As contractors and consultants in the environmental field, Peninsula Environmental is very diversified in our knowledge and application of tools. As a Certified Arborist myself, I cherish the instruments that can help deepen my understanding and provide a more-comprehensive picture of environmental factors onsite. The air spade and the Microresistance drill are two of these tools. I like to describe the AirSpade as similar to a pressure washer, but it shoots out air instead of water. It is one of the least-invasive tools used to investigate tree roots and to improve compacted soil from construction and heavy machinery. The Microresistance drill measures resistance to detect and evaluate decay and cavities in trees and building timber with minimal impact.


Tree roots – though one of the most important elements of trees – can often be overlooked. Roots can account for 50 percent of a tree’s entire mass. Roots provide crucial stability and are integral for moving water and nutrients throughout the tree. When roots are torn and their soil compacted, their ability to perform these essential functions is reduced. Torn roots take longer to heal than cut roots and provide an entryway for disease. Compacted soil decreases aeration, root penetration, and water infiltration. This can lead to suboptimal root growth and instability, and this stress can cause or accelerate a number of different problems.
Figure 1: Kia Sutter of Peninsula Environmental uses the AirSpade to remove piled-up soil from around the trunk of this Douglas-fir and to improve compaction from machines.

The AirSpade is such an important arborist tool since it can help aid in these issues both before and after they occur. Ideally, before any construction, a tree protection zone would be installed around a tree so roots would generally be free of damage and compaction. But in practice, trees are often located close to construction and in pathways of heavy equipment.

If this has to be the case, what can we do? We can use the AirSpade to excavate the roots and blow out the soil so we can see where the major structural tree roots are heading. If possible, we can work with architects to design a plan that works around these major roots, instead of tearing them up in the process of excavation with giant machinery. Often, it’s hard to predict where roots are growing underground, so this takes the guesswork out of in in the least invasive way possible.

Another great way to use the AirSpade is post-construction by breaking up and aerating soil that has been compacted from heavy machinery. This can be achieved through radial trenching or vertical trenching (radial trenching creates trenches from the trunk outward while vertical trenching loosens pockets of soil all around the tree). These trenches get backfilled with native soil and compost, and then the entire surface is mulched. This breaks up the compaction, allows for more airflow, and adds important nutrients to the soil. It is also important to give a nice deep watering after any of this trenching.

Figure 2: Example of radial trenching post-construction. These trenches were later backfilled with a mixture of native soil and compost.

Microresistance drill

As arborists, when we look at trees and try to understand their health and disposition, usually in the form of a Tree Risk Assessment, we have a number of tools at our disposal. Just by looking at a tree, we can gather so much vital information. Is the crown thin or full, typical in growth and color? Is there any discoloration in the bark or branches that could signal stress? Are there obvious wounds or defects? Are there signs of pests or disease? We can sound the tree, dig around the roots, and use our magnifying glass to check for fungal activity. We can even climb the tree to get a better look at what’s going on in the crown.

These visual investigations can lead to great insight about the state of the tree, but sometimes we need more advanced tools to give us a better picture of what’s really going on inside the tree. While sometimes there can be obvious signs of internal decay, other times the tree outwardly appears to have typical health and vigor while battling decay on the inside. That’s where the Microresistance drill comes in. It is an instrument that allows us to drill a very small hole in the trunk of a tree to test internal decay. It gives us a picture of what’s going on inside with non-invasive techniques.

Using a very fine drill (1.2-millimeters wide and 500 long) made of high-quality steel, we drill into the trunk of a tree to test resistance. We change the speed of the drill depending on the species of wood. For example, if the wood is harder (like oak), we drill at a higher speed. Conversely, if it’s a softer wood, we drill at a lower speed. This results in the fullest possibly image. The resistance tells us the structural conditions of cell walls (have these walls broken down and decayed or are they structurally sound?), the variation between early and late wood in the annual growth rings (what season did the tree grow the most?), and the tree’s response to environmental factors like drought and excess rain.

Figure 3: Figure 4 from Frank Rinn’s article:Typical Trends in Resistance Drilling Profiles of Trees” overlaying Microresistance drill test results with tree rings.

Properly interpreting these results, and using those results alongside all the other information given, helps us to provide sound recommendations for all the trees we assess.

While the Microresistance drill and the AirSpade are two tools out of many at our fingertips, they both greatly increase our ability to make the best possible assessment of the situation. They give us a better idea of tree risk, and they can help reduce tree risk. They are incredibly useful in helping us help clients keep as many healthy trees alive as possible.

We can consult, design, and carry-out many of the projects that we propose.

For more information on our services and how we can help assess your trees, visit our website at