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‘Sudden Branch Drop’

Beware the scary 'sudden branch drop,' when trees start self-pruning during the summer

Houses and cars can be smashed and people can be seriously injured and even killed by a strange tree action — a curious natural “pruning” that is happening right now in North Texas.

Big limbs just fall out of the blue with a loud “crack.” Often these failed limbs curiously reveal no obvious external defects, and the inner wood is broken bluntly, with no sharp splintering. The absence of obvious visual warning signs like cracks and color changes make this danger hard to predict. This scary tree reaction is usually a complete surprise. It is called “sudden branch drop,” “high temperature limb drop” and “summer branch drop.”

Theories about the cause include branch movement and tissue shrinkage, internal cracking, internal moisture changes, gas releases inside limbs, and microscopic changes in cell wall structure. Consistent warning signs are lacking and there are still no definite answers.

Susceptible trees 

Trees that are subject to sudden drop are usually mature, with limbs that are very large, mostly horizontal, sweep upward toward the end and extend out beyond the main canopy.

Mostly these failures occur on hot, still days with no wind. My best theory is that this is a tree’s response to demands when transpiration exceeds the capacity of the roots and vascular system. This imbalance of moisture causes the tree to abort limbs.

Trees reported to be susceptible to sudden drop include, but are apparently not limited to, oak, poplar, willow, elm, ash, cedar, pecan, walnut, hackberry, sycamore, tree of heaven and maple. In other words, most trees.

Protecting your trees

To reduce the risk of this happening, arborists and tree owners should consider the following preventive tree measures:

1. Properly exposed trunk flares (also known as root flares). Buried trees are always in stress.

2. Quality pruning that avoids flush cuts and excessive thinning. Cabling is recommended by some but not by me. Unless done perfectly, cabling just moves the weak points to new locations and adds to maintenance.

3. Timely plant health care: organic fertilization, soil aeration and the use of the Sick Tree Treatment.

4. Appropriate watering practices in the form of deep, infrequent irrigation.

Proper watering is key

Surprisingly, proper watering is the biggest challenge. Stress in trees caused by improper watering (under watering or over watering) is very common. Light, frequent watering cycles weaken trees by promoting shallow, weak root systems.

Limb failures are simply not always predictable. Arborists don’t have crystal balls and cannot always detect conditions that lead to failures. Trees are living organisms, and they may fail in ways we do not fully understand. Sudden branch drop is a perfect example.

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