In the Garden:
Northern California Coastal & Inland Valleys
The tissue between the two trunks is subject to decay and splitting.
Double Trunks Mean Double Trouble
I always knew that two trunks were trouble, but now I know why. Some trees, for whatever reason, create a second leader when they are saplings. A leader is the main vertical trunk that the scaffolding (horizontal) branches grow from. Liquidambars are notorious for creating a second leader, as are silver maples, oaks, and some evergreens. Redwood trees will frequently create a second leader if the main leader is damaged or broken. Trees with double trunks take on a unique form that is appealing to some people, take my mom for example.
Mom planted a small, double trunk silver maple about twenty feet from her kitchen window 22 years ago. The tree grew quickly and provided shade to the eastern side of the house. It also was home to squirrels that amused everyone with their acrobatic antics, and it supported several bird feeders that were a delight to observe from the kitchen window. The tree was bursting with life and provided shade, everything you want from a tree.
My dad and I could see that the silver maple was going to cause trouble eventually. As it grew over the years, the two trunks enlarged and pushed sideways, against each other. A large fissure began to appear where the trunks were joined.
The mountains of North Carolina where they live are notorious for sudden, violent storms, both winter and summer. In the winter, ice adds to the burden of weight the branches carry, and it's not unusual for trees to topple unexpectedly.
I was visiting one summer when a particularly violent storm blew through. Dad and I conferred and decided that the tree needed to come down. If one of the trunks should break off in a storm, it would be a disaster. The prevailing winds would bring it directly onto the house! It was too far late to remove only one of the trunks -- the tree would never survive the massive surgery, and besides, it would then be heavy on one side. The tree had to come out. But not without wailing and ranting from my mom. Boy, she was mad! Dad and I were in the dog house for months afterwards, and I heard about how hot the kitchen was, how much she missed the squirrels, and the poor birdies had to eat on the ground. However, not long after the tree was removed, a particularly violent storm blew through. It took out trees throughout the region and no doubt the silver maple would have caused significant damage.
We recently shot a segment with the Bartlett Tree Experts on this very subject. Arborist Jesse Hesley explained that dominant leaders and co-dominant leaders can be trained over time. When selecting which trunk will be dominant, look for potential scaffolding branches and remove any sideways-growing twigs on the secondary leader that might develop into scaffold branches.
Another reason for removing the small branches on the secondary leader is to allow light into the main trunk. The secondary leader should be gradually reduced in size by constant pruning to encourage the development of scaffolding branches on the primary leader. At the same time, gradually reduce the height of the secondary leader.
Prevention is the Key
The best way to prevent damage from a double trunk is to select young trees that are in good shape, with one leader and well-formed scaffolding branches. An ounce of prevention is much better than being yelled at by your mom!
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