Growth and Physiology of Six Horticultural Crops Irrigated with Municipal Reclaimed Water

Reclaimed water from the South Treatment Plant in Renton, King County, Washington can be a valuable source to supply irrigation water for landscape plants and urban farming in the area during the summer dry season. Currently most of the treated water at the Renton Plant is discharged into Puget Sound while only a limited portion of the water is treated to meet Class A reclaimed water standards by sand filtration and used for turf irrigation. Potentially important users of reclaimed water in the Seattle area and King County are small-scale urban farms, commercial greenhouses, and nurseries for production of a range of ornamental and vegetable crops. Since irrigation water is limited in many areas in King County, reclaimed water may prove to be a key factor in maintaining the viability of these operations.  Although the use of reclaimed water for irrigation is commonly practiced in many parts of the country as well as overseas, each area is likely to have unique concerns and management requirements that need to be addressed for efficient and safe use of this resource.  In addition to public and environmental health concerns, there are horticultural questions that need to be addressed such as how to integrate the use of reclaimed water with existing fertilization practices, potential impacts of reclaimed water on plant growth and soil properties, and potential salinity issues. The objective of this study was to evaluate plant growth and physiological responses of three ornamental (Amaranthus hypochondriacus cv. Burgundy, Helianthus annuus cv. SunSpot, Delphinium grandiflorum cv. Blue Mirror) and three vegetable crops (Lactuca sativa, Daucus carota subsp. sativus, Fragaria ananassa) to irrigation using reclaimed water from the Renton Plant. Compared to control (tab water only) and fully fertilized (using half strength Hoagland solution) plants, all six crops showed moderate growth and photosynthetic capacity when grown with reclaimed water. This indicates that there are limited nutrients available for crops to use in reclaimed water. The reduction in growth and photosynthetic capacity in plants irrigated with reclaimed water compared to the fully fertilized plants appear to be a result of limited nitrogen availability. Based on these results, a recommendation may be made to practice reduced fertilization in urban farming and horticultural production when using reclaimed water from the Renton Plant.