Summer Session: Watering 101
Previously published in the Davis Enterprise
Drooping, wilting plants?
Sprinklers set to come on daily, for five minutes?
Drip-system watering for 15 minutes?
There's a connection!
Think back to how Dad watered, when you were a kid. Every weekend he moved a sprinkler around the yard: once a week, an hour each spot. With the advent of automatic sprinkler timers, unfortunately, water goes everywhere, every day, and plants don't get what they need.
After some extremely hot weather, nursery professionals and master gardeners see a steady stream of plant samples suffering from "summer leaf scorch," usually caused by improper watering during hot weather. The accompanying question can be predicted:
"How should I water my plants in the summer?"
There's no simple answer, but most folks water too often, and not long enough. Few plants need water daily, except maybe those in small pots during hot weather.
How often? How long?
That depends on the watering mechanism, because systems put out water at dramatically differing rates. Symptoms of overwatering and underwatering can be very similar: yellowing or scorched leaves, or wilting, dropping leaves. "Overwatering" actually is watering too often. You really can't apply "too much" water at any one time -- up to a point -- but you can cause root rot if you water too frequently and keep the soil surface wet all the time. "Underwatering" can mean applying too little water at a time (common with drip irrigation systems), or not watering often enough (very uncommon).
How much water do plants use?
To figure out how much water to apply, scientists refer to the "ET rate," or the evapotranspiration rate, measured in inches (as is rainfall). The ET rate varies by temperature, wind, and humidity, and tells us how many gallons of water are being used by the plant each day. (For ET readings, go to the UC Davis IPM web site for readings from California weather stations.). We then adjust the amount by a factor (the landscape coefficient) that is based on how drought-tolerant the species is. (For more information about landscape coefficients, visit the Water Use Classification of Landscape Species page.)
Wind is a significant factor. Dry wind evaporates water from the plant leaves quickly. ET rates during windy weather may exceed ET rates during hot, still weather.
From May through September, the ET rate is about 1/5 to 1/4 of an inch of water every day in the Sacramento Valley. The number of gallons that equals per week depends on the size and type of the plant, or the area of the lawn or border:
Lawn or border, 1000 sq. ft.: 800 - 1,000 gallons.
Large trees may use 500 to 1,000 gallons or more of water in a week, but their roots go deep in this area and use water stored in our soil from winter rains. A deep, slow overnight soaking every few weeks may be all you need to keep them happy.
Small shrub or perennial: 5 - 8 gallons per week.
Medium shrub: 15 - 25 gallons
Large shrub: 30 - 50 gallons
Small tree, 10 feet across: 50 - 80 gallons
Medium tree, 20 feet across: 250 - 350 gallons
Note: landscapes carefully designed for very low water use, using plants native to arid regions, can be watered as much as 50% less than the amounts shown, and in some cases can go without summer irrigation at all.
How often should I water?
Remember, you don't need to water daily to replace the water used by established plants, because the soil stores water, and clay soils store it very effectively. A slow, deep soaking can provide landscape shrubs, perennials, and small trees enough water for an entire week.
Even newly planted plants can go three or four days between good soakings, because the soil stores water. Just be sure to water slowly and thoroughly. If you're using a hose at a moderate flow, set it down and walk away. Do something else for a few minutes. You probably don't have the patience to stand there long enough to give a new plant a good drink!
Lawn roots are shallower -- except with Bermuda grass! -- and only go deep enough to retrieve water stored for three or four days, so lawns typically need to be given one inch of water twice a week. Light, daily waterings lead to shallower root systems and encourage fungus diseases.
How long should I water?
The trick is to apply the water slowly enough so that it soaks in, rather than running off. Soils that contain clay absorb water slowly, about 1/4 inch per hour. A hose running at full throttle puts out about eight to 10 gallons per minute (gpm), which is way too fast for the soil to absorb in one spot. The water runs off unless redistributed over larger areas, by running it through a sprinkler on a hose, or via a sprinkler or drip system.
Drip systems, in particular, must run a long time to give a good deep soaking. An emitter on a drip system puts out one to four gallons per hour (gph) onto a very small area. Drip systems need to run for several hours to give a thorough soaking. Mini-sprinklers and microsprayers have become very popular, because they're flexible and easier than sprinklers to install and repair. They put out water about twice as fast as drip systems, so they usually need to run for a few hours once a week.
Lawn sprinkler systems vary greatly in how much water they apply, but it's easy to measure an inch of water using tuna cans or cake pans. Little pop-up heads take about 30 - 45 minutes to put out one inch of water, while impulse sprinklers -- Rainbirds are the best known brand -- may need to run for a couple of hours in order to cover a large area. To avoid wasting water, it's important to measure how long your system takes to get that inch of water twice a week.
Sprinklers placed on the end of a hose also vary greatly, with respect to how much area is covered. A little spot sprinkler may cover only a 20-foot circle, putting out an inch of water in only 25 minutes, and probably running off as it does. An oscillating (also called wave) sprinkler covers a huge area, and can take as much as two hours to provide an inch of water.
If you have a water meter, it's not hard to learn how much water you use. Your city may have experts who can help reduce your water use, and nursery professionals can calculate how much water would be optimal for your landscape.
Aside from perhaps lightening the monthly water bill, your plants will thank you...and reward you with vigorous growth, flowers, fruit and shade.
© 2008 Don Shor, Redwood Barn Nursery, Inc., 1607 Fifth Street, Davis, Ca 95616
Feel free to copy and distribute this article with attribution to this author.
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