by Craig Lauridsen
As you probably already know, 2013 was a dry year. With a total of 10.68 inches for the calendar year, 2013 set a new record low for rainfall in Marin. The previous low was 19 inches back in 1929. So a logical question would be: If the rain is not watering my plants, how much do I need to irrigate? MMWD’s website currently states:
Please turn off your irrigation system for the season. If dry weather continues, check newly planted, container and high-water-use plants for stress and water if needed. Note that, even without rain, most plants require little water this time of year.
I would like to add a personal touch to this statement. My backyard has a small, 500-square-foot patch of 90/10 tall fescue. We get a lot of use out of this lawn: I have a black lab that loves to play fetch, and my 14-month-old toddler is quickly learning that it’s less painful to fall on grass than on the patio. Even though grass is among the highest water-use plants, usually our rains provide all the winter irrigation a lawn needs. This year, though, I started to get concerned when one dry day followed the next. However, as a water conservation specialist, my training and experience told me it’s not that low rainfall equals thirsty plants but that low evapotranspiration (ET) equals plants that aren’t very thirsty. ET is the loss of water to the atmosphere by the combined processes of evaporation (from soil and plant surfaces) and transpiration (from plant tissues). It is a good indicator of how much water your lawn, garden, and trees need to stay healthy—and in winter it tends to be pretty low.
The California Irrigation Management Information System (CIMIS) pulls data from over 120 automated weather stations throughout California and has been around for over 30 years. MMWD uses CIMIS data from the weather stations within our service area to create the Weekly Watering Schedule. CIMIS uses many variables to help users determine how much to water:
- solar radiation
- vapor pressure
- air temperature
- relative humidity
- dew point
- wind speed
- soil temperature
According to 2013 data from the Marin weather stations—and despite less than two inches of rain during December—plants needed about 80 percent less irrigation in December than during the average summer month. This information allowed me to comfortably decide to water my lawn only one day in the month of December.
I also have several other plants (lavender, rosemary, heavenly bamboo, breath of heaven, lantana, various grasses, etc.) that received zero irrigation in the month of December, all of which are doing just fine. I also have rosemary in a pot on my front porch that gets plenty of afternoon sun and starts to brown if I don’t water it one to two days per week in the summer; however, it was completely happy with only one watering in December. My brother used a few sprigs of this plant to prepare his famous garlic rosemary mashed potatoes that received lots of positive attention during our Christmas dinner.
Different species of plants handle dry and/or freezing conditions differently, so it’s important to learn about your own plants. But my earlier point still stands and is worth repeating: It’s not that low rainfall equals thirsty plants but that low evapotranspiration (ET) equals plants that aren’t very thirsty.
Another thing you can do to help manage your landscape water use is to replace your standard irrigation controller with a smart irrigation controller. MMWD is offering rebates for smart controllers, toilets, and clothes washers. Visit the rebate section of our website for more information.