Well let's look at the lowly comfrey plant. It is a perennial who self sows readily, although valueable in it's own right can quickly overtake a garden space. You are best to catch the little seedlings before they develop much of a root. Once established they are very hard to get rid of. I cut my flowers off 3 times a year.
They are a wonderful plant for attracting bees to the garden. Comfrey is used as a medicinal herb. It also speeds up decomposition in the compost. I use it to mulch, or smother, weeds around other plants too.