Image by US Department of Agriculture via Flickr

Frost Injury

Many trees are damaged by early fall and late spring frosts.

Trees prepare for winter as fall approaches through a process
called hardening off. They change the chemical composition of their sap so that it is similar to antifreeze. Trees will also reabsorb minerals from the leaves (causing leaves to turn fall colors) and direct energy reserves down to the roots. 

Unexpected hard frosts freeze and destroy conductive tissue so that food and water can no longer be transported. This results in branch die-back, and sometimes whole trees can be killed.

After frost injury has occurred, nothing can be done to reverse the effects. It is best to wait until summer to see if the tree has sustained serious damage. Sometimes trees will take additional time to leaf out in the latter part of spring.

Dead branches should then be removed to prevent insects and fungi from establishing in the dead material and possibly invading the living part of the tree.

Overwatering

Overwatering is a common problem in Colorado, particularly in irrigated turfgrass areas. Trees do not show symptoms of rotted roots in waterlogged soils until their health is seriously compromised. Symptoms include:

• Leaf fall in midsummer
• Interveinal discoloration of leaves
  
• Marginal leaf scorching

Ash, aspen, honeylocust, birch, and maple trees are the most susceptible to wet soil conditions.

Wet soils can cause trees to grow girdling roots, which may eventually constrict the flow of nutrients and water up the tree. Sometimes girdling roots are visible above ground, wrapping around part of the base. 

Chronically-stressed trees are more susceptible to infestations and decline more quickly.

Drought Injury

Drought injury is a dehydration of leaf or branch tissue when more water is lost than replaced through the roots. 

Injury can be caused by multiple factors:

• prolonged hot, dry summer weather and wind
• dry winters
• high salt concentrations in the soil
• compacted soil
• new construction nearby

Drought injury in decidious trees appears as leaf scorch and leaf fall. Evergreens also change color to yellow or red as needles die from the tip downward. Evergreens can experience this in winter when chinooks (warm winds) come through and rapidly increase the temperature.

Severe dehydration can cause death of branches or even the whole tree. Wait until the following summer to see if the tree leafs out before removing it.

Diseases

Diseases caused by contagious bacteria or fungi may require that infected parts of tree be removed. An example is fire blight on apple trees.

Not all abnormal changes in appearance are cause for alarm.

Examples:

• Galls (enlarged, round structures such as are common on the hackberry) are caused by insects and are not contagious nor harmful to the tree.
• Browning needles on pine trees do not mean the trees are dying. Pine trees shed their needles every few years.

Emerald Ash Borer and Pine Beetles

The emerald ash borer is an invasive species that destroys ash trees after adults lay eggs in the bark. The larvae bore into the conductive tissues of the tree, girdling and killing the tree within a year if it is small, and over the course of 3-4 years if the tree is large. Symptoms include thinning, yellowing leaves, branch die-back, and holes in the bark. Early detection may save the tree if appropriate pesticides are used. Usually, the tree must be removed.

Pine beetles often attack already-ailing trees, which may be stressed by competition from other trees, drought, or diseases. A tree infested by pine beetles will have its needles turn yellow/ red and holes will be present in the bark. Infected pine trees do not recover and will have to be removed.