I asked our friends Gab and Geneviève at La Ferme Älska their opinion. Here was their response:
In order to limit damage, we use a 1/4” drill bit, whereas the old-timers used a 5/8. They used to go 2” deep; we go barely 1” deep because research has shown that yield of sap has nothing to do with size or depth of the drill hole. Healthy trees will heal by the following spring and often we are not even able to find the hole from the previous year or years. We all know that an open wound has a much higher chance of infection which brings me to my next point.
Research has also shown that the actual hole is indirectly not the problem. It’s the bacteria that can enter from the hole that causes all the problems (by not drilling deep we never reach the heart [coeur]; if the bacteria reaches the heart, it’s game over, as it will spread down into the roots). That is why we use seasonal taps so the part that goes into the tree is always free of bacteria, and we clean our tubes with alcohol after every season to create an environment where bacteria just cannot grow.
As far as treating the hole after the season, we tend towards letting the trees heal on their own. I know of some studies that have been done where they filled the hole with some sort of a “putty” after the season. The results were quite interesting because it led to some major rotting issues. The tree produces an enzyme that eats bacteria in the wound (self defence), but that enzyme is moist. So when they filled the hole with putty, the enzyme became trapped because of the lack of air flow, which resulted in the wood starting to rot around all the filled holes.
Location of the tap is the most crucial factor in the amount of sap we can possibly harvest from the tree. Basically picture an old hole in the tree: draw an imaginary rectangle 3” on either side and 6” above and below the hole. That is the No Tap Zone . The wood in that rectangle is compartmentalized and shut off for future sap flow--perhaps another self-defence mechanism in order to prevent spreading bacteria.