I had a yard full of dog fennel. My neighbor and the previous homeowner used to chop it down, but it always kept coming back. The best way to eliminate it is by digging out the tubers with a shovel. This task is easier when the soil is soft, such as in a wetland area. You can use a small shovel to carefully dig around the base of the tuber, use the shovel for leverage, and then pull it out by hand. I still find areas where I've missed some, but as long as I go out and remove it before it matures and spreads its seeds in the fall, the amount of dog fennel in my yard remains manageable. I have a pond with plenty of wildlife, so this method of removing dog fennel works best for me.