I'm still fighting these damn willows in my dam.. Completely dug them out when the water dropped from drought trees that have been gone underwater for the last 2 yrs are all growing again.. Really don't know if I'll ever beat them..
It has been my experience that some willow roots can grow up to near 40 feet. Pretty much as long as they can get near water and be exposed to open air / sunlight they can grow. I auctually physically walk around ponds from time to time pulling any new trees up and get all the roots I can. I then lay them out to dry before disposing in the dumpster. You may want to consider getting an excavator and removing them and refilling area with clay. It has been my take that a bulldozier will only spread some roots around while removing the problem it may create a later problem.
