Mark, I’ve had similar issues when grooving Pine. It tends to tear out for me at the start of the cuts. Sharpness is the best thing you can do to combat this. The next thing is to change the direction of the cut if you have that option – you may not. Sometimes you have to cut against the grain. Lastly, don’t bear down on the top of the plane. Gentle, shallow cuts at first really help. If you can get a small groove going, you can run your knife along the wall of the groove to help separate the fibers and reduce tear out.