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.

Take a look at the chart for a moment. Notice the seasonal changes to ET. During August the ET, or inches of water needed, was just more than six. If six inches of water is being applied to these plants during August, they will be just right; healthy and happy. Any more water is wasteful, perhaps even detrimental to plant health.

Now notice that in October the inches of water needed is just over three inches; about half of what was needed just sixty days earlier. Wow! During November, December, January and February water needs are even less. Automatic sprinkler timers, of course, are slaves to a calendar; ignorant to a plants need for water.

But there’s more. During summer, all of the water going onto a landscape is from irrigation. During the winter, irrigation water is supplemented by rainfall. In fact, in a normal year, rainfall, will provide a majority of the water used by plants during the coolest months of the year. Why are our sprinkler systems going on so much?

I hope you’re not confused, because this is a very important lesson in plant health, as well as water conservation.

Try to follow me: Six inches of irrigation water in August may be as little as one inch in November. Look at the chart; plants only need 2 1/2 inches of water. Historically, rainfall averages almost 1 1/2 inches, leaving an inch for your sprinklers to provide.

With such a small amount of irrigation water needed, it just makes sense to turn your system off completely and only turn it on manually every now and then, depending upon the weather, or ET.

The Municipal Water District of Southern California is asking us to save 20 gallons per day. Consider that local home landscapes use about 500 gallons of water per application and many of us still have our system set to turn on twice a week, maybe more.

As you can see, by understanding a plants real need for water, using the ET data on this chart and a little common sense, saving 20 gallons a day is a breeze. And our plants will be healthier as well.

ASK RON

Question: I enjoyed your column on fluoridated water and plants. With my dracaena and other indoor plants, should I use bottled water?

Wendy

Newport Beach

Answer: Probably not, since bottled drinking water often also contains fluoride. Distilled water is better on sensitive indoor plants. Distilled water contains almost no fluoride, chlorine or dissolved salts. But be sure to fertilize your potted indoor plants to provide them with their nutritional needs.

ASK RON your toughest gardening questions, and the expert nursery staff at Roger’s Gardens will come up with an answer. Please include your name, phone number and city, and limit queries to 30 words or fewer. E-mail stumpthegardener@rogers gardens.com, or write to Plant Talk at Roger’s Gardens, 2301 San Joaquin Hills Road, Corona del Mar, CA 92625.

RON VANDERHOFF is the Nursery Manager at Roger’s Gardens in Corona del Mar