Chemical Composition of Mentha arvensis var. piperascens and Four Hybrids with Mentha crispa Harvested at Different Times in Indiana and Michigan 1

Unlike Mentha piperita , the oil from juvenile and immature herbage of Mentha arvensis and four M. arvensis ✕ Mentha crispa F 1 hybrids may not have substantially higher amounts of menthone and lower amounts of menthol/menthyl acetate than oil from mature herbage. In contrast, oil from very overmature and senescent herbage usually has more menthyl acetate (ester) without marked reduction of menthol (alcohol) content. Flower oil has as much menthol as total herbage oil but less ester. Oil differences due to the time of harvest are minor compared to those due to inherited differences. In comparison to the cultivar, each illustrated hybrid has a distinctive characteristic, namely, higher isomethone, piperitone, menthyl acetate, or neomenthol with these differences present in juvenile, mature, and overmature herbage. Biogenetic reasons are given to explain the absence of major ontogenetic trends in limonene/menthone/menthol, but the colder temperatures of early spring or late fall are believed to have a direct effect on increasing the amount or efficiency of the enzyme that converts menthol to menthyl acetate. Low temperature‐induced increases in menthol and ester may occur in 3 to 6 days in juvenile plants. From an agricultural viewpoint, harvest should be at time of maximum oil yield and prior to severe leaf loss due to Puccinia rust or Verticillium wilt.