Facing a second year of drought, Napa Valley gardeners have to make some tough decisions about what to water. All plants need water, even cactus and succulents. If you have to ration water, give priority to your biggest investments.

     Focus on your garden’s most significant trees and shrubs. Even established trees need water, and since water tables are lower than usual, trees that haven’t needed water in the past may need some this summer.

    The bigger the plant, the more leaf area it has. That means it will use more water on a hot day. Temperature, humidity and wind are all important in determining how much water plants need. Water-stressed plants often have a dull look even before they wilt. Try not to let plants get water stressed because they will stop growing and may not recover. Stressed plants are also more susceptible to pests and some diseases.

     For some general guidelines on watering, consult “Wise Watering of Landscapes,” part of the Healthy Garden Tips series available free from the University of California Cooperative Extension office or from the Napa County Master Gardener website (both addresses below). 

     Most large perennial plants need soaking to a depth of three feet to wet the entire root zone. How often to water depends on the type of plant. Established fruit and nut trees and flowering perennials require deep soaking every other week, ornamental shrubs once a month and landscape trees every other month.

     This watering schedule should begin as soon as the weather warms and the soil dries out,  usually around mid May, and continue into August or September.

     How do you tell if you have watered three feet deep? One way is to dig a hole or to push a dowel or metal rod into the ground. Moist soil does not provide much resistance. When you feel resistance, you have probably reached dry soil. But you might also have hit a rock, so it’s a good idea to probe in a couple of places.

     If you are watering with a sprinkler, place a small empty can, such as a tuna can, under the sprinkler and see how long it takes to fill with one inch of water. For clay loam or loam soils, an inch of water will moisten the soil to a depth of about one foot. So if you leave the water on three times as long, you will wet the soil three feet deep—assuming all the water soaks in and does not run off or evaporate.