MINT WEED RESEARCH

Authors: Boydston, Rick; BAKER, R - WSU-IAREC, PROSSER, WA

Submitted to: Proceedings Tri State Mint Convention
Publication Type: Proceedings
Publication Acceptance Date: 12/1/2002
Publication Date: 1/1/2003
Citation: BOYDSTON, R.A., BAKER, R. MINT WEED RESEARCH. PROCEEDINGS OF THE 3RD ANNUAL TRI STATE MINT CONVENTION, SPOKANE, WA. P. 16-37. JAN 2003.

Interpretive Summary: Weeds lower mint oil yield and quality. Weed control represents a major production cost and investment of time for growers. The goal of this research is to develop improved weed control methods in spearmint and peppermint and to identify promising new herbicides on spearmint and peppermint. Several herbicides were identified that were safe on mint while controlling several problem weeds. Flufenacet was selective on spearmint and warrants further testing on peppermint and spearmint. Clomazone and sulfentrazone controlled two common broadleaf weeds, kochia and pigweed, equal to or better than a standard treatment of pendimethalin. Flumioxazin and sulfentrazone controlled prickly lettuce, marestail, and common groundsel well when split applied to dormant mint in the fall and spring. MCPB applied twice at 0.25 lb ae/a to Scotch spearmint infested with field bindweed increased oil yield of mint compared to nontreated checks. Western salisfy often escapes control by herbicides in mint fields but was controlled with clopyralid oxyfluorfen, terbacil, flumioxazin and sulfentrazone when applied at specific growth stages in greenhouse tests. Western salsify was susceptible to all five herbicides at the one leaf stage of growth.

Technical Abstract: Weeds lower mint oil yield and quality and weed control represents a major production cost and investment of time for producers of mint oil. The goal of this research is to develop improved weed control methods in spearmint and peppermint and to identify promising new herbicides on spearmint and peppermint. Flufenacet applied preemergence at 0.5 lb ai/a only slightly injured native spearmint and did not reduce mint hay or oil yield. Mesotrione, cloransulam, and sulfosulfuron applied preemergence or early postemergence injured spearmint, delaying spring growth. The efficacy of clomazone, sulfentrazone and pendimethalin on kochia and redroot pigweed were determined in established peppermint. Clomazone at 0.38 lb ai/a and sulfentrazone at 0.19 lb ai/a controlled kochia and redroot pigweed equal to or better than pendimethalin applied preemergence at 1.5 lb ai/a in peppermint. Flumioxazin, sulfentrazone, and pendimethalin were applied in fall or early spring to dormant peppermint and native spearmint, and immediately after the first cutting in July. Herbicides caused only minor injury to spearmint and peppermint at each application date, but controlled weeds to varying degrees. Split applications of sulfentrazone improved prickly lettuce control and split applications of flumioxazin improved marestail control. A split application of flumioxazin or sulfentrazone in fall and spring resulted in the greatest spearmint yield of spearmint oil. Weed control was not consistent with a microencapsulated formulation of pendimethalin (3.8 lb/gal formulation) compared to the standard 3.3 lb/gal EC formulation. MCPB suppressed field bindweed well in Scotch spearmint when applied twice at either 0.25 or 0.5 lb ae/a. MCPB at 0.25 lb ae/a increased oil yield of Scotch spearmint (2nd cutting) compared to untreated field bindweed infested checks or spearmint treated with 0.5 lb ae/a MCPB or 2,4DB ester at 0.5 lb ae/a. Terbacil, oxyfluorfen, sulfentrazone, flumioxazin, and clopyralid were applied to Western salsify from preemergence to 4-leaf growth stage in the greenhouse. Salsify was most susceptible to oxyfluorfen, terbacil, and flumioxazin at the 1 leaf stage. Sulfentrazone controlled salsify well when applied preemergence or 1-leaf stage. Clopyralid controlled nearly all salsify regardless of the stage of growth when applied. Salsify seedling emergence peaked at 1 to 2 months after planting seed either in October or July. Seed longevity in soil was short-lived lasting only twelve months.