Dry summer water needs
by Mary Robson, WSU Extension Agent
The Seattle area measured about 3/4 inch of rain during June. The average garden requires an inch of water a week, or about four to five inches a month. Since rain doesn't provide this during the average summer, irrigation has to make up the difference.
Large, established trees and shrubs selected for drought-tolerance won't have to be irrigated. Some plants, such as madrona trees (Arbutus menziesii), will suffer and decline if they get summer water. Don't allow lawn irrigation to sprinkle madronas. Even large, old rhododendrons can do with less than the monthly four inches.
Areas that do require summer irrigation are vegetable gardens, flower gardens, and plants that have been installed less than two years and do not yet have established root systems. Symptoms of water stress include leaf curling or rolling, droopy leaves, a slight "graying" or "silvering" of leaves.
Choosing a method of watering is crucial. Sprinklers, with the familiar summertime "thwip, thwip, thwip" sound, are thoroughly inefficient. The water sprayed up into the air, especially if applied during a sunny period, evaporates rapidly and not all of it reaches the roots of plants.
Standing with a hose and dampening the top of the soil doesn't get enough water to roots, either. If water goes on very frequently in light sprays, plant roots won't grow deep but will hang out at the surface of the soil, waiting for their sprinkle. This prevents growth of deeper, sustaining roots that take advantage of soil moisture several inches below the surface.
Soaker hoses and drip irrigation systems work well. My small garden is threaded with the hoses made of recycled tires that resemble black sponges; these hoses release water gently all along their lengths. Some gardeners bury them about three inches down, though this isn't necessary. It's also possible to stretch them around plants and along the surface of the soil without burying. They work best if the hose lines are about 16-18 inches apart along the soil surface.
Soil amended with organic matter receives and holds irrigation better. Mulchers on the soil surface also help. July is a great month to weed, stretch out irrigation hoses if they haven't been installed, and mulch over the soil. Homemade compost, bags of last fall's leaves, aged sawdust, bark chips, or even pine needles can be effective as mulches, applied about 2 inches deep. Keep mulch away from the crowns of the plant.
Keep notes of which plants seem to be located badly for irrigation needs. In fall, when transplanting, group "water needy" plants together. A lawn, for instance, will need irrigation, and the beds near it can be "wet zones." Rhododendrons, azaleas, and Japanese maples--the staples of many Northwest gardens--need summer irrigation and should be grouped. Some perennials like astilbe, primrose, ligularia, and rodgersia will die fast without enough water.
Choose plants that don't require as much water for "drier" areas. Lilacs, rugose rose, mock orange, butterfly bush, forsythia, rock roses (Cistus. sp.), cosmos, poppies, bearded iris, and most herbs are all plants that get along well on restricted summer water.
Get a free publication from Washington State University Cooperative Extension: KC-125, "Low Water Use Plants." Send your request with a self-addressed stamped (32 cents) business-size envelope to WSU Cooperative Extension, 506 Second Avenue, Room 612, Seattle, WA 98104-2394.
Visit WSU's Agriculture Site on the World-Wide Web at http://www.cahe.wsu.edu.