by Charlene Burgi
The air temperatures in Marin have been pushing close to and into the 90s for more than a week now. The urge to increase our irrigation times seems natural to compensate for the heat, yet the Weekly Watering Schedule is saying to water less now than was suggested at the beginning of May. At that time, the evapotranspiration (ET) was 1.57 inches with a 106% Watering Index, compared to 0.97 inch with a 66% Watering Index this week.
You might ask why plants need more water in cooler spring than when the temperatures soar in early fall. The truth is air temperature only plays a small role in determining how much water our plants need.
But first, some background on how we determine the Weekly Watering Schedule: The California Irrigation Management Information System (CIMIS) is a network of weather stations throughout California. Our CIMIS weather station is located near Point San Pedro Road in San Rafael and collects data points throughout the day that determine plant irrigation needs. Besides recording air temperature, our CIMIS station has instruments that measure soil temperature, hours of solar radiation, wind speed and direction, humidity level, and rainfall. We use all the collected data to calculate how much water is evaporating from the soil and transpiring from plants. Those results are transposed into inches. This tells us how many inches of lost water we need to replace with irrigation to maintain the optimal health for our gardens.
The question still remains why we irrigated more in late spring/early summer than we are now in the heat of fall. By examining the data collected by CIMIS, you will find that solar radiation is the key difference. Daylight hours play a critical role in a plant’s growing conditions and cycle of growth. In short: Even with the warm autumn weather we’ve been having, shorter days mean your plants don’t need as much water this time of year. (Without further investigation, I suspect daylight hours play a role in our animals’ cycle of winter survival as well; I notice the horses’ and donkeys’ sleek coats are beginning to come in heavier right now, despite the continuing warm weather.)
If you are following the suggested irrigation runtimes but find that some of your plants are wilting during the fall heat spells, check the soil moisture level. Oft times, the soil has adequate water available to the plant, but the plant cannot draw the water up to the leaf fast enough (usually found in very large leaf plants). In this situation the plant will droop but recover by the following morning.
If you are uncertain how much water your plant is receiving, you can experiment by placing an empty low can (such as a tuna can) near your plant before you irrigate. After the station is complete, measure the amount of water in the can and multiply that times the number of days you irrigate in that area each week. Check that number with the given ET loss per week to see if you are replacing what the plant loses. For example, if the can collected 0.25 inch and you irrigated three times a week, you are replacing 0.75 inch. Last week the ET was 0.91 inch, so if you only applied 0.75 inch last week you are not applying enough water to sustain the health of the plant.
To really do this test correctly, it would be best to distribute several cans throughout the area being irrigated and total the number of inches in the cans and divide that number of inches by the number of cans you have placed out. That test is known as distribution uniformity and is getting into a whole different topic for next week’s blog!
Have a great weekend, and stay hydrated if you are working outdoors.