2 4 D Amine is the cheapest method of control and it absolutely will smoke dog fennel. 24-32 oz per acre in minimum 10 GPA will get it. If it's well developed go with a quart per acre or 3 1/2 oz per gallon of water. Add surfactant at 1/4 of 1% by total volume or 1/3 oz per gallon of water.