From 2002 Mint Research Report Rick Boydston, USDA-ARS, Prosser, WA

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1 From 2002 Mint Research Report Rick Boydston, USDA-ARS, Prosser, WA Fall, Spring, and Summer applied Herbicide Trial on Peppermint and Native Spearmint with Valor (flumioxazin), Spartan (sulfentrazone), Prowl, (pendimethalin) and Sinbar (terbacil). Four herbicides were tested at various application times to determine best timing to maximize weed control and minimize injury to native spearmint and peppermint. Herbicides were applied in November 30, 2001 and in March 4, 2002 to both native spearmint and peppermint on a Warden sandy loam soil. Native spearmint was harvested on July 3, A third application timing was tested in native spearmint on July 17, 2002 following the first harvest. Peppermint had an infestation of root borer so no summer treatments were tested and no hay or oil yield was recorded. Treatments were applied with a bike CO 2 sprayer delivering 25 gpa with 8002 flat fan nozzles. Valor and Sinbar treatments included crop oil concentrate (COC) to improve burn-down of existing weeds. Prowl treatments applied in the spring included Gramoxone (paraquat) at 0.5 lb ai/a to burn down existing winter annual weeds. Treatments were replicated three times in a randomized complete block design in both peppermint and native spearmint. Spartan and Valor slightly reduced early spring growth of native spearmint, but by late spring, growth appeared normal (Table 7). Crop injury in peppermint was not measured due to an infestation of mint root borer, Spartan controlled common groundsel, kochia, flixweed, Russian thistle, and horseweed (Conyza canadensis) well (Tables 5, 6, & 7). Spartan also suppressed small salsify and prickly lettuce. A split application of 0.19 lb ai/a of Spartan in both the fall and spring improved prickly lettuce control compared to spring applications. Fall applied Spartan did not persist long enough to control late emerging pigweed and grasses (Tables 5, 8, & 9). Pigweed and barnyardgrass control in the second cutting of spearmint was excellent when Spartan was applied in July prior to mint regrowth after the first cutting (Table 9). Valor applied in fall or spring controlled prickly lettuce, common groundsel, flixweed, tumble mustard, pigweed, kochia, Russian thistle, and suppressed salsify and barnyardgrass (Tables 5, 6, 7, 8, & 9). Horseweed was not controlled well by Valor applied in the spring (Table 6). Valor applied at 0.25 lb ai/a in the fall or split applications of lb ai/a in the fall and spring, controlled horseweed. Pigweed and barnyard grass in the second cutting of spearmint was controlled better when Valor was split applied at lb ai/a in both fall and spring or when Valor was applied at 0.25 lb ai/a in the spring (Tables 8 & 9). Valor applied in July after the first cutting, controlled pigweed 100% in the second cutting (Table 9). Prowl failed to control most of the winter annual weeds (prickly lettuce, common groundsel, horseweed, flixweed, and salsify), when applied to peppermint or native spearmint in the fall, but when applied as a tank mix with Gramoxone in the spring, smaller emerged weeds were controlled (Table 6 & 7). Flixweed control with fall applications of Prowl 3.3 formulation was superior to Prowl H 2 O 3.8 formulation in native spearmint (Table 7).

2 Sinbar-treated peppermint plots were free of prickly lettuce and horseweed, but spring applications of Sinbar in native spearmint only partially controlled prickly lettuce (Tables 6 & 7). Control of groundsel, kochia, salsify, barnyardgrass, and pigweed with Sinbar was marginal and inconsistent (Table 6). Hay and oil yields were determined on the native spearmint trial only. Hay yields from the first cutting and hay and oil yields from second cutting were not significantly different among treatments (Table 8). However, there was a significant treatment effect on oil yield of the first cutting (Table 8). Most oil yields from herbicide-treated plots were more than double that of nontreated weedy checks, which averaged 23 lbs oil/a. Spearmint treated with either Valor or Spartan split applied in the fall and spring averaged the highest oil yields, 59 and 62 lb/a, respectively (Table 8). The two treatments in which spearmint was treated with Prowl in the fall (1.5 lb ai/a) averaged only 46 lb oil/acre, probably due to poor control of winter annual weeds. Three treatments containing Prowl applied in the spring (treatments 6, 10, and 13) averaged 48 lbs oil/a (Table 8). We plan to repeat Valor and Spartan fall and spring treatments on a commercial peppermint field in since no yield data or crop injury data was obtained from the peppermint trial. Prowl, Valor, and Spartan applied in fall, spring dormant, and after first cutting caused minor injury to native spearmint and peppermint and controlled weeds to varying degrees. A split application of Valor or Spartan in Fall and Spring controlled most weeds well and had high spearmint oil yields. Prowl H 2 O, the microencapsulated 3.8 lb ai/gal formulation has caused slightly less mint injury in various trials than the 3.3 lb ai/gal formulation currently registered in mint. The new formulation may be useful for weed control in newly planted peppermint, which is susceptible to injury from Prowl. However, weed control was somewhat erratic with the new formulation in 2002 trials.

