Peppermint productivity and oil composition as a function of nitrogen, growth stage, and harvest time
The commercial production of peppermint (Mentha x piperita L.) is concentrated in more northern latitudes worldwide (north of the 41st parallel), including the United States. This 2-yr field study in Mississippi evaluated the effect of N (0, 80, and 160 kg/ha), growth stage (bud formation and flowering), and harvest time or cut (first cut in mid-July, second cut beginning of October) on peppermint yields, oil content, and composition. Biomass and oil yields were higher from the first cut than from the second. Overall, N increased biomass and oil yields. Contrary to literature reports that peppermint requires long days north of the 41st parallel to reach flowering, peppermint in Mississippi (at 34°43'22' N lat) did reach flowering. The average oil yields at bud formation and at flowering were 165 and 122 kg/ha, respectively, and were greater than the average peppermint essential oil yields for the United States in 2008. Generally, (–)-menthol concentration in the oil from the 2007 harvest was lower than in the oil from the 2008 harvest. The average (–)-menthol concentration in the oil from the fertilized plots harvested at flowering in 2008 was 43 to 46%, but (–)-menthol in the other treatments was below 37%. Our results suggest the first harvest in Mississippi should be delayed until the end of July to promote conversion of (–)-menthone to (–)-menthol. Peppermint could provide two harvests per growing season under the Mississippi climate, with oil yields and composition similar to those from other peppermint production regions.