Loose knots are a pain in the planer.  I've had them cause problem too.  

I've turned some ash juniper (aka mountain cedar)  which is native to Central and part of West Texas and it is a joy to turn.  As cedars go, it is actually fairly hard.  Eastern red cedar is pretty nice to turn too but you gotta turn it before it cracks if you want to make a bowl from it.  

--Nathan, TX. Hire the lazy man. He may not do as much work but that's because he will find a better way.