Summer is here, and the drier weather requires more attention to watering the lawn. Following a few basic principles of good water management can result in a more efficient use of our water supply and a healthy lawn.

The best time of day to water is in the early morning, an hour or two before sunrise if possible. Water pressure is higher and it is usually less windy so water distribution is more even. Watering in the middle of the day can result in more than half of the water either blowing away or evaporating. Late evening or nighttime watering can lead to the grass staying wet too long, which may encourage fungal and bacterial diseases.

Deciding how often to water should take into account seasonal variations in weather and plant growth rates. Hawai'i winters are wetter than summers, and in addition, most turf grasses require less water in winter because they grow more slowly. Plant roots grow in the top 12 to 18 inches of soil. This area is called the root zone and is the plant's only source of available water. Each watering session should wet the root zone. Hawai'i's soils are either clay or sand. Sandy soils hold about one inch of plant-available water in the top 12 inches, while clay soils can hold four to five times as much. The general rule is to water clay soils less often, but give longer watering times. Every two to three days is an adequate cycle. Sandy soils will dry out more rapidly and require more frequent irrigation, possibly every day in summer, but for shorter periods of time.

One irrigation period should deliver one inch of water onto a sandy soil and about two 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 to get that much water. 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 20 gallons per minute over an area of 1,000 square feet. A zone must therefore stay on for 30 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.

Runoff can occur with any irrigation system when water is supplied faster than the soil can absorb it. This is not only wasteful but can carry away fertilizer and pesticides, making it a potential source of pollution. It is more common with clay soils because of the slower rate of water absorption. Runoff can be controlled by changing the type of sprinkler heads to reduce the rate of application, or by reducing the length of time that each zone operates and irrigate for several successive shorter periods to give sufficient time for the water to infiltrate into the soil without runoff.

The worst irrigation program, however, is to water 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 the turf has available. On the other hand, overwatering can saturate the root zone, resulting in water draining through it and being lost. This drainage can also move fertilizers and pesticides into the ground water.