Q. I notice that trees in my yard are already dropping leaves, and I see the same thing along roadsides and in the parks. Is there some kind of insect problem or blight, or is it just the dry weather?
A. What you are seeing is an environmental problem rather than an insect or disease issue. They are usually pretty species specific, and you would not see similar damage on so many unrelated plants.
Spring started out fairly warm and dry, and then we started getting a lot of rain, but it remained warm. Temperatures remained above average through spring and beyond. The growing season seemed to speed up, with plants blooming earlier than we remember from past years.
Some vegetables and annual flowering plants do very well in this weather, as long as they have sufficient irrigation to make up for the lack of rain. It is a different story for trees and shrubs.
This has been a rough growing season for many woody ornamentals. The stress they are under shows up as yellowing and falling leaves, and premature fall coloration. Much of the stress was caused by the wet weather we had in May and June followed by hot, dry conditions since then. The saturated soil early on may have damaged the root systems of many plants by causing some root rot.
The rain also promoted a lot of quick, succulent growth. When those plants were hit by the hot, dry weather, that succulent growth lost moisture faster than more moderate growth would have. Their compromised root systems were unable to take up sufficient moisture to support that growth after the rains stopped coming so frequently.
If your established woody ornamentals are showing signs of stress 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 so, until you have watered the entire circumference of the plant. Start near the trunk and work your way out to the drip line, if possible. Even if you cannot water, trees and shrubs have set their buds for next year’s growth by this point in the growing season. Although it is not ideal for them to go into winter drought-stressed, they should be fine next year.
A two to three inch layer of mulch around the circumference of a tree, out to the drip line if possible, helps conserve soil moisture and moderate soil temperature. Replacing competitive lawn grasses with mulch reserves water and nutrient resources for the tree. Mulch also helps keep weeds in check that would otherwise compete with your trees for water and nutrients.
Just as important, a circle of mulch around the base of trees and shrubs 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. The thin, light green layer of tissue under the bark is called the cambium. This where water and nutrients absorbed by the roots are transported up to the top of the tree and nutrients produced by photosynthesis are transported down to the root system. If this transport system is damaged around the circumference of the trunk (girdling), the plant will die. Even less severe damage from mowing equipment creates an entrance into the plant for insect and disease problems.
Avoid mounding excessive mulch up around the trunk of trees and shrubs. Doing so can cause the bark to rot, and it creates a perfect environment for insect and animal damage to occur out of sight. Mulch can be close to the trunk as long as it does not physically touch it.
Thick mulch also tends to become water repellent (hydrophobic) once it dries out completely. When it does rain, plants mulched in this manner may not benefit as much as plants mulched more moderately.
Avoid pruning or fertilization now, since this will push new growth that will not harden off before winter arrives. It is never wise to fertilize drought-stressed plants – the concentration of salts in most fertilizers can send stressed plants over the edge. It is always best to take a soil test and fertilize according to the resulting recommendations.
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.