I have a very large oak tree in my yard that has very few leaves this year with some dead wood. There is also some bark coming off. We are in Brazoria county, Texas. The tree has been fertilized and sprayed for pests. The tree is estimated to be 75 years old. We also have a tree in the back that is much older and also much healthier.

It sounds like you are describing a tree in decline. Decline could be described as a progression towards mortality, caused by a mounting of various stress factors. The first stressor could be when the roots suffer a disturbance, such as soil compaction, trenching, grade change, or nutrient breakdown due to turf grass competition. Following root disturbance, diseases such as root rot can set in, and pests such as mites, caterpillars, or sucking insects could pile on. Each stressor causes a percentage of the leaf population to fall off unseasonably early, until over time, the tree's leaf population cannot provide adequate photosynthesized food to meet the energy needs of the organism. At this point, the tree tends to have more dead wood than its neighbors, and often we see tip dieback and aggressive sprouting on the trunk and scaffold branches.

The questions to ask are:

  1. How often has it been fertilized, at what time of year, using what application method and what formulation?
  2. When the tree was sprayed, what pests were being targeted? What product was used? Were any beneficial insects harmed at the same time? How often has it been treated, at what times of year?
  3. What is the irrigation schedule? Often we overwater our oak trees in an attempt to reverse the decline spiral. If the organic content of the soil is poor, the soil is compacted, the internal drainage is poor, or the tree shares all or part of its root zone with turf grass, then overwatering can actually speed up the decline spiral, rather than help. Most trees need to dry out thoroughly during waterings, so oxygen can work into the pores of the soil.
  4. How often has the tree been trimmed? What was the technique? Was care taken to minimize the amount of live foliage removed with each pruning session? Was interior foliage left intact, or was the interior stripped out, leaving only foliage in the upper 1/3 of the tree's canopy? Over-zealous trimming can be one of the stress factors mentioned above.
  5. Does the tree still have at least one-third of its ideal leaf population intact (judging against nearby, healthy trees, like the one in back)? If so, then make sure the tree has a large tree ring of mulch, 3-4 inches thick, out at least 8-10 radial feet from the trunk. Take care not to bury the root flare. Put a soaker hose out at least 8 radial feet from the trunk and leave on for 4-6 hours. Depending on the composition of your soil (clay retains water, whereas sand usually drains internally), you probably don't want to water more than once a week, even in the heat of summer.
Lastly, please make sure pesticides are not being used without a clear understanding of what specific pest or disease you are trying to suppress!

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