Splitting a wishbone may bring good luck to the person who gets the larger piece, but when a wishbone-shaped tree rips in half, it’s bad fortune to all. One might think the trunk lying on the ground is the loser, but the tree left standing is toast as well; too badly damaged to be safe.
In the majority of cases, we can keep a double-trunk tree from splitting through preventive care.
Welcome to Narrow Forks!
There should be a town called Narrow Forks—it seems like a cute name. For trees, narrow forks or unions occur when the angle of attachment between two competing or codominant trunks is acute (as opposed to cute).
The narrower the angle at which two trunks meet, the weaker they are attached to one another. Narrow unions always get worse with age, and usually end up failing in a storm or under an ice load. Strong unions are open and U-shaped.
In part, having more than one trunk (in trees, anyway; unsure about elephants) is genetic. Environment plays a part, too: forest trees usually develop strong central leaders*. But open-grown trees get “confused” and many of their branches become leaders.
*Leader: the vertical stem at the top of the trunk.
The bulk of tree imperfections are safe, but some are not. If a building, play area, or other “target” is within striking distance of either half of a large wishbone tree, action is needed. Obviously, trees with major defects can be removed. However, many times a weak union can be safely braced.
The dormant season is the best time to have landscape trees evaluated, as tree architecture is easier to see without leaves. That said, a hazard assessment at any time of year is better than none.
Although a tree with additional problems like damaged roots may have to be removed, judicious pruning along with the right cable system can save most multi-trunk trees.
No DIY, Please
A word to the do-it-yourself crowd: An improperly installed cable can make the situation even more dangerous. Cable systems must conform to the American National Standards Institute (ANSI) A300 Part 3 Supplemental Support System Standard (apologies; I would’ve picked a shorter name). While it sounds like a government project, the ANSI A300 is industry-written and research-based. It specifies minimum size and load ratings for each element; cables, eye bolts, anchors, etc., depending on tree size.
Sometimes, a synthetic line is preferable because it allows more movement. Other situations call for galvanized steel cables and bolts. Either way, a Certified Arborist who knows the ANSI standards should do the design and installation. Proper cable systems are inconspicuous, located two thirds to three quarters between the fork and the treetop.
Save Money, Save a Tree
For a fraction of the cost of removal, most wishbone trees can get an extended lease on life this way. Recent storms remind us that microbursts and derechos can trash structurally perfect trees.
Extreme weather events aside, I’ve never seen a properly installed cable system fail. On the other hand, I’ve seen improvised or substandard ones break.