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Measuring Decay with a Resistograph

The Resistograph test is the most current and reliable method available for determining if your tree is stable. This test is invaluable for further diagnosis when a tree shows signs of instability or weakness, such as cracks, root decay, and leans. Not every tree needs the resistograph test, but there is no substitute for this crucial test when it is indicated.

The Resistograph machine uses a 19-inch long, 1/8-inch diameter drill bit. When drilled into the tree, the wood’s resistance to the drill is recorded on a strip of paper (it looks like an EKG). We usually drill four points at the base of the trunk and the major root flares. Sometimes it is advisable to climb the tree with the Resistograph to test branches, crotches, and trunk cavities. Using a mathematical formula, we can then determine the precise amount of hollowness and/or decay in the area drilled. This test is slightly invasive but does not harm the tree.

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