ResistoSometimes a tree develops problems that cause it to be unstable or weak. These problems may be visually obvious, such as a cavity or mushrooms at the base. They may also be detected when the trunk or root flares (the part of the root coming off the trunk above ground) sound hollow when tapped with a mallet (lots of experience is so very helpful here!). To find out whether the tree is in danger of failure (falling over), it's very important to know if decay is present, and how extensive the decay is. The Resistograph® machine is used to answer these questions.

How It Works

The Resistograph uses a stainless steel drill bit that is 1/16 inch in diameter and about 20 inches long. When the drill bit is pushed into the tree by a gear in the machine, the wood’s resistance to it is recorded on a strip of paper or displayed on the machine's computer screen. It looks like an EKG. By analyzing the results, the arborist can tell whether the tree is hollow, decayed, or solid. A flat line indicates no resistance to the drill: a hollow or decayed area. A jagged line indicates resistance and stronger wood.

Does it Damage Trees?

Drilling a tree with a Resistograph is considered only slightly invasive. It causes no long-term damage. The hole which is left by the Resistograph's small-diameter drill bit can be easily sealed off by the tree during a couple of years' growth.

Where Do We Drill?

Heritage white oakThe most common areas that are drilled are the lower part of the trunk and where the roots come off the trunk (the root flares). The tree is sometimes drilled at two points, but occasionally more drillings are required, depending on the problem. Occasionally the junction where a large branch meets the trunk higher up in the tree needs to be drilled if the inspector suspects decay there, such as when there is a cavity or mushrooms can be seen.

Test Results

Our arborists depend on the results of Resistograph testing to help them decide whether or not to condemn a tree. When we make a recommentation for removal, we want to be able to provide our customers with solid evidence of why it's necessary. So even though performing this test adds to the cost of a tree inspection, doing it is far better than relying on guesswork.

Southern Red EAST

With the computerized Resistograph, we can generate graphs like this to be able to visualize and quantify the amount of decay inside a tree. For this 47" DBH (diameter at breast height) Southern red oak, Peter drilled 4 inches up the trunk. Reading from right to left, the graph shows that the trunk is solid (blue) until 4-1/2 inches into the tree. At that point to the end (yellow), the tree is hollow. Sadly, this tree presented high risk, and had to be taken down.

Core Sampling is Not the Same as Resistograph Testing

Foresters use core sampling to determine a tree's rate of growth. Resistograph testing does not provide this information.