That is a tough knot! You treated it as if it were not there, but it looks pretty solid and is probably harder thn the wood itself. Plus, with the grain riding up on the back, you need to hollow it on the belly to keep the thickness somewhat consistent. As it is, the area is twice as thick and twice as wide. That makes it 16x as strong as it surroundings. Not counting it being harder which only makes it worse.
Looks like you need to drill it out and go with the surrounding wood. I don't think you could trust it to stay put. Besides, thats the point of big knots like that right? Making the limbs with gaping holes.
Jeez is there that big a shortage of hedge over there in east IL? Poor feller.