Weed Control in Peppermint and Spearmint

Charles E. Stanger and Joey Ishida

Malheur Experiment Station

Oregon State University

Ontario, Oregon, 1995

Introduction

Peppermint and spearmint crops have been removed from the Prowl (pendimethalin) label. Mint growers are left without a soil active herbicide with residual activity to control many species of summer annual broadleaf and grassy weeds. The purpose of these studies are to evaluate experimental herbicides in small replicated plots and to compare registered herbicides for weed control and crop tolerance in field strip plots. Observations were taken on herbicide performance from different rates, tank-mixes, and timing of applications. Our objective is to find herbicide treatments that will control summer weeds with the same effectiveness as Prowl.

Procedures

Herbicides evaluated in the replicated trial included Command EC and two slow release formulations of Command identified as PL 95-087 and PL 95-014. Each formulation was applied at 0.5 and 1.0 lb ai/ac. The experimental FMC herbicide F 6825 was included at rates of 0.187, 0.25, and 0.5 lb ai/ac. Prism was tank-mixed with all treatments for grass control at the rate of 0.094 lb ai/ac. These herbicide treatments were applied to 3-year old spearmint. The field was located near Nyssa, Oregon. The field was rotary corrugated in the fall for furrow irrigation. Buds of mint were starting to emerge when herbicides were applied. Soil surface was in excellent tilth. Soils were silt loam texture with 1.1 percent organic matter and a pH of 7.3. Herbicides were applied on March 7. Air temperature was 50°F, soil temperature at 4-inches 36°F, skies were clear, and the wind was calm. Spray equipment was a single bicycle wheel plot sprayer, 8.5 foot boom, and 10-inch spacing between 8002 teejet fan nozzles. Spray pressure was 42 psi using a water volume of 34 gal/ac. Spray pattern was broadcast double overlap. Weed species present when herbicides were applied included downy brome, prickly lettuce, blue mustard, tansy mustard, and tumbling mustard. Downy brome was tillering, prickly lettuce had 4 leaves with 2 inch wide rosettes, and the mustard species were 2 inches tall and 2 to 3 inches across rosettes.

The strip plots were applied to peppermint and spearmint in fields located near Nyssa, Oregon, New Plymouth, Idaho, and Nampa, Idaho. All these sites were on two and three year old mint. Herbicides included Command, Sinbar, Stinger, Karmex, Buctril, and Gramoxone. Fusilade and Assure II were applied at certain sites to evaluate for control of downy brome. Treatments were two and three way tank-mixes applied at varying rates. Herbicide treatments were applied using a Rear's manufactured pull type sprayer equipped with a fifteen foot spray boom and teejet fan nozzles size 8002 spaced ten inches apart spraying herbicides in a broadcast double overlap pattern. Spray pressure was 35 psi and water volume 28 gal/ac. The sprayer was pulled with a 4-wheel recreational vehicle. Each treatment consisted of plots 30-feet wide for the length of the field. Herbicides, rates, and results are included in separate tables for individual sites, because the time of application and weed species varied between locations.

Results

The emulsifiable formulation of Command herbicide was slightly more active than the encapsulated formulations of Command. Weed control was better from the emulsifiable concentrate formulation at the 0.5 lb ai/ac. Symptoms of chlorosis from Command on crop and weedy plants in plots adjacent to Command treated plots were about equal between the two formulations indicating drift from Command vapors can still occur from the encapsulated PL 95-087 and PL 95-014 formulations. Mint and weedy plants in plots adjacent to the encapsulated treated plots may have outgrown the symptoms sooner, but the initial symptoms were as pronounced as those on plants adjacent to plots treated with the emulsifiable concentrate formulation. Weed control was excellent for both formulations at the one pound ai/ac rate. Command controlled all mustard species of weeds and persisted in the soil to give preemergence control of pigweed, lambsquarters, kochia, barnyardgrass, and green foxtail. It was less active on prickly lettuce giving only partial control of that species. The FMC herbicide F 6825 did not perform well in this trial. At the lower rates it did not control the annual weeds effectively, and spearmint was injured at the higher rate. F 6825 did show good activity for control of yellow nutsedge. Prism was compatible with Command and controlled downy brome. Prism did not control downy brome when tank-mixed with F 6825.

The outstanding treatment applied on grower fields in strip applications was the tank-mix combination of Karmex, Sinbar, and Gramoxone (0.8, 1.0, and 0.33 lb ai/ac respectively). A crop oil concentrate (MorAct) was added to the preceding tank-mix at the rate of 1 quart per acre. Both spearmint and peppermint were tolerant to the tank-mix treatments. No herbicide injury was observed in the crop, and weed control was excellent. Gramoxone with Sinbar and Karmex gave excellent control of all emerged weed species including downy brome and persisted to control spring and summer germinating annual weeds and even weeds germinating postharvest. Stinger was very active on prickly lettuce and gave excellent control when used in combination with Karmex and Command. Command was compatible with Karmex, Stinger, and Sinbar tank-mixes. Command vapor drifted to adjacent fields for a distance of 300 feet or more causing severe chlorosis to wheat, but the wheat recovered with normal color and growth. Fusilade tank-mixed with other herbicides did not give complete control of downy brome. Downy brome was severely injured initially, but some plants recovered and continued to grow. This did not occur with Gramoxone tank-mixes. Buctril tank-mixed with Karmex was also effective on emerged species of mustards and prickly lettuce. Karmex plus Sinbar did not control all prickly lettuce. Stinger added with Karmex and Sinbar gave excellent control of prickly lettuce from both contact and preemergence activity. Results from these studies show that Karmex tank-mixed with Sinbar and Gramoxone or other herbicides including Command and Stinger in tank-mix combinations can effectively control both winter and summer annual weeds. If Prowl is relabeled, Prowl/Karmex combinations with Gramoxone, Sinbar, or Stinger may be very effective and useful combinations in both established and new plantings of spearmint and peppermint.