On my walk up my street this afternoon, every other verge had agapanthus, Agapanthus praecox, growing healthily. Definitely the predominant species round these parts.
Why it's a pest plant
It seeds prolifically, spreads seeds effectively which germinate densely. Fragments of the roots easily regrow. Plants live a long time and can handle a wide range of conditions and soil types.
Wherever it grows, it forms solid clumps. I've seen layers and layers grow on top of each other. It bullies all other plants out and takes over.
For information on getting rid of agapanthus see Weedbusters,