Q. I have noticed that many of the trees in my yard are dropping leaves, some still green and others yellow. I have a sweet gum, a linden and several black cherries. I imagine the very hot, dry weather is to blame but want to be sure it is not some kind of blight.
A. You are absolutely right that the weather is to blame. What you are seeing is an environmental problem rather than a disease problem. Diseases are usually species specific. It is unlikely that the same disease would attack these unrelated trees.
This has been a rough growing season for woody ornamental plants. The warm spell in March forced many trees and shrubs to put out new growth much earlier than normal, followed by frosts and freezes that damaged or killed the tender new growth. As if that wasn't enough stress, June was very dry and July started off with record high temperatures.
The early growing season also promoted a lot of quick succulent growth. When those plants were hit by the hot, dry weather, that growth lost moisture faster than more moderate growth would have. The stress trees are under shows up as yellowing and falling leaves and premature fall coloration.
There are a few things you can do to help them. Provide an inch of water a week if there is no rain. Soaker hoses and drip irrigation are the most efficient ways to water. If that is not an option, remove the nozzle from your hose and allow it to trickle slowly around the base of the plant. Move the hose every hour or so until you have watered the entire circumference of the plant. Place the hose within 18-24 inches of the base of the tree.
A two- to three-inch layer of mulch around the base of the tree, out to the drip line (ends of branches) if possible, helps conserve soil moisture and moderate soil temperature. It also helps keep weeds in check that would otherwise compete with your trees for water and nutrients. A circle of mulch around the base of trees and shrubs also keeps lawn mowers and string trimmers away from their trunks. Many woody plants (especially young ones) are killed outright by lawn mowers and string trimmers damaging the bark.
Avoid mounding excessive mulch up around the trunk of the tree. Doing so can cause the bark to rot, and it creates a perfect environment for insect and animal damage to occur out of sight. Keep mulch a few inches away from the trunk. Excessive mulch is difficult to re-wet once it dries out completely, so even when it rains, trees with mulch "volcanoes" are not benefiting from it as much as they should.
Also avoid pruning or fertilization now because this will push new growth that only will add to the stress. It also may not harden off before winter arrives. Do not fertilize without taking a soil test first. Trees growing in a lawn situation receive more than enough when you fertilize your lawn. They should not require additional fertilization unless a soil test reveals a deficiency. If you do not fertilize your lawn, or if the trees are growing in a bed, they may benefit from a late fall application of fertilizer. The roots of woody ornamentals put on a lot of growth late in the season. The best time to fertilize is when they have lost their leaves and gone dormant, but the soil is still warm enough for root development. This is usually late fall, right around Thanksgiving.garden
Send questions to Sandy Feather by email at firstname.lastname@example.org or by regular mail c/o Penn State Extension, 400 N. Lexington Ave., Pittsburgh 15208.