. The cooler temps at that time will reduce the likelihood of chemicals volatilizing and drifting onto your garden plants.”
Several factors may affect the ability of herbicides to move from the site targeted for application to a non-target site, including:
1. Formulation – The form of the herbicide’s active ingredient determines how it should be applied and its likelihood of causing plant injury. Some formulations have the ability to volatize (turn into a gaseous vapor), which can be carried with or without wind to non-target plants.
2. Application method – Fine spray droplets have a greater potential to drift from the application site. So, using application methods that produce larger droplets, such as lower pressures or sprayers with large orifice nozzles, can help minimize drift potential. Additionally, in some cases, garden sprayers are often used for application of other pesticides, such as those to control insects. If the sprayer has not been thoroughly cleaned after herbicide application, garden plants and trees can inadvertently be injured by remaining herbicide residue still in the sprayer.
3. Temperature – High temperatures (above 85F) during or immediately after application may cause some herbicides to vaporize and, like highly volatile formulations, move to areas outside the site of application.
4. Wind – Even on seemingly calm days, small wind gusts can move herbicide spray droplets away from the intended site and cause injury to non-target plants. Larger droplets and spraying closer to target plants can reduce drift.
5. Soil factors – Depending on the herbicide, location of roots in the soil, soil type, and moisture, some herbicides – whether soil-applied or not – are capable of moving through soil, especially following rain or irrigation. Other herbicides do not persist in the soil long. Herbicide labels will specify if the product has a potential to move in the soil and injure adjacent plants due to root uptake.
Always read, understand, and follow the product label, as pesticide labels provide specific instructions that must be followed whether the product being applied is a herbicide, insecticide, or fungicide.
Also, keep in mind that herbicide injury of non-target plants can also occur via the use of manure or grass clippings in gardens because each of these can contain herbicide residue.
Pyridine herbicides are widely used to control weeds in pastures. The herbicide can persist in straw or manure for months or years. When pyridine-treated grass is consumed by livestock, the pyridine can pass through the animal’s system without breaking down. When you add such manure to your garden you are adding the herbicide along with it.
If using lawn clippings as mulch where herbicide has been applied, it is recommended that lawns should be mowed at least three times before using the clippings for mulch.
Another common culprit behind herbicide injury in home landscapes – especially in trees and shrubs – are weed-and-feed products applied to lawns. Many weed-and-feed products use 2,4-D as their sole herbicide. While this chemical will kill dandelions, it is not effective at controlling many other weeds, including clover, black medic, ground ivy and bindweed, AND many trees are sensitive to this and other common weed control products in this herbicide family.
Dicamba, a more powerful herbicide available to homeowners in broadleaf herbicide spray mixes, is in the same family of growth regulator herbicides as 2,4-D and can build up in the soil, leading to damage of shrubs, young trees and perennials.
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