“Can my tree be saved?”
— Paul Grimes, Aurora
The photo indicates that the cause of the damage to your tree was from an improper pruning cut. The top of the wound is smooth from the downward cut by a chainsaw, and once the cut was about three-quarters of the way through, the weight of the limb caused a massive rip as it fell off the tree.
Two mistakes were made in the pruning process. First, the weight of the limb needed to be reduced before making the final cut at the main trunk. To do so, start by making a cut about a quarter of the way through the limb on the underside of the limb. Then cut through the limb from the top, starting a few inches out away from the undercut.
As the limb begins to fall, any tearing of the bark will be stopped by the undercut, and the limb will cleanly fall from the tree. Then the final cut can be properly made without damaging the tree.
The second mistake was making the angle of the cut too steep — the photo shows it angled almost straight down. This tree limb likely had a bark inclusion, which would have required a very carefully positioned cut to do properly.
A bark inclusion occurs when two branches grow too close together, creating a very narrow angle in a V-formation. As the tree grows, the narrow angle gets deeper, which increases the weakness of the junction and increases the chances of failure.
Since this limb had included bark, the damage caused by the improper pruning was significant. Snow load and wind can also cause limbs with included bark to break off.
There is nothing to do for your tree, as the wound will never heal properly. It does not appear to be an immediate hazard, but the best practice will be to cut the entire tree down as soon as possible and replace it with another shade tree.
The tree to the left in the photo and near the stop sign may also have an issue with included bark. Consult an arborist for recommendations, such as possibly cabling the limbs to increase stability.
For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at [email protected]. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.