I think of this quote, from Wizard of Earthsea, often when I am working to eliminate Russian Knapweed and Dock from the property.
You don't just spray them with something you grabbed in home depot and walk away. To get rid of them you need to understand them. Their lifecycle. You need to be able to recognize them in all stages. To know their smell and the color of their roots. To know how they change the soil. To know when they grow upward and when they grow down.