My pear tree I planted this spring is dropping yellow leaves. My thirty-foot tall hackberry is doing the same thing. I'm watering regularly with a sprinkler and the grass looks okay under the trees. What's wrong with them?
Newly planted trees must be carefully watered. Even then they may experience establishment problems including leaf drop. Always check the soil before watering by digging down to see how moist it is. Do not water by a calendar schedule. Soils that are too dry or too wet each present problems for newly planted trees. Consistent watering is as important as applying the correct amounts of water. Note that new trees should be watered not only during the growing season, but also during the fall and winter. Even under the best circumstances trees require at least one year to become established for each inch of trunk diameter. For example, a transplanted tree with a trunk 2 inches in diameter will require 2 years for establishment. Water carefully year round during this establishment period.
Hackberries and many other trees are now showing signs of stress from drought and water restrictions. Trees that were not watered last winter show more stress. Stress typically appears after extended periods of 90-degree temperatures that increase trees' demand for water. Although spring snowstorms recharged soil moisture, subsurface moisture supplies have dwindled. Dig down with a trowel in several places in your yard. You may find the soil is moist for 3 or 4 inches but dry below. This would explain why trees might not be performing well even if the grass looks okay. Hackberries are a drought evader species - they allow some leaves to yellow and drop under drought stress in order to save the rest of the tree. Take advantage of opportunities to consistently water your tree as water restrictions allow. Some water districts are considering changing restrictions and encouraging hand watering of trees this summer. If permitted, use a soil needle attached to the end of a hose to wet the subsurface soil. Insert soil needles no more than 8 inches deep and allow water to flow for no more than 2 minutes before moving to a new location. Pay particular attention to the roots in the area under the branches of the tree. Also, this fall and winter be certain to water trees that show signs of stress this summer.