Submitted to: Washington Mint Drops
Publication Type: Popular Publication
Publication Acceptance Date: February 1, 2005
Publication Date: March 1, 2005
Citation: Boydston, R.A. 2005. Timing of dormant herbicide applications. Washington Mint Drops, Spring 2005, p. 4. Interpretive Summary: Winter annual weeds are often poorly controlled with spring applied herbicides in peppermint and spearmint production. Fall applications of preemergence herbicides are often more effective in controlling winter annual weeds in mint, but may not adequately control summer annual weeds that germinate later in the spring. Grower knowledge and good record keeping of weed species present in fields will improve herbicide selection and application timing decisions for optimal weed control. Advantages and disadvantages of fall and spring herbicide applications in mint are discussed.
Technical Abstract: Many winter annual weeds (prickly lettuce, mustards, sowthistle, salsify, henbit, common groundsel) germinate from late fall through spring depending soil moisture and temperature conditions and present problems in spearmint and peppermint which are dormant during winter months. Late winter applications of preemergence herbicides should include paraquat to burn down existing winter annual weeds that germinated several months earlier. However, many winter annual weeds are several inches or more in diameter by this time and are difficult to control with paraquat. A hand weeding or another herbicide application may be required to remove weeds that regrow and escape control. Once mint is not actively growing in the fall, usually in mid November, and when winter annual weeds are very small or haven’t germinated yet, a preemergence herbicide can be effective in controlling many winter annual weeds. Paraquat can be included if weeds have emerged and are larger than 0.5 inch. Treatments of flumioxazin, sulfentrazone, or oxyfluorfen applied in November with paraquat have usually resulted in weed-free fields the following spring. However, control of summer annual weeds that germinate in April and May may not be adequate with fall applied herbicides. Growers must keep good records of the main weed species present in fields and select and time herbicide applications to control their dominant weed species.