stump inoculation

I have some freshly cut poplar trees. The stumps are almost 24" in diameter. I'm not sure how much plug spawn I need to use. Does anyone have a rule of thumb?

If you place the plugs 4" apart, then you will need 56 plugs per foot of log.

If you place plugs 6" apart, then you will need 25 plugs per foot of log.