Shade Tree Decline

Shade tree decline, or shade tree disorder, is a term used to describe branch and twig dieback in the crown of a tree. These trees are not affected by pests or disease, per se, but they are sick.


In winter, most deciduous trees are bare. By late spring, a thick blanket of sugar-producing machinery covers those trees. Every leaf surface should be cranking out energy as fast as it can. In recent years, especially in heavily populated areas, somewhat healthy-looking trees tend to be a bit thin on top. This thinning is not a hormonal problem treatable with hair tonic. Shade tree decline is a serious problem. Those trees are dying.

Canopy of healthy sweetgum tree (Kate Russell)

Symptom of shade tree decline (Kate Russell)

Drought (or lack of proper irrigation) is the number one cause of shade tree decline, but there are other causes. And ornamental trees are not the only victims. Canada’s sugar maple trees have been exhibiting shade tree decline for decades. That decline is due to air pollution blowing northward along our Eastern seaboard. [My sincere apologies, Canada.] There are many other causes (and few solutions) to the problem of shade tree decline.


Symptoms of shade tree decline

Symptoms of shade tree decline appear gradually. Like the frog in the proverbial pot, we often don’t notice the damage until it is severe. Initial signs of shade tree decline include:

• Reduced new growth
• Die-off of new twig and leaf buds
• Leaves are smaller and fewer
• Premature fall color change
• Early leaf drop

Later symptoms include:

• Thinning of the crown
• Excessive seed production
• Dieback of larger limbs and branches
• Increased sucker growth

These are all signs that the tree knows it is dying. It is frantically putting what little energy it has left into the next generation. Most trees affected by shade tree decline die within a few years.


What causes shade tree decline?

Imagine being pummeled all day, every day. Professional boxers do it, but they have strict nutritional support, medical teams, and proper hydration. Drought, pollution, and injuries have plagued our trees for many years, but most never noticed. Some of the more common injuries include the following:

• Extensive bark damage from lawnmowers and weedwackers
• Herbicides and other chemical damage
• Soil infertility
• Soil compaction
• Extreme temperature changes
• Flooding or changes in the water table
• Root damage due to rototilling and digging
• Girdling by tree supports left in place too long
• Sprinkler misplacement (never let the spray hit tree trunks)

The downside of shade tree decline

Trees are an investment in the future. They cost money and take time to grow. They also prevent erosion, provide shade and food, store carbon, create the oxygen we breathe, and filter pollutants out of the air. They provide habitat and food for many other living things, promoting biodiversity. As a tree begins to suffer shade tree decline, it becomes less able to defend itself against pests and diseases, becoming more likely to suffer sunburn damage, providing easy entry to borers and other insect pests.


The bigger problem

The Carnegie Institute for Science reported that California lost more than 58 million large trees between 2011 and 2015. Even when heavy rains appear, the damage has already occurred, and tens of millions more trees will die. The strain on the trees has been too much. Dead and dying trees harbor pests and diseases in epidemic proportions. The large-scale die-off of trees also makes it easier for invasive plants to get established. And sick trees can be fire hazards.


Preventing shade tree decline

Prevention is much easier and more likely to succeed than treatment. You can help your trees stay healthy by recreating the conditions they evolved in to prevent shade tree decline.


These tips can help you protect your trees against shade tree decline:

• Prune shade-loving trees to keep them in the shade of a more heat-tolerant tree.
• Coil a soaker hose in a spiral around the tree trunk, starting a few inches from the base, ending at the drip line, and watering deeply regularly during summer.
• Fertilize regularly and only as needed, depending on lab-based soil test results.
• Mulch under trees, keeping the mulch six to eight inches from the trunk.
• Avoid overspray of herbicides and other chemicals.
• Install trees a safe distance from concrete surfaces that produce heat islands.
• Plant trees in soil with a suitable pH.
• Remove tree supports when they are no longer needed.
• Install footpaths around trees to reduce soil compaction.

Treating shade tree decline

Prospects for an affected tree are not good. The damage has already occurred. Even the most diligent care may not be enough. If you are determined to try, these actions may save your tree or help it to survive a little longer:

• Fertilize the tree in spring.
• Prune out all dead branches and twigs.
• Mulch.
• Provide extra water throughout the growing season.

How to irrigate trees

Most tree roots are in the top 12 to 18 inches of soil. When these roots get too dried out for too long, they die. Fewer roots reduce the amount of water available to the tree for life processes and cooling (and growing new roots). Frequent, light watering encourages shallow root development. Deeper roots are less likely to be damaged by the elements and may access groundwater. 


Properly irrigating trees means getting the water to a depth of at least five to seven inches every three or four days during the peak of summer. Before we knew better, we used to plant trees in bowl-shaped depressions we would fill with water. Sounds good, but it’s a bad idea. This practice leads to crown rot and several other fungal diseases. Better methods include soaker hoses and irrigation rings. Irrigation rings can be mechanical devices or nothing more than trenches dug around trees at the drip line. Fill the irrigation trench with water every few days during the peak of summer, and your trees will thank you.


Selecting the right trees

If it is too late to save your tree, invest in a reputable arborist for professional removal. Trees can be deadly. Seriously. Then, use the following information to select trees better suited to your microclimate:

• Get an inexpensive, lab-based soil test.
• Identify your USDA Hardiness Zone.
• Select trees suited to your soil structure and microclimate.
• Plant trees where they can reach full size (big trees in small spaces are rarely healthy).
• Plant trees where they will get the correct sun exposure.
• Irrigate trees properly.

Fighting Mother Nature is a full-time job, and it’s mostly a waste of time. Rather than setting ourselves up for tons of maintenance in a never-ending battle, put the things that occur naturally in your landscape to work for you and your trees.