JoAnne Skelly: Fall lawn watering

A reader recently asked me, “How much do I water my lawn at this time of year?” It depends. Influencing factors to consider include: temperature, wind, solar radiation, humidity, age of the lawn, soil type and method of irrigation.

When it is hot, dry, sunny and windy, plants, including grass, use more water than when it is cool, humid, overcast and still. Newly installed lawns need more water than established lawns in order to develop strong, deep roots, which an older lawn already should have. Clay soils take longer to absorb water, but once it sinks in, they have a better water-holding capacity than a sandy soil. In a sandy soil, water goes in and moves through the soil quickly, leaving little behind for roots to access. To water a clay soil efficiently, it is best to water in shorter, but frequent watering cycles, rather than all at once. This will allow the water to penetrate and move through the soil rather than sit on the surface. Sandy soils require the addition of organic matter to hold water.

Sometimes when people ask me the reader’s question, they expect me to tell them how long to run their sprinkler system. Actual watering needs vary according to water pressure, applied gallons per minute, nozzle size, coverage and even slope. I don’t think people realize that timing irrigation is a scientific process determined by not only these variables but also by individual plant characteristics.

“Evapotranspiration (ET) is a term used to describe the water consumed by plants over a period of time.” “Evaporation occurs when water changes to vapor on either soil or plant surfaces. Transpiration refers to the water lost through the leaves of plants” (https://coagmet.colostate.edu/extended_etr_about.php).

For maximum water use efficiency, you can set your sprinklers’ timer based on weather data, which allows you to water only as much as a plant, in this case – grass, has used in a day or week.

The Washoe Evapotranspiration Project (https://wrcc.dri.edu/washoeEt/index.html) provides daily water use statistics that can help you set your irrigation timers for what the lawn needs. By doing the ‘Can Test’ you can measure how much water your sprinklers put out and set your timer accordingly. You can find out how to do the test in the “All Seeing, All Knowing Lawn Care Manual” at https://wrcc.dri.edu/washoeEt/docs/sp9302.pdf. Or, call University of Nevada Cooperative Extension, 775-887-2252 in Carson City and 775-782-9960 in Douglas County for them to print you a copy.

JoAnne Skelly is associate professor and Extension educator emerita with University of Nevada Cooperative Extension, skellyj@unce.unr.edu