The Coastal Gardener:

On Oct. 8 at 11 a.m., at the offices of The Metropolitan Water District of Southern California, gardeners were put on notice that the current water shortage may lead to cutbacks, higher water rates and more. Every homeowner was asked to reduce their water use by 20 gallons per day. Sounds like tough times for gardeners.

It’s not. Saving water is easy — really! Just turn your sprinkler system off for the next five or six months.

Having talked to thousands of homeowners over the past 25 years, I am convinced that gardeners waste far more water during the cool half of the year than the warm half. Our gardens need drastically less water during the cool months of the year. Since all of our irrigation water in southern California is stored water, we all draw upon the same supply during the winter as we do during the summer. Conservation in the winter helps in the summer.

Scientists measure the water use of a plant with a term called evapotranspiration. Let’s call it ET. ET is the loss of water from a plant through evaporation (from the plant and the soil) and transpiration (from plant tissues, mostly leaves). I don’t want to get too complicated, so let’s just say that ET is how much water your plants need for healthy growth.

ET is measured in inches of water; the same way that rainfall and irrigation water is measured. ET, or use of water by a plant, changes constantly. As you might expect, temperature, sunlight, humidity, wind, day length and other factors change throughout the year. As these factors change, a plants use of water changes accordingly. Of course, you’re right if you assume a plants use of water, or ET, would be higher in summer than in winter. A plant uses more water on a hot day than on a cool day. ET is higher in summer than winter, higher on a hot, dry, windy day than a calm, cool, humid day.

Agriculture and horticulture professionals in Orange County have been calculating and plotting local ET for years. Farmers rely on this data to know when to irrigate. The graph here shows Orange County’s 10-year average ET. Remember, ET is the baseline for how much water plants need.

Homeowners, for the most part, set their automatic sprinklers and forget about them. Of course, ET is changing continuously; automatic sprinklers are not.