In this section
Established Tree, Shrub, Rose, and Ground Cover Landscapes Revised March 2021
Bulb and Flower Beds Revised March 2021
General Maintenance around Ornamental Plantings Revised March 2021
In this section
This section is primarily a reference for commercial enterprises that manage landscapes or garden areas, often with the use of herbicides. Please refer to other University and Extension guides that cover homeowner uses https://catalog.extension.oregonstate.edu/em8742. The herbicides listed in this section are intended for commercial use unless noted otherwise.
Weed management in professionally managed landscapes requires a thorough knowledge of the weeds, weed control practices, application options, plant tolerances, and management strategies. Success with any method depends on planning, properly selecting control methods, and effective timing. When selecting chemicals for weed control, you must understand the subtle differences in application techniques, residual activities, susceptibilities, and timing for control of target species, tolerance of desirable plants, potential for weed shifts, and long-term effects of integrating site objectives and year-round weed management strategies. Often, treatments must be repeated or combined with other practices for maximum control.
Weed shifts Weed infestations are dynamic and change depending on previous cultural and weed control practices. For example, routine use of the same or similar herbicides will result in a shift to weed species that tolerate the treatment. Repeated use of simazine in nurseries was first demonstrated to cause a shift within a natural population of common groundsel. Since then, more than 72 species represented in 40 genera have been reported to tolerate triazine herbicides. Globally, there are currently 413 unique cases of herbicide resistance (species x site of action), found in 222 species.
Preventing weed shifts Weeds that survive repeated use of the same or similar herbicides must be eliminated before the tolerant species or biotypes become established. Spot-treat or hand pull weeds and combine these methods with other weed management practices to minimize the occurrence of resistant biotypes.
Managing weedy vegetation Successful weed control in landscapes requires a comprehensive or year-round approach whereby a combination of weed control practices is employed and alternated over several years. Development of these strategies requires knowledge of each weed and weed control practice. Weeds must be identified and information gathered about the effectiveness of each weed control practice. Consider costs and select herbicide combinations that can be applied together or in split applications that control the weeds present in the landscape. Note the action of each herbicide or how the chemical works in the plant. Then tank mix, and alternately use these products to reduce the chance of developing resistant species or biotypes. Often a combination of mechanical, herbicidal, and sometimes hand removal or spot treatment with herbicide sprays or wipers will provide the most effective year-round control.
Soil-active herbicides Persistent, soil-applied herbicides can be applied to weed-free soil during winter when rain will activate the chemical. Some compounds may be applied throughout the year if irrigation is available. Apply lower rates on sandy soils having lower clay or organic matter, or cation exchange capacities. Existing vegetation can be controlled by mixing the soil-active herbicide with a post-emergence contact or translocated herbicide, provided such a mix is not prohibited by either label. Consult labels for listed species and duration of expected control. Avoid disturbing the soil when applying.
Postemergence herbicides In landscape plantings, postemergence weed control requires precision since few options exist. Postemergence treatments either selectively control susceptible weeds or are applied with selective equipment. Frequent scouting aimed at identifying susceptible weeds and the correct stage of weed growth must be combined with appropriate weather conditions and labeled spray additives to maximize control. Consult each label for numerous precautions or information about crop or cultivar tolerances. Always test new products on selected plants before adopting their use throughout your operation.
Caution! Information provided in this handbook is not intended to be a complete guide to herbicide use. Before using any chemical, read the recommendations on the label. Before a chemical can be recommended for a specific use, it must be thoroughly tested. The recommendation on the manufacturer’s label, when followed, can prevent many problems from arising from the wrong use of a chemical.
Note Herbicides must be applied at the correct rate and time to selectively control weed growth with minimal chance for injury to landscape plants. Obtain more consistent results by reading the herbicide label and other information about the proper application and timing of each herbicide. Suggested rates listed in this guide are stated as pounds active ingredient per acre (lb ai/a) or pounds acid equivalent per acre (lb ae/a). Calibrate your equipment precisely and verify exact application rates, especially to irregular landscape sites.