Plants’ irrigation needs vary by season

Plants’ irrigation needs vary by season

Vincent Lazaneo. Special to the U-T, Oct. 24. 2014. California.

The plants need less water in winter than in summer, but people often water their landscapes the same year-round. An irrigation system that applies enough water for plants in midsummer may apply twice as much or more water than the same plants need in midwinter. There is a transition from lower to higher Evapotranspiration in spring and from higher to lower ET in fall. Adjusting your irrigation time clock in October could save a lot of water through winter and spring. Adjusting it again in mid- to late spring will give plants enough water during the hotter summer months. Can be seen calculated evapotranspiration (ET) for each month of the year at coastal and inland locations in the county.

We are in a drought, and adjusting your irrigation time clock now is an easy way to save water. It may seem obvious that plants need less water in winter than in summer, but people often water their landscapes the same year-round. An irrigation system that applies enough water for plants in midsummer may apply twice as much or more water than the same plants need in midwinter.

The seasonal difference in water use by a well-irrigated, cool-season lawn like tall fescue can be seen by calculating evapotranspiration (ET) for each month of the year at coastal and inland locations in the county. Evaporation occurs when water is lost from moist soil, pavement and other surfaces. Transpiration occurs when plants lose water vapor from their foliage through tiny pores. Monthly ET is lowest in midwinter when we have the shortest days and coolest temperatures, and it peaks in midsummer when we have the longest days and warmest temperatures. In midwinter, ET is almost the same (about 2.0 inches) at coastal locations like San Diego, Chula Vista and Oceanside as it is in inland locations like Escondido, Santee and Ramona.

Monthly ET increases dramatically in summer and is higher as you go inland from the coast: San Diego 5.1 inches, Escondido 6.8 and Ramona 7.3. There is a transition from lower to higher ET in spring and from higher to lower ET in fall. Adjusting your irrigation time clock in October could save a lot of water through winter and spring. Adjusting it again in mid- to late spring will give plants enough water during the hotter summer months. How long an irrigation system should run to apply enough water for plants at different times of the year depends on the type of plant being grown, the type of irrigation head (sprinkler, bubbler, drip, etc.) the type of soil and other factors.

You can create a personalized irrigation schedule by visiting www.sandiego.gov/water. On the water utility home page, select the water conservation tab then click on Landscape Watering Calculator near the bottom of the page. It will ask you to enter some information: your ZIP code, type of plant, soil type and type of watering system. For a more accurate schedule, you could also enter your water application rate if known, or view information on how to measure it. The calculator will then tell you the maximum number of minutes to run your system and the number of days per week to water for each month of the year. When your time clock is reset and the system has operated for a couple of weeks, you can check to see if it is applying too little or too much water by digging down in the soil near plants being watered. You can then adjust your time clock if need, to apply less or more water at each irrigation and adjust how often it runs.

Plant roots should not be in either saturated soil or dry soil for too long. During irrigation, enough water should be applied to wet the plant’s entire root zone. The depth roots penetrate is about 6 to 12 inches for turf grass and herbaceous perennials, 1 to 2 feet for small shrubs and trees and 2 to 3 feet for large shrubs and trees. After irrigation do not water again until soil near the surface begins to dry. This allows oxygen, which roots need, to enter the soil. The frequency of irrigation can vary from as often as every few days during summer for shallow-rooted plants to as little as once or twice a month during winter for deep-rooted plants. If adequate rainfall miraculously occurs, you can turn your sprinklers off for a while.

Vincent Lazaneo is an urban horticulture adviser emeritus with the University of California Cooperative Extension.