Most of us could use a little help in determining how much to water our gardens during a drought—both to conserve water and, if possible, save the trees, shrubs and other plants we cherish. Here are a few guidelines and tools to help you determine watering needs with the drought in mind.
Watch for Plant Stress A quick, easy way to ensure plants are getting just enough water to survive is to check them visually before irrigating, rather than relying on preset timers or a fixed watering schedule. For established plantings, the rule of thumb is to wait until you see the first sign of plant stress: typically, slightly wilted leaves (the operative word here is “slightly”). This is a simple, effective method to use in mature gardens and landscapes. The “stress test” is not a good method to determine water needs for new plants in the garden, though, even if they are billed as drought tolerant once established. Most new plants need heavier, more constant watering the first year or two to grow a strong root system.
Remember the 50-Percent Rule Many established landscape plants can survive on about half of their ideal water needs for at least one season of severe drought. But first you need to know how deeply a particular plant likes to be watered. The University of California Master Gardener Handbook offers this general guideline for determining how deeply three different classifications of plants should be watered under non-drought conditions:
Leafy vegetables and annual bedding plants: 6 inches to 1 foot
Small shrubs, cool-season turf grass, corn and tomatoes: 1 to 2 feet
Large shrubs, trees, warm-season turf grass: 1.5 to 5 feet
Using a rough guide like this, you can approximate what watering depth represents 50 percent of ideal. (The caveat is, depending on the plants or landscapes in question, plus state and local water restrictions, you may have to settle for even less than half of an ideal watering depth.)
Know How Deep it Goes Once you determine how deep to water, how do you know if your irrigation system is reaching that depth? You can buy portable soil meters. Or you can drive a long screwdriver into the soil, then pull it out and check to see how much of the tool holds moist soil. If you like to get your hands dirty—and you’d also like to learn more about your soil composition—go for the “feel test.” With a trowel or shovel, dig to roughly half the plant’s ideal moisture depth. Place a handful of soil from the bottom of the hole into your palm, then close your hand and squeeze. You’re at roughly the 50 percent depth if the characteristics of the soil in your hand match one of the following:
Sandy Loam: Still appears dry, will not form a ball with pressure
Clay Loam: A little crumbly but will hold together under pressure
Clay: Will form a ball under pressure
If rivulets of water run from the soil when squeezed, or if the soil easily ribbons out between your fingers and feels slick, you are overwatering. You should only need to test for soil-moisture depth once or twice during the growing season if your watering schedule is consistent.
Get Technical If you are mathematically inclined, there is a UC-developed system for determining irrigation rates called “The Landscape Coefficient Method.” Simply put, by using mathematical formulas that include published evapotranspiration rates by region and month, plus plant species, landscape density and microclimate factors, you can determine more precisely how much to water. The accompanying “Water Use Classification of Landscape Species (WUCOLS)” guide is an exhaustive alphabetical list of plant species that indicates whether a given plant would have low, moderate or high water needs, by region. Visit http://ucanr.edu/sites/WUCOLS.
With the right tools and practices (and the right plants), we have a better chance of saving the gardens we love.
Rachel Oppedahl is a UCCE Tuolumne County Master Gardener who is mathematically challenged, so she will be watching for wilting leaves and digging holes to manage landscape watering.