Probably the most common question I am asked is “How often should I water.” Because I am very educated and experienced in horticulture, I always have the correct answer. “I don’t know.” There are too many factors to consider and correct irrigation can only be determined by observation. Every species has specific requirements which vary with maturity and growth rate. Weather conditions such as temperature, humidity and wind affect how much water is used by plants. Soil conditions are also important considerations, as well as method of irrigation.
Some plant species have very high evapo transpiration rates. Translated into English, this means that they lose considerable amounts of moisture to evaporation from foliar surfaces. Willows for example, are often used to dry swampy conditions as they consume large volumes of water. Although they can survive less than swampy conditions, willows are easily damaged if their soil dries out. It is therefore often necessary to give willows extra water to compensate for that lost through the foliage.
Large, mature willows have more extensive root systems and are more resilient than young trees with more confined roots. Of course, during 
the winter when the trees defoliate, they draw no moisture from the soil, and only require enough moisture to keep the roots from desiccating.
Warmer temperatures, low humidity and increased breeze all enhance evapo transpiration, increasing the need for water. All three of these factors affect foliar surfaces much as they affect laundry out on the line. Everyone knows that laundry dries best on warm, dry days, especially with a breeze.
Soil is best well drained, but with reasonable moisture retention. Sandy or gravely soils may drain very well, but dry out very quickly. Heavy clay soils may hold too much moisture so that when the surface indicates that irrigation is necessary, lower strata may by too moist to support healthy root growth.
Development of healthy roots requires a good ration of moisture and air, (as well as nutrients.) Dry soil, causes wilt. Saturated soil will drown roots and provide and environment ideal for root rot organisms. If upper soil frequently dries, most roots will grow at lower levels. Conversely, if lower soil is too saturated (frequent and generous irrigation), or irrigation is only sufficient to moisten upper soil, leaving lower soil too dry (frequent irrigation), surface roots will develop.
In most situations, it is best to irrigate generously, but as infrequently as possible. This will allow for good air penetration between irrigation and also enough water to reach lower soil. Of course, if plants are wilting, frequency should be increased. Lawns and a few plant species, such as rhododenrons and azaleas, are exceptions to this rule because almost all of their roots are near the surface; in which case irrigation may need to be more frequent than for the rest of the landscape. Mulch helps keep the top surface of soil moist between watering.
Sprinkler irrigation is essential for lawns, and usually most effective for most parts of the landscape. Although not as thorough, drip irritation (and its relatives) are useful for conserving water. Slow application of water, as from drip emiters, usually moves vertically through the soil faster than it moves horizontally. Roots outside the volume of soil affected by such irrigation do not get any water. High volume emiters also conserve water and are a good compromise because they flood a basin, distributing water over a wider area, but do not irrigate the area outside the most dense part of the root zone. Spray emiters also affect a wider area than drip emiters, but they require more maintenance and often clog.
If irrigation is automated, it is best done at night or in the morning, when less water is lost to evaporation. Roses or other plants which are sensitive to fungal diseases may require that water be applied about dawn; so that leaves and surroundings dry as quickly as possible. Some may prefer to water later in the morning when everyone is out of the house, to avoid plumbing noise or lack of pressure when it is needed most.
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