The crop water use, or evapotransporation, of most field crops is the dominant factor controlling yield in arid climates, and this is especially true for peppermint. In the dry summers of the Northwest, irrigation is necessary for peppermint to reach maturity. Inadequate irrigation reduces yield by limiting transpiration and plant metabolism. On the other hand, excess irrigation can reduce yield. For example, standing water limits the supply of oxygen to plant roots, and may promote root diseases, such as rhizoctonia. Too much water may carry plant nutrients, especially nitrogen, below the root zone. Excess water may cause the plant to lose more leaves than normal, and thus oil yield suffers because the oil concentration is reduced.

Irrigation scheduling requires knowing how much water to apply, and when to apply it. Knowledge of a soil's water-holding capacity and the crop water requirements can provide the answers. The U.S. Bureau fo Reclamation maintains an AgriMet network of weather stations throughout the Northwest that automatically collect site-specific weather data and calculate crop water use daily. To view the specific crop water use in your area, visit the U.S. Bureau of Reclamation site.

For more information on irrigating peppermint, order Publicaton EM 8662, Irrigating Peppermint, A. R. Mitchell, Oregon State University Extension Service, January 1997, and PNW Publication 288, W. L Trimmer and H. J. Hansen, Irrigation Scheduling, Oregon State University Extension Service, 1994. Order these publications.