Posted on: Friday, November 18, 2005
Healthy lawn begins with good irrigation
By Jay Deputy
Watering may seem the most basic part of keeping your grass green, yet it is one of the most abused and least understood aspects of lawn care. Improper watering is wasteful, costly and can damage the landscape. Plus, as Hawai'i continues to grow and develop, water use for landscape is increasingly restricted. Here are watering tips to keep your lawn and our water supply healthy.
The best time to water is early morning, an hour or two before sunrise, if possible. That's when it's cool, water pressure is usually higher and the wind is low, which means water is distributed more evenly and evaporation loss is less. If you water in the middle of the day, more than half of the water can blow away or evaporate. Late-evening or night watering is not recommended because the grass stays wet too long, which encourages fungal and bacterial diseases, particularly in areas that stay shady during the day.
How often? Take into account seasonal variations in weather and plant growth rates. Hawai'i winters are wetter than summers, and most turf grasses require less water in winter because they grow more slowly. We are entering those winter months, so cut back now.
Soil type is important. Roots grow in the top 12 to 18 inches of soil. Called the root zone, this area is the plants' only source of available water. Hawai'i's soils are usually clay, with sandy soils more common close to the beach. Clay soils can hold as much as 4 inches of water in the root zone, while sandy soils hold about 1 inch. Generally, Hawai'i turf grasses require about 2 inches of water a week in the summer and 1 inch a week in the winter. Sandy soils will dry out more rapidly and require more frequent irrigation. Without rain, a sandy soil may need watering every other day in the summer and a clay soil only every three or four days.
Determining how much water to spray on your lawn is essential. One irrigation period should deliver 1 inch of water onto a sandy soil and 2 inches onto a clay soil. One inch is equivalent to 620 gallons of irrigation water for every 1,000 square feet of lawn. The type of irrigation system will determine how long you will need to irrigate. An in-ground automatic system is usually divided into separate sections called zones. Each zone consists of a group of sprinklers that come on at the same time. An average zone of sprinklers will deliver about 15 gallons per minute over 1,000 square feet. The zone must therefore stay on for 40 minutes to deliver one inch of water. A single hose-end sprinkler will deliver about 4 gallons per minute over a much smaller area and will need to be moved around often enough to prevent runoff.
No matter what kind of irrigation system you use, runoff occurs when water is supplied faster than the soil can absorb it. This is wasteful and can carry away polluting fertilizer and pesticides. Runoff is more common with clay soils because of the slower rate of water absorption. You can control runoff by changing the type of sprinkler heads to reduce either the rate of application, or the length of time that the system operates. It may be necessary to irrigate a lawn for several short periods during the day to allow sufficient time for the water to infiltrate into a clay soil without runoff.
An irrigation session should wet only the root zone. The worst irrigation program, however, is to water lawns daily for 5 to 10 minutes, particularly clay soils. This wets only a few inches of soil and results in a shallow root zone, reducing the amount of water-storage soil. The grass depends on the daily drink and wilts or turns brown quickly if it doesn't get it. On the other hand, overwatering can saturate the root zone, resulting in water draining through it and being lost.
Jay Deputy is an education specialist in landscape horticulture and turf at the University of Hawai'i-Manoa, Department of Tropical Plant and Soil Sciences, and state administrator for the Certified Landscape Technician Program sponsored by the Landscape Industry Council of Hawaii. Send lawn care or turf questions to deputy@hawaii.edu.
