Nursery, Landscape, and Turf - Avoiding Herbicide Injury

The following is information on avoiding herbicide injury where they are being applied or from off-target movement.

Steps in Avoiding Herbicide Injury in the Nursery or Landscape

1. Read the label and confer with your Extension educator for current pest control recommendations. Choose correct herbicides that least affect the plants and surrounding environment. Avoid using soil sterilants around ornamental plantings. Soil sterilants or selective herbicides applied at soil sterilant rates should not be used within the nursery or landscape. Never apply long lasting, high toxicity soil sterilants under new pavement or sidewalks. Tree roots from nearby species will eventually grow into the site and translocate the chemical throughout the plant. These trees will be severely injured or killed as a direct result of the chemical weed barrier applied. Purchase herbicides from nurseries or garden centers that are staffed by qualified individuals who can recommend the appropriate chemical control when necessary.

2. Calibrate application equipment before each use.

3. Apply herbicides around desirable plants that are healthy and vigorously growing. Use caution around nontarget plants with thin or green bark. However, in some cases, dormant, nontarget species will be less susceptible.

4. Do not plant in sites suspected to be contaminated with herbicides. Any herbicide may be used successfully for several years on one or more nontarget species with little or no injury. Problems may arise, however, when plants are finally harvested from a field and the grower decides to plant a different crop or crops. A sufficient buildup of herbicide residue, capable of injuring or killing newly introduced species, may be present in the soil.

5. Avoid drift by applying herbicides on calm days. Strive to apply herbicides when the wind speed is 10 mph or less. When possible, use a coarse droplet spray applied at low pressure. Strive to use herbicides early in the morning when winds are likely to be calm. Excessive temperatures can also be avoided in the morning hours. Intense heat can cause phenoxy type herbicides to volatilize and injure nearby nontarget species. When available, choose herbicide formulations that pose the least risk of drift or other forms of contamination. For example a low volatile ester or amine type formulation of 2,4-D is appropriate when spraying around sensitive, broadleaf ornamentals. Follow this precaution even when plants are dormant. Buds are capable of absorbing phenoxy-type herbicides in sufficient amounts to produce distorted leaves. Nurseries are often located in rural areas close to agronomic enterprises. It is a good idea to plant nursery crops as far as possible from fields of grain or other agronomic species. Herbicides used in these operations are often broadleaf herbicides that can be injurious to broadleaf ornamentals, should they drift or volatilize over to the nursery. The nursery operator should discuss any concerns with the farmers.

6. Injury may occur to certain plants when herbicides are applied to liners that are not well rooted. This may be addressed on the chemical label.

7. Although excellent grass control herbicides are available, they also have the potential to injure some broadleaf species. They are not totally selective between grassy weeds and desirable broadleaves.

8. The presence of a nontarget species on a herbicide label may imply that it will not be injured by the chemical. Cultivars of that species, however, can sometimes differ drastically in their tolerance to any given herbicide. Always experiment with new cultivars by only treating a few plants to establish their tolerance to the herbicide.

9. Avoid applying herbicides at their highest labeled rates in soils low in organic matter. Less soil applied herbicide is required for proper weed control in soils low in organic matter. Unfortunately, a smaller margin of tolerance exists for desirable plants growing in these soils. In general, Oklahoma soils are low in organic matter.

10. Avoid water contamination. Note nearby water sources that are potential targets for drift or runoff. Not only can water spread herbicide damage to nearby plants, but fish deaths and injury to other aquatic life may occur.

11. Be aware of where roots of desired plants really exist. Remember that root grafts of like species are quite common. For example if one stump is “poisoned”, a nearby tree of the same species may also be killed. Phenoxy type herbicides, glyphosate, and ammonium sulfamate are capable of translocating long distances and injuring desirable plants. Additionally, herbicide damage frequently occurs when harmful chemicals are applied just outside the dripline. Roots extend far beyond the dripline of trees and often absorb these chemicals. Absorbing roots that translocate herbicides throughout ornamental trees and shrubs are located in the top few inches of soil. Lastly, use caution when treating suckers as the herbicide may translocate back to the “mother plant” causing injury.

12. Air circulation is imperative due to volatilization that may occur. Nursery stock, treated in a greenhouse, can be injured in a short amount of time before being placed outside with proper air circulation. This is why few herbicides are labeled for indoor use. They are not phytotoxic if proper ventilation is provided around treated plants.

13. Keep chemicals in their original containers. Never switch containers, since this can lead to accidental contamination and/or personal injury.

14. Chemicals should only be applied by conscientious, certified applicators who follow label directions. Violating the label in any fashion is against the law.

15. Keep records of chemicals used, date, rate, wind speed, temperature, humidity, etc. that can increase the impact on both target and nontarget species.

16. Be sure to follow label directions for excess chemical and empty container disposal.

17. Apply pesticides in the cooler time of day; avoid applications during hot periods.

18. Always label and dedicate a sprayer to herbicides only to avoid phytotoxicity.

19. Choose water-based pesticides over oil-based chemicals when possible. When drift occurs, water-based pesticides are less likely to injure nontarget plants.

Information from Herbicide Injury in the Nursery and Landscape by Michael A. Schnelle, Professor, Extension Ornamental Floriculture Specialist and Janet C. Cole, Professor, Nursery Management, Oklahoma State University.