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Geoff Sherman: Respect post oaks' space to keep them healthy

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Geoff Sherman, Texas A&M AgriLife Extension
<br>Geoff Sherman

Geoff Sherman

Being on temporary assignment with Texas A&M AgriLife Extension here in Denton County has given me the opportunity to visit with county residents and provide assistance with tree, turf and ornamental questions and concerns. Recently, I have visited sites where the canopies of post oak trees have completely or are beginning to brown from the branch tips inward. 

Unfortunately, post oaks have the most sensitive root systems of all of the native trees in our area, and with this species, it is very common for any disturbance of the root system to lead to a fast, and often irreversible, decline and eventual death. In some cases, decline is very quick; in others it may take a few years as the tree canopy thins and then totally browns out.

A number of factors are associated with the disturbance of the soils and root systems around post oaks. Construction damage from the addition of driveways, patios, remodels, swimming pools and even parked vehicles and foot traffic can upset the root zones through disruption and compaction of the soil. This reduces the soil pore space where oxygen and water should be present together to support the trees. Excessive moisture needed to support other vegetation such as St. Augustine grass in hot Texas summers can also upset the oxygen-water balance in the post oak root zone and begin the decline process.

Even if we are able to minimize the damage from soil compaction and excessive moisture in the root zone of post oaks, weakened trees that are not showing signs of decline can be affected by a fungal pathogen called Hypoxylon canker. 

The spores of this opportunistic fungus are spread by beetles and wind from tree to tree. Unfortunately there is no cure for this fungal disease. The spores of Hypoxylon canker are, for the most part, already present on all post oak trees. This fungus will not infect the trees until they are stressed by some environmental factor, such as soil compaction, drought, ice storms, excessive moisture, lightning, defoliation from hail, etc.

While there is nothing that will totally guarantee the survival of the post oaks you may have in your landscape, consider the following actions to help reduce the possibility of environmental stresses and disease:

  • Keep any construction activity outside the root zone of any post oaks.
  • Minimize foot traffic and vehicular traffic in the root zone of the trees.
  • If installing an irrigation system, keep piping outside of the tree root zone.
  • Replace any vegetation within the root zone with 2 to 3 inches of mulch, but away from the trunk.
  • Water during drought periods.
  • Remove broken and damaged limbs from uninfected trees to help prevent or reduce avenues for infection.
  • Keep the trees happy by keeping them as undisturbed as possible.

Contact us with your landscape and garden questions at, 940-349-2892 or at

GEOFF SHERMAN is on assignment with Texas A&M AgriLife Extension in Denton County.