3 Table 7. Mint Injury and Weed Control in First Cutting Native Spearmint with Fall, Spring, and Summer Applications of Valor (Flumioxazin), Spartan (Sulfentrazone), Prowl (Pendimethalin), and Sinbar (Terbacil). Applic. Native Spearmint Weed Control Herbicide Rate Timing Injury Flixweed Prickly lettuce 2/7/02 4/10/02 6/17/02 4/10/02 5/9/02 4/10/02 5/9/02 (lb ai/a) (%) (%) Valor Fall Valor 0.25 Fall Valor Fall + Spring Valor Spring Valor 0.25 Spring Prowl/Valor 1.5/0.125 Spring + Summer Spartan 0.19 Fall Spartan Fall + Spring Spartan 0.19 Spring Prowl/Spartan 1.5/0.19 Spring + Summer Prowl 3.3 EC Fall + Summer Prowl 3.8 H2O Fall + Summer Prowl 3.3 EC 1.5 Spring Sinbar 0.75 Fall Sinbar 0.5 Spring Weedy Check LSD (P=0.05) N.S. N.S MorAct COC added at 1% (v/v) to treatments 1-6 and Gramoxone at 0.5 lb ai/a plus R-11 nonionic surfactant added at 0.5% (v/v) to all Prowl spring treatments. Fall, Spring, and Summer treatments applied November 30, 2001, March 4, and July 17, 2002.

4 Table 8. Native Spearmint yield (first cutting), Injury, and Weed Control with Fall, Spring, and Summer Applications of Valor (Flumioxazin), Spartan (Sulfentrazone), Prowl (Pendimethalin), and Sinbar (Terbacil). Applic. Hay Oil Mint Weed Control Herbicide Rate Timing Yield Yield Injury BYG PW ----7/3/ /31/ /31/ (lb ai/a) (T/A) (lb/a) (%) (%) Valor Fall Valor 0.25 Fall Valor Fall + Spring Valor Spring Valor 0.25 Spring Prowl/Valor 1.5/0.125 Spring + Summer Spartan 0.19 Fall Spartan Fall + Spring Spartan 0.19 Spring Prowl/Spartan 1.5/0.19 Spring + Summer Prowl 3.3 EC Fall + Summer Prowl 3.8 H2O Fall + Summer Prowl 3.3 EC 1.5 Spring Sinbar 0.75 Fall Sinbar 0.5 Spring Weedy Check LSD (P=0.05) N.S MorAct COC added at 1% (v/v) to treatments 1-6 and Gramoxone at 0.5 lb ai/a plus R-11 nonionic surfactant added at 0.5% (v/v) to all Prowl spring treatments. Fall, Spring, and Summer treatments applied November 30, 2001, March 4, and July 17, 2002, respectively.

5 Table 9. Native Spearmint yield (second cutting), Injury and Weed Control prior to Second Cutting with Fall, Spring, and Summer Applications of Valor (Flumioxazin), Spartan (Sulfentrazone), Prowl (Pendimethalin), and Sinbar (Terbacil). Applic. Hay Oil Mint Weed Control Herbicide Rate Timing Yield Yield Injury BYG PW ----9/16/ /06/ /06/ (lb ai/a) (T/A) (lb/a) (%) ----(%) Valor Fall Valor 0.25 Fall Valor Fall + Spring Valor Spring Valor 0.25 Spring Prowl/Valor 1.5/0.125 Spring + Summer Spartan 0.19 Fall Spartan Fall + Spring Spartan 0.19 Spring Prowl/Spartan 1.5/0.19 Spring + Summer Prowl 3.3 EC Fall + Summer Prowl 3.8 H2O Fall + Summer Prowl 3.3 EC 1.5 Spring Sinbar 0.75 Fall Sinbar 0.5 Spring Weedy Check

6 LSD (P=0.05) N.S. N.S. N.S MorAct COC added at 1% (v/v) to treatments 1-6 and Gramoxone at 0.5 lb ai/a plus R-11 nonionic surfactant added at 0.5% (v/v) to all Prowl spring treatments. Fall, Spring, and Summer treatments applied November 30, 2001, March 4, and July 17, 2002, respectively.