Irrigation of Peppermint for Optimal Yield

Little field research has documented the water use of peppermint ( Mentha piperita L.), an important irrigated crop of the Pacific Northwest grown for oil and tea leaves. Our objective was to determine a peppermint yield—water use function and to develop a crop coefficient (Kc) to relate crop evapotranspiration (ET) to potential ET. Trials were conducted from 1992 to 1994 at Madras, OR on a Madras loam mesic Aridic Argixeroll (fine‐loamy, mixed, mesic Xerollic Duriargid). A line‐source sprinkler irrigation system was used to impose five irrigation treatments ranging from 30 to 120% of the ET required for maximum yield. Evapotranspiration was measured by monitoring the water balance components of irrigation, precipitation, drainage, and soil water storage. Peppermint yield was sensitive to irrigation, with both deficit and excess irrigation lowering oil yields compared with the optimum treatment, which was 324, 388, 364, and 487 mm season ‐1 for the four trials. Peppermint ET‐yield functions exhibited a greater transpiration component (89%) than other crops due to its thick canopy that reduces soil evaporation. The highest yielding irrigation treatment kept the soil water pressure between −18 and −50 kPa in the top 0.3 m. The Kc followed the general three‐lines model with an initial value of 0.3 that increased to 0.85 at mid‐season and declined to 0.5 at harvest. Thermal units were used to normalize the inflection points for the Kc curve for 2 yr of the trial (1993 and 1994). Late winter tillage in 1992 delayed peppermint growth and shifted the Kc curve.