Understanding the water needs of plants in the summer
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Karen Gideon
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Plants are a lot like people. As the summer weather warms and days grow longer, a plant's need for water increases and they sweat a bit more, just as we do. Only they draw the water from the soil with their roots and it travels up to the rest of the plant.
Plants sweat through a process called transpiration — the loss of water primarily from pores on their leaves, but also from stems and flowers. Transpiration serves to cool plants as escaping water vapor carries away heat energy.
While this need for increased water is a gradual build up from spring to mid-July, it's not unusual for homeowners to turn on their irrigation systems in late March. They let it run
until October, most knowing at some level that the plants need more water in the summer, but are unable to gauge just how much.
When you consider the true, incremental rate of increase in water needs, this blast of irrigation is overkill. Since outdoor water use accounts for more than half of all water use in Marin, it adds up to a monumental waste of water.
Smart homeowners adjust the amount of water based on what plants really need and make sure the water-wise requirements of the plants are being met. This can be helped by the use of smart irrigation controllers and rain shut-off valves.
How do we know just how much water plants need during our dry period? Scientists call this balance of water intake and transpiration "Reference Evapotranspiration" or ETo. They evaluate geographical areas based on temperature, relative humidity, air movement and soil type and label them by ETo zones. Marin has diverse microclimates, areas that are hot and dry and areas that are foggy and moist. It's challenging for homeowners to understand their home zone. For this purpose, Marin Municipal Water District has simplified the ETo zones into three areas monitored by high-tech weather stations labeled Northern, Central and Southern.
The MMWD website at marinwater.org provides an ETo shortcut for Marin homeowners. Once you're in the MMWD website, click on the "Conservation" tab, click on "For Your Landscape" on the left, and then click on "Weekly Watering Schedule." The three zones are listed with the number of minutes of irrigation that plants require based on temperature, prevailing conditions and solar radiation. The water district also provides weekly email updates and tweets to homeowners to let them know how long they should be watering their plants based on the current climate conditions.
The amount of watering is critical, but the cultural care for your plants also will influence how much water they have available in the summer. Cultural care can include the soil, wind, evaporation, mulch and hydro zones.
Spend time in your garden and watch your plants for signs of water stress. Unfortunately, the signs of overwatering and dehydration can be similar. Look for browning, limp and drooping leaves, and slow growth. If you see your plants are showing signs of stress, it's time to run down the checklist above and make sure you have the cultural requirements in order. Then determine how much water your irrigation system is delivering to the plants.
As the hot weather subsides in the fall and the days grow shorter, plants need less water, just like people. You can decrease the amount of water they receive through your irrigation system until the first rains fall and maintain soil conditions that promote water retention. And keep your eye on the MMWD website; it will tell you when to stop irrigating in the winter. You can also arrange to have a Marin Master Gardener evaluate your garden and suggest water-wise tips for conservation by calling 473-4204 to schedule a free visit.