Orchard Floor Management: Chemical Weed Control
Weeds may be managed either mechanically or chemically. Each approach has advantages and disadvantages (Table 1). Mechanical weed control usually requires more energy, is short lived, non-selective, typically selects for perennial weeds and has a higher risk of damaging tree trunks and shallow roots. Chemical weed management, if not practiced carefully, can expose applicators, fruit trees, and the environment to toxic materials. Always read, understand, and follow herbicide labels.
The advent of chemical weed killers (herbicides) has given fruit growers many choices for controlling orchard weeds. erbicides have various modes of action, but can conveniently be grouped as either pre-emergent or post-emergent herbicides. Pre-emergent herbicides are active in the soil against germinating seedlings and require incorporation into the soil with tillage or precipitation. Some pre-emergent herbicides can slow root growth of trees. Pre-emergent herbicides that are more mobile in the soil are often more effective but are not labeled for use in newly planted orchards. Post-emergent herbicides are active against vegetation that is already up and growing. Post-emergent herbicides can further be categorized as contact (e.g. paraquat) or systemic. Post-emergent systemic herbicides can also be divided as selective (e.g. 2,4-D only controls broadleaf weeds and Sethoxydim only controls grasses) or non-selective (e.g. glyphosate) that affect any plant.
Effective chemical orchard weed control programs utilize both pre-emergent and post-emergent herbicides. When the initial fall or spring herbicide application includes both a pre-emergent and post-emergent material the flush of weeds that have already germinated is controlled and the new weeds are kept from growing—providing longer control. If a second herbicide application is required later in the season, a post-emergent product usually is sufficient. Check the product labels for allowable tank mix combinations.
Herbicides are active at relatively low concentrations. Herbicide application must be made with a well maintained and accurately calibrated sprayer. Fixed boom applicators with flat fan or low-drift nozzles at low pressures (15-25 psi) are best for herbicide application, unless the label specifies otherwise. Low pressure application reduces the number of very small droplets. Small droplets are prone to drift.
Use extreme caution when applying herbicides to newly planted and young trees. Ensure the soil has completely settled before applying pre-emergent herbicides. Post-emergent herbicides can be absorbed through the tender bark of young trees resulting in tree damage or death. Some product labels restrict application to newly planted trees. Carefully read the label before application.
If an herbicide is being used for the first time, treat a small area of the orchard first to gain experience and to check for effectiveness.
Rotate the herbicides used. Rotating herbicides reduces “weed shifting” where weeds that are tolerant to the materials being used can thrive. This is particularly important for pre-emergent herbicides. Be sure to rotate to an herbicide with a different mode of action. For example, simazine and diuron have the same mode of action, so alternating these products would not have the desired result.
Herbicides Best Use Practices
- Use an accurately calibrated fixed boom sprayer, flat fan or low drift nozzles, low pressure and constant speed.
- Read the full herbicide label before making an application. Labels change over time and applicators must follow the label for the product they are using. Wear the personal protective equipment specified on the label for the product(s) being used. Store herbicides in a secure location in accordance with label requirements.
- Use herbicide sprayers only for herbicides and clean sprayers thoroughly following use—especially following 2,4-D.
- Dispose of excess spray material properly. Avoid contact with non-target vegetation.
Pacific Northwest Weed Handbook. 2011.
West, B.C. and T.A. Messmer. 2010. Voles. Utah State University Cooperative Extension publication NR WD 009. 4pp.