Effect of irrigation and harvest timing on peppermint oil yield in California

The effect of delay of onset of spring irrigation and harvest date of peppermint (Mentha piperita L. cv black mitcham) grown in Western Lassen County was studied in 2003 and 2004. We measured applied water in sprinkler-irrigated plots with catch cans and related applied water and rainfall to local CIMIS reference ETo. Highest oil yields of 118.9 kg ha−1 in 2003 and 119.2 kg ha−1 in 2004 occurred for harvest dates of July 29 and August 13, respectively. Date of onset of irrigation had no impact the first year, but did cause a reduction the second year in relation to length of delay. Soil water sensors at the 152 and 305 mm depth provided similar results in measuring water potential in response to crop use and water application by both rainfall and irrigation. Yield reductions were strongly associated with the time intervals during which soil water potentials were less than −20 kPa and the sum of total irrigation + rainfall was less than reference ETo. Measurement of canopy temperatures and plant height were useful only as indicators of severe crop water stress. Both menthol and menthofuran increased steadily with date of harvest. Menthol levels of 42–44% were associated with highest yields. Pulegone peaked with the mid-August harvest in both years.