Large, established trees may begin to deteriorate over a period of years for no apparent reason. Trees most commonly affected include oaks, ash, sugar and Norway maples.

When no specific disease, insect or cultural problem can be determined as the cause, the tree is said to be "in decline." Generally, a tree declines because it is under stress. Symptoms develop slowly and may be so subtle that most people don't notice them until the problem is in advanced stages.

Keys symptoms of decline include:

  • fewer leaves develop on the tree

  • leaves are smaller than normal in size

  • early leaf drop in late summer

  • premature fall color

  • tips of branches die back

  • small branches die

  • leaf scorch (browning on the edges)

  • trees becomes more susceptible to other insect and disease problems

 

What Causes Decline?

Decline is not the result of a specific insect or disease. It is the buildup of several stresses that combine to weaken a tree.

Generally, trees decline because they can no longer move enough water from the roots to support the leaves. Or, for some reason, they are not able to manufacture enough energy in the leaves to sustain the tree through the winter.

The most common cause of decline in large trees is a change in the root zone of the tree including:

Restriction: As the tree grows, it needs more room. The roots of most trees are concentrated in the top 18 to 24 inches of soil and extend out past the "drip line" of the canopy. Large trees often out grow their space and can no longer support their canopy without help. Sidewalks, driveways, streets, structures, sewers, etc. all tend to restrict root zones.

Soil Compaction: Construction activities, vehicle traffic or just foot traffic tend to compact soils. Water cannot penetrate and low oxygen levels may kill roots.* Excavation: Any digging near trees damage their roots. Trenching, laying foundations, sewer repair, underground utilities, etc. all can cause problems.

Drought or Floods: Too little water causes roots to die back. Some large trees may never be able to reestablish roots lost in a drought. Wet sites where water stands may cause root rot.

Soil Fertility : In extreme cases, low soil fertility or incorrect pH may contribute to decline.

Vessels in the trunks of trees transport water up to the leaves or carbohydrates down to the roots. Factors that may lead to damage to these vessels include:

Trunk Wounding: Damage by lawnmowers, weed whips, etc. can cause wounds in tree trunks.

Girdling: If the cambium layer located just beneath the bark is damaged, the tree will be unable to grow in girth. Minor damage will interfere with the movement of carbohydrates to the roots. Major damage of the cambium will kill the tree. Some trees, especially Norway maples, develop roots which circle around the trunk just below ground level. As the tree grows, these roots restrict the cambium layer and may result in decline and death.

Diagnosing Shade Tree Decline

Determining the cause of decline may be difficult since it is not the result of a single attack by an insect or disease. Key factors to consider include:

  • Time: Decline is a gradual process that normally occurs over years as individual stresses multiply.
     

  • Symptoms: The key symptom of decline is not what is present but what is missing. Fewer leaves, smaller leaves, dead branches and a general "unhealthy" appearance usually indicate decline. Leaf spot diseases, insect damage and other symptoms may be secondary or may indicate that the problem is something other than decline.

Dealing With Decline

The sooner the treatment begins, the better the chances of survival. Unfortunately, most cases of decline are not noticed until advanced stages of the disorder. After several years of decline, trees may be so weak that nothing will save them. The key is to eliminate or minimize the conditions that are stressing the tree.

Plant the Right Tree: When planting new trees, choose the right tree for the site. Avoid planting large trees such as sugar, Norway and silver maples, and sycamore in small, restricted sites.

Deep Root Watering: In compacted soils, water applied to the surface may not penetrate deep enough to help the tree. Special attachments are available for garden hoses which will deliver water into the root zone of most trees. Aerating and vertical mulching discussed below will also help with water penetration.

Fertilize: Periodically fertilize trees using tree spikes or other methods for placing fertilizers 12 to 18 inches into the root zone of the tree.

Aerate the Root Zone: Drill or drive holes into the soil 18 inches deep in many places beneath the canopy to allow water penetration.

Vertical Mulching: Using a post hole digger, make several holes 18 inches deep under the canopy of the tree. Fill the bottom 12 inches of the hole with a porous material such as woodchips, gravel or coarse sand. Replace the last few inches of top soil and the sod plug on top. This allows moisture and nutrients to penetrate to a depth to be useful to the trees.

Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.

 

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