Peppermint and Spearmint in Wyoming Can Survive the First Fall Frosts

Peppermint (Mentha x piperita L.) is grown mainly in the northwestern United States, while spearmints ('Native' spearmint, Mentha spicata L. and 'Scotch' spearmint, M. gracilis Sole.) are mainly grown in the Midwest. There is no information on how these crops would yield under the Wyoming short growing season conditions. Field experiments were conducted in 2011 to evaluate the effect of harvest time on peppermint  'Black Mitcham' and 'Native' spearmint oil content and yield. The Native spearmint plots were harvested every week commencing on September 14 and finishing on November 1. Whereas, the peppermint plots were harvested on September 20, September 28, and October 5, 2011, (peppermint plants were damaged and later killed by subsequent frosts). The last two harvests yielded reduced oil content and oil yield of ‘Native’ spearmint relative to the first two harvests. The concentration of carvone in spearmint was also reduced during the last two harvests. Harvest time did not change oil content or oil yields of peppermint. However, later harvests of peppermint resulted in greater accumulation of menthol in the oil relative to the first harvest. Overall, the concentration of menthol in peppermint oil was relatively high, and much more than menthol in peppermint grown in more southern latitudes, such as in Mississippi, as recently reported. This result supports the notion that for best oil quality, peppermint should be grown in more northern latitudes. Our preliminary results indicate that both peppermint and spearmint could be grown under the environmental conditions of north central Wyoming. Peppermint may be harvested after the first light frosts, but will be killed by late fall frosts. Native spearmint was much more resistant to frosts, it can be harvested after few fall frosts and just before the first snow.