Narrow tree forks can cause split trees, St. Lawrence County expert warns
Saturday, September 6, 2014 - 8:31 am


Narrow Forks. It sounds like a picturesque town in the Cascade Range, or maybe in coastal Maine. Unfortunately, narrow forks are the reason that a tree splits during a storm, much like a wishbone. Well, high wind plays a role, too, I suppose.

No tree is perfect. Saplings may grow up leafing through “Cosmopolitree” magazine, hoping to look like their flawless Arbie dolls, but sooner or later they develop imperfections.

A common and potentially fatal (to the tree as well as any passer-by when it fails) problem is structural weakness caused by narrow forks. A narrow fork (also called union or crotch) develops when the angle of attachment between trunk and limb or between two competing (codominant) trunks is very small. The strongest attachments are open and somewhat U-shaped, with a wide angle approaching 90 degrees. Narrow forks get weaker with age and most eventually split. This type of defect can be corrected with a mere snip of a hand pruner when the tree is young.

Once the narrow union reaches a certain size, it’s usually too big to correct through pruning. In the forest, such a tree may split and subsequently die with no great consequence, but in our yards and parks, people and property are at risk when large limbs or codominant trunks crash to the ground.

When a tree with one or more narrow forks is identified, it really should be assessed by a tree care professional (preferably a Certified Arborist). In some cases the tree is too fraught with problems to be saved, but more often than not it can be saved with a properly installed cable brace system.

My hat’s off to all the ambitious do-it-yourselfers out there, but it’s important to have cabling done professionally. The American National Standards Institute (ANSI) has standards for—let’s see—sewer pipe, steel bridges and, here it is, tree cabling. Factors like cable diameter and type of eye bolts to use are critical. The correct hardware is not available at any home-improvement store; it has to be ordered from specialty suppliers. Also, since proper height of a cable system is essential, equipment and training for working aloft safely are needed.

It’s vital that a cable be installed between two-thirds and three-quarters of the way from the weak fork to the top of the tree. Of course, a cable is not wrapped around the trunk, since that would weaken and eventually kill the tree above that point. Either a drop-forged eye bolt or a J-shaped lag screw is used to secure the cable end to the tree. The correct-sized hole is drilled through the tree (for bolts) or into the tree (for lags). Since bolts are much stronger, they’re used for larger wood and for cases where decay inside the trunk is suspected. Lags are cheap and easier to install but are used only for small wood where there is no danger of internal decay.

Lest you fear you’ll end up with a Frankentree in the yard, don’t worry. A proper cable system is inconspicuous to the point that you may have to squint through binoculars to be sure it was actually installed. For a fraction of the cost of a removal (and a tiny fraction of the cost of emergency removal plus damage repair) your narrow-fork tree can get an extended lease on life.

While under extreme conditions even an ideal system may fail, I’ve never seen a properly installed cable system break. I have, on the other hand, seen many homemade or substandard ones snap or even rip out of the trunk. It’s best to find a tree-care professional who belongs to organizations like the International Society of Arboriculture (ISA) or the Tree Care Industry of America (TCIA). They should be able to produce a copy of the ANSI-568 cabling standards, as well as proof of insurance. This way you can visit Narrow Forks if you so choose, instead of having a narrow fork “visit” you during a storm.

Paul Hetzler is a forester and Cornell Cooperative Extension of St. Lawrence County horticulture and natural resources educator.