A: The best time to water is early morning, since that is when plants are “awakened” by the sun and begin making their own food in a process known as photosynthesis. Plants photosynthesize when there is light, which opens their leaf pores, known as stomates. When the stomates are open, carbon dioxide is absorbed from the air. This carbon dioxide then combines with the hydrogen in water (taken up by roots) to form sugar, which is a plant’s only true source of energy and nutrition. What we purchase at the nursery under the label “plant food” is actually a mineral supplement that allows the plant’s own food-making process to take place.
Stress reduces the efficiency of all biological and biochemical processes, and photosynthesis is no exception. As the day progresses, a plant faces intensifying heat and wind. These weather variables increase plant stress and make photosynthesis a laborious process. Early in the morning, when the temperature is cool and the air is still, photosynthesis (or plant-food manufacturing) can go forward at a steady pace.
Watering in the evening does nothing for a plant or lawn until the following morning, when the moisture in the ground can be taken up by roots and utilized in photosynthesis. In the meantime, all the problems associated with standing water come into play, especially the proliferation of certain disease organisms. Evening watering can be lethal to California natives and other drought-tolerant plants such as rosemary, lavender and cactus of every description.
Except in highly sandy soil, spray sprinklers should be left on for no more than five minutes at a time. After five minutes, runoff occurs, and you will just be watering the sidewalk. A lawn, however, will require up to 10 minutes of spray sprinkling to replace the water lost during the previous day. The way to get your 10 minutes of water with minimal runoff is to divide the watering time into two five-minute periods, say at 5 a.m. and 9 a.m. In the interests of efficiency, it is highly recommended that you automate your watering system.
In hot weather, lawns will require 10 minutes of daily watering, with spray sprinklers, five or six days a week. If you have slow-moving rotary sprinklers, which cover more ground than spray heads but apply water at a slower rate, leave them on for 15 minutes at a time. Fifteen minutes of slow-moving rotary sprinkler water delivery are roughly equivalent to five minutes of spray-head delivery.
Annuals and vegetables require roughly the same amount of water as lawns. Flowering perennials vary widely in their water requirement, depending on habitat, but require ample water no more than six months out of the year. Ground covers also vary widely but, generally speaking, require about one-eighth the amount of water needed by a lawn.
TIP OF THE WEEK: The worst mistake made when the weather warms is to significantly increase water allotment to plants that originate in the dry regions of Australia, South Africa, the Mediterranean or our own back yard. Plants native to these areas should not require more than a single weekly soaking throughout the year. Iceplant, gazania, geranium, perennial herbs such as rosemary and lavender, and many types of sages and bulb plants are in greater danger of contracting fungus diseases from excess water than they are of dying from thirst.