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Guide for Monthly Irrigation Schedule Adjustment

Reprinted from an article I wrote for the Contra Costa County UC Master Gardener newsletter in June 2021.

by Lori Palmquist

Since we are officially in drought again (August 2021), and the water companies are asking us to cut back, I have some suggestions for reducing your water use in the landscape.

I’ll bet you’ve heard about adjusting your controller’s watering schedule seasonally. It makes perfect sense. Winter is when we get rain in the Bay Area. So our controllers are off in the winter. In the spring, it rains a little. In summer, it’s hot, and we don’t get any rain. In fall, it’s still hot, and the rain starts mid- to late fall (usually November).

But I’ll bet you didn’t know that the amount of water plants require is different enough from month to month that we should be adjusting our irrigation schedules monthly, instead of seasonally.

Let’s take Walnut Creek as an example. Lawns in Walnut Creek require 6 inches of water in July, 5 inches in August, 3.6 inches in September, and 2.6 inches in October. The difference in amount of water required between July and October is a whopping 56%!

Standard behavior in irrigation scheduling I usually see is that people enter their July “hotter than Hades” irrigation schedule in July, and leave it in place all the way through October. Sure, the temperatures remain high July through October in the Bay Area. But after July, plant water-needs drop significantly, regardless of the temperature. This is mainly due to shorter days and a lower angle of the sun.

The factor that is most influential on plant water needs is solar radiation or sunlight. Leaving the summer schedule in place through October wastes a great deal of water.

The graph below shows the monthly water requirement in blue for lawns in Walnut Creek. The red bars reflect common irrigation scheduling practices. You can see that in August through October, there is significant water waste that happens due to this practice. For every 1,000 square feet of lawn, almost 4,000 gallons are wasted during those three months in Walnut Creek. And this would be for just one property.