1. Placement of the trench: Place the trench at least 100 feet from the last known symptomatic tree. There is a buffer zone of healthy trees between the diseased trees and the trench. This is very, very important. 2. Sever all of the roots. This is where we got into trouble in Tierra Linda. The trench must be dug to a depth sufficient to get all of the roots cut. In the past, most of the equipment used out here was a bulldozer pulling a ripper bar behind it. Some of you probably saw the machine and the big mess that it made. That was the best thing that we had at the time. The problem is the rocky soil out here. There is so much rock, ledges, shelves, and fractured rock, so there are roots that were impossible to reach and sever using a bulldozer with a ripper bar. The ripper bar could hit a rock and them ride up over some roots, leaving some root connections that spread the oak wilt. That was in the past.
Now there is new equipment available. There are now rock saws that have the capability of cutting 54 to 60 inches deep. This is a giant machine that does excellent work and can cut through just about anything. These new rock saws have been in use for the past two or three years, so it is a little early to know the effectiveness of these rock saws versus the early saws that did not cut as deep. In 6 or 7 years, sufficient data will be available to evaluate the effectiveness of these large rock saws. Preliminary indications are that the effectiveness of trenching will increase from 65 - 70% to 75 - 80%. Trenching with a 54 or 60 inch rock saw costs $3.25 to 3.50/linear foot. The trench can be covered immediately after it is installed (it does not need to stay open).
The Forest Service does not recommend using the smaller 36 inch rock saws you can rent because they can only cut 28 inches deep, which does not sever all of the roots. Bulldozers pulling a ripping bar are not an option for Tierra Linda because they do not sever all of the roots in our rocky soil, which results in a high frequency of trench failure.
The large belt trenchers used for highway work to install large sewer lines work well, but they cost $3,000/day.
Backhoes can be very effective where there is deep soil and not a lot of rock. There are many backhoes available so the cost is only $1.50/linear foot. A backhoe can trench up to 4 feet deep, so it can achieve the depth required for effective oak wilt control. The trench is wide enough that you can climb down into the trench and inspect for roots. You just keep digging deeper until you don’t see any more roots.