Hi! We laid down a patio in our back garden last summer, but only a couple of months later, a plant we thought was dead and done for came back to life and lifted the patio on one corner.

The plant is (I think) Acanthus mollis (Bear's breeches?). It must have been there for more than 10 years. I thought a cold winter had killed it, then we cut it right down, and finally we hired a guy to dig it out completely. The thing was that some of its roots must be under the patio we have at the back, so it would be impossible to dig it out completely.

Photo #1 is the hole, photo #2 is an internet search and characterisation of the plant's inner self.

So my question is: 1) What poison actually kills this? 2) Will filling the hole with concrete help stop it?

Thank you so much!