Why Post Oaks are Sensitive to Change

The post oak is a large tree, which grows to some 75 feet tall, with a dense, rounded canopy width equal to its height. It produces acorns that are about one inch long, and are a favorite of squirrels. The post oak has a high heat tolerance and a low water requirement, making them a common tree around Austin and the Central Texas area. It likes a dry, sandy soil, and is often seen growing with blackjack oak. The post oak leaf has a unique cross shape, not unlike the Maltese cross. When seen growing “wild,” it can be considered an indicator tree of poor soil.

Sensitivity

The post oak is very sensitive to having its roots disturbed and having a lack of oxygen. This can happen during construction, if the soil around the tree gets tamped down or run over by heavy equipment. Paving over the roots will upset the tree, as can over watering, adding or removing soil around the tree. Any of these activities can throw the post oak into distress and can result in the death of the tree.

Cause of Sensitivity

It seems that the reason that post oaks are so sensitive to change is the way the root system grows. When the post oak is growing in a shallow sandy loam that lies atop a clay hard pan, the root system can’t penetrate the hard pan, so it grows laterally in the sandy soil. This places the root system very close to the surface, thereby making it very vulnerable to damage. The root system has to develop where there is enough soil air, water, and nutrient uptake. When the root system is disturbed by construction activity, it is damaged often beyond rehabilitation.

Cause of Post Oak Death

In studies done in Central Texas, it was discovered that the majority of post oak deaths were centered around new construction zones. When this was discovered, the arborists theorized that the cause was the disturbance of the root system during construction. Recommendations were made to the authorities that developers and builders be required to learn how to develop their properties without damaging the post oaks.

Precautions

Whenever new construction is planned, or any remodeling, upgrading, or addition of utilities, sidewalks, or other work of this nature is to be done, the property owner, in cooperation with the construction foreman, should consult with a certified arborist to determine the best way to proceed with the desired work without doing harm to the post oak(s) on the property. The arborist will be able to give sound advice as to the placement of new construction in relation to the post oak(s), or to recommend how to proceed with other types of work.

Preventive Measures During Construction

Some rules to follow in doing any type of construction or improvements to your property are as follows:

1. Determine the location of the root system boundary of your post oak tree(s).
2. Stay clear of this area. Do not allow any type of machinery or equipment to drive in this area. Do not allow any type of material, whether it is dirt, tools, broken concrete, or anything to be dumped within the root boundary.
3. Do not dig to remove dirt from this area. Conversely, do not add any dirt to the root system boundary. Any disturbance of the soil atop the root system will result in damage and probable death to it.
4. Don’t place sidewalks, paved driveways, or other concrete or asphalt work within the root system boundary. Compressing the soil in this manner will kill the roots. Another thing to consider is that no matter whether you avoid the root boundary or not, will the new sidewalk/driveway/patio, etc. change the flow of water runoff in the yard or other area so that it backs up over and around the root system? Even if you have placed the concrete improvement sufficiently far away from the root system, it may still cause problems if it causes water backup. There are many angles to consider!
5. Change your plans, if at all possible, to accommodate your post oak tree(s). In light of the rapid decline in numbers of post oaks in Texas, it is more important to adjust your plans than to risk killing another of our precious trees.

Other Preventive Measures

Some other things you can do to protect your post oak trees include not over watering the trees. Post oak trees are very drought-resistant, which means they don’t need much water. Keeping the soil too moist around the tree causes root rot, which makes the roots decay and die, thereby killing the tree. Don’t place mulch around the post oak tree, either. Mulch holds in the moisture.

Watering

It may be a difficult balance if you have lawn or flower beds around and near your post oaks. Determining drainage can be a key to solving both issues. If you can manage to situate your lawn and flowers so that when you do water them that the water drains away from the post oak, you may have solved the problem. Not that post oaks never need any water, but they should be fine with just the rain watering them, unless there is a significant drought going on. Again, here is where you’ll want to consult with a certified arborist to determine the needs of your post oak(s) and how you can best meet those needs.

Impact of Trees

The contribution of trees to our landscape and our lives cannot be underestimated. Trees prevent erosion, provide shade, provide a home and shelter for a variety of birds and small animals, provide food, purify the air by emitting oxygen and absorbing carbon dioxide, and enrich our lives with their beauty. Although this article has focused on the post oak, which is a very important tree to the ecosystem of the Austin area, and Central Texas in general, all trees are important with their variety of gifts to us. Educating yourself on how you can best care for trees will be a lasting benefit to this generation and those that follow.

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