Why do trees change color in the fall?

by Dan Carney, City of San Diego Water Department Water Conservation Program

ou've probably noticed leaves changing colors – from green to shades of yellow, purple, and red – on native trees such as sycamores, cottonwoods, and deciduous oaks. But, did you know that sunlight plays a key role in changing the leaf colors? The autumnal equinox, on September 22, marks the beginning of cooler days and longer nights in the northern hemisphere. Because the sun spends fewer hours in the sky and the light is less intense, the pigments in the leaves begin to change. The pigment chlorophyll causes the green leaf color we usually see. Because chlorophyll absorbs all of the colors in the light spectrum except green, which it reflects, we only see the green color.

    Chlorophyll requires sunlight and warmth to remain active in many deciduous trees. So when temperatures drop, so does the production of this green pigment. As a result, the other pigments that are not as dependent on light and temperature get a chance to show off their colors.

    Each type of plant has a unique mixture of pigments. Yellow leaves are caused by the pigment carotene, which absorbs blue-green and green light and reflects yellow. Another interesting group of pigments are the anthocyanins. They make the leaves look red and purple, and are the same pigments that make apples appear red and grapes purple.

    Along with changing color, plants also decrease the amount of water they need during the fall. In fact, between September 22 and December 21, an average lawn needs 25 percent less water than during the summer months. It's a great time of the year to save money on your water bill and help your plants stay healthy.

    To help you adjust your watering during the fall, the City of San Diego offers an easy-to-use online landscape-watering calculator designed to custom produce a weekly schedule that will estimate the right amount of water to give your garden. The calculator considers weather patterns, soil, plant, and irrigation types when creating the schedules. Create your watering schedule today by accessing the calculator at www.sandiego .gov/water/conservation.

    The Water Conservation Program reduces water demand through promoting or providing incentives for the installation of hardware that provides permanent water savings, and by providing services and information to help San Diegans make better decisions about water use. For more information about Water Conservation, visit www.sandiego.gov/water or call (619) 515-3500.