The need for water by apple trees increases with warmer temperatures, wind, intense sunlight and low relative humidity. About 95 percent of water taken up by an apple tree is lost to transpiration, similar to evaporation, in which water is lost through openings called stomata on the leaves. The leaves need these openings to obtain carbon dioxide from the air for photosynthesis.
Spring and Summer Watering
Apple trees absorb water in the spring as the leaves begin to grow. Most of this water is in the warm upper surface of the soil and is taken up by feeder roots. Washington State University researchers say apple trees do not need a lot of spring-time moisture to grow and produce apples.
Water penetrates the soil less with each irrigation. The soil becomes increasingly dry after each irrigation in the summer. Apple trees need more water beginning in early to late June when the soil should be moistened to about 50 percent of its capacity.
Average Water Needs
Apples need 1 to 1½ inches of rain per week over the course of a growing season, according to Ohio State University horticulturists.
One acre-inch of soil moisture will last about two weeks in April. An acre-inch is the amount of water needed to moisten 1 acre 1 inch deep. An acre-inch of water might last about six days in May, four days in June and three days in July; that rate drops to four days in August, seven days in September, and more than 10 days in October.
Roots and Water Needs
Apple tree roots are extensive, often going 6 feet deep. In sandy soil, apple roots extend two to three times the spread of the branches. The roots grow about 1½ times the branch spread in clay or loam soil. If apple tree roots are shallow, such as found in high-density planting, the trees need to be watered more often.
Effects of Drought
The stomata stay and open for a shorter period of time on trees in dry soils and during high temperatures. In periods of severe dryness the stomata may not open to prevent water loss. When the stomata are closed during drought both transpiration and photosynthesis are reduced by as much as 40 percent before the leaves begin to wilt. Wilted leaves cause transpiration and photosynthesis to drop by more than 90 percent.
Drought and Trees
If there is not enough moisture in the soil during drought, apple trees produce shorter shoots and smaller leaves. Shoot growth occurs during the first six weeks of spring growth. If there is adequate water in the soil in the spring followed by a late season drought, the growth of shoots will not be affected, but the growth of the trunk might be reduced.
When less water is available, an apple tree receives less of the necessary nutrients. Most affected are nitrogen, potassium, boron and magnesium.
Drought and Fruit
A lack of water during mid-summer and fall might result in fewer apples the next year. An English study cited by Ohio State University concluded that trees receiving no irrigation or rain from March to June produced 65 percent fewer apples.
Apples suffer corking when they are water stressed. Corking is a condition in which apple flesh takes on the appearance of a dried cork.