Expert opinions on watering vary widely. An agriculture student of the twenties and thirties would never water during the middle of the day. Today it is common to see golf courses as well as farming operations watering during the most intense heat of the mid day. The only steadfast rule for home lawns is, do not water in the evening hours. All other answers to the water questions can be categorized by the period in which the expert was educated, and the goal to be achieved with the watering.
The first concern when irrigating lawns is getting sufficient water to the grass plants roots. This requires time and plenty of water. The second concern is that many lawns are simply over watered. A third concern in lawn irrigation is watering sufficiently while avoiding the encouragement of fungus outbreak. We have just about determined that most fungus problems occur when the grass plants stay moist longer than a given period of time. No one is exactly sure what that period of time is, but, a good guess is, begin to water after midnight when the dew would normally form on the turf. End irrigation at a time that allows the lawn to dry out at it's normal time, say, eight or nine in the morning. In other words, don't water at seven in the evening when you get home from work or at start at nine am when you leave for work. These two scenarios would extend the length of time the grass plants remain wet, increasing the chances of fungus outbreak.
Early morning irrigation is often ideal. From midnight to eight am the wind is less and the grass is already wet from dew. Evening irrigation enhances the environment for insects and diseases.
In the pre World War II era, irrigation was much more expensive and far less sophisticated than it is today. It was believed that if plants were irrigated by sprinkling over the top during the middle of the day the droplets would act as magnifying glasses and scorch the leaf thereby reducing the plants ability to carry on photosynthesis. Today we hold to the theory that the water would evaporate before causing any harm. The tide on mid day irrigation turned for good during the seventies when it was agreed that we could use irrigation to cool plants, particularly cool season grass plants, and curtail desiccation during times of heat stress. Most of the water sprinkled on at midday will evaporate, thus the cooling effect to the plant, and not very much actually gets to the roots to be used by the plant for life processes. If you can afford to air-condition your lawn with irrigation by all means, call in sick the next hot day and spray away. In other words, if your goal is to combat heat stress, then mid day irrigation is perfectly valid, especially if you have submerged ledge or a southern exposed slope.
Some general starting points for watering the lawn.
If you live in New England and only plan to water once in a while, use well water and can only water for a short period of time, just don't water at all. The grass may go dormant, fold up its little blades and turn brown, during hot dry periods but will usually turn green with the fall rains. Septic tanks, rocks and ledge can radiate enough heat back onto turf at night to actually kill a section of turf. For the most part, shallow, infrequent watering is more harmful to lawns than it is beneficial. Water deeply and infrequently. Begin to water when the soil starts to dry out and the grass plants begin to lose turgor pressure (blades fold and lose their resilience).
If you have an in ground irrigation system, start by watering each section of lawn for forty minutes, three times per week. If you are pulling a half inch garden hose and using an above ground sprinkler start at about an hour and a half per section either two or three times per week. Don't bother to attempt lawn watering with a hand held hose end sprayer. A hose end sprayer just doesn't do any good for getting water to grass roots. Now take a look at the micro-climates in your lawn. Areas that are naturally wet or shaded may require less watering time. Areas that tend to be hot or well drained will require a longer watering period.
For the home owner using a garden hose and sprinkler I highly recommend using quick connect couplings, an adjustable pulsating sprinkler, and an inexpensive timer. You will learn quickly how to set your pulsating sprinkler to most efficiently water each section of lawn. The timer will allow you some freedom from the chore of turning the sprinkler on and off.
To accurately gauge the amount of water your sprinkler system is delivering set out empty tuna cans and measure the amount of water they collect during a timed period. If you collect a half inch in forty minutes of watering and you water three times per week it's easy to see that your lawn is receiving an inch and a half of water per week. Watering recommendation for New England cool season grasses range from one and a half inches per week to three inches per week. My personal observation is that at three inches per week the humidity on these lawns resembles that of a subtropical New Orleans summer and is decidedly uncomfortable. An inch and a half per week is generally sufficient.
Is there an economic return for all this watering? Yes. The average healthy lawn has the cooling capacity of about sixteen tons of air-conditioning, and provides an organic cooling of the environment. A healthy lawn filters pollutants from the air we breath. Turf also acts as a giant outdoor dust mat, trapping dust, pollen and other irritants. Grass plant roots can extend two feet or more into the soil and combat the global problem of top soil erosion. A healthy lawn is the first step in the filtering process for acid rain that will one day enter our ground water system. Studies over the last ten years indicate that when applied at label rates, pesticides and fertilizer used to maintain established turf are completely used up by the turf leaving none to leach into streams or ground water even during extreme rainfall situations. Our number one source of water contamination appears to be our own sewage handling systems.
Please, do your part for our planet, maintain a healthy established lawn. My real-estate agent claims that the eye appeal of a lush green lawn will increase the initial offer of a potential buyer by $5,000.00.