Extension Educator, Horticulture
Good gardeners are good waterers. Through practice we learn the skills of knowing when to water and how much to water.
What time of day is best for watering? If a plant is wilting due to lack of water, water it no matter what the time of day. Wilting trumps every watering "rule". Otherwise the best time of day to water is early morning. There is less water wasted due to evaporation and plants are better able to use the water when temperatures are lower than midday. Watering late in the day can lead to more leaf diseases if leaves remain wet through the night.
Some garden myth information refuses to die. The old adage that watering during midday will cause leaf scorch due to the water droplets acting as tiny magnifying lens continues on websites and over garden fences. If this were true we would see a great deal more leaf scorch since the sun often shines after a rain.
Hungarian researchers recently took a scientific look at this adage. The magnifying lens theory just did not hold water. However with a perfect storm of some tropical plants with hairy leaves they did find they could produce some leaf scorch, but it was more likely to occur late morning and early afternoon when the sun was at an angle as opposed to midday. The bottom line is there are reasons outlined above detailing why watering early morning is best and the magnifying lens theory is not one of them.
How much water is needed and how often? As a general rule garden plants need about an inch of water a week, but that depends on soil type, plant species, and if the plants are well established. Clay soils tend to stay wet longer, but are harder to rewet once they turn hard as a brick patio. Anything planted this year and any trees or shrubs planted over the last two years need extra water. New plantings and containers often need daily watering. Plants listed as drought tolerant will also need extra water if newly planted. Vegetables and fruit plantings need water especially during fruiting.
Generally new trees purchased as balled and burlapped should be watered with one gallon of water per diameter inch of trunk every 5-7days if rain is not adequate. For example a 2-inch diameter tree should be given 2 gallons. Trees and shrubs purchased in containers will need to be watered more often, perhaps daily, depending on temperature and wind. Established trees (more than 3 years in present location) should be watered once a month during dry periods. Apply water at the dripline, not at trunk.
With lawn watering it depends on whether you want the lawn to stay green or just alive. Cool season grasses such as Kentucky bluegrass naturally go dormant when it gets hot and dry. Once they are dormant, it's best to leave them there during hot dry periods. Cycling between green-up and dormancy drains plant reserves. A dormant lawn may not look as nice, but is also less prone to insect attacks.
Applying at least 1/3 inch of water every 3 weeks should be enough to maintain moisture in the grass crowns and roots for turf to survive. Water in early morning. Use empty tuna fish cans in area covered by sprinklers to determine how long the sprinklers need to run.
Use soaker hoses to efficiently water landscape plantings. Check soil after half hour of soaking to determine proper watering period. An inch of water will wet an average of 6-15 inches of soil, depending on soil type.
Use 3-4 inches of wood chip mulch around trees and landscape plantings to conserve moisture and reduce soil temperatures. Consider plants such as many native prairie plants that once established will tolerate drought periods.