A large pink flowering plant has grown up in my garden which has spread all over the garden. I believe it is some sort of invasive weed ( I have checked the Himalayan Balsam but it does not seem the same). I cut back the stems and flower the leaves are a diamond shape green leaf and the flower is a pink cluster, the base of the stem is bamboo like hard, brittle with a hollow centre. The root has several tentacles and it spreads far and deep. When dug up I found clustered around lots of white pod like shapes I have never seen before, almost like small bones in various sizes, when broken apart they had a hollow centre, they broke cleanly, when stamped on they had a sticky substance. I have dug them all up but am terrified it will come back. Can anyone tell me what it is and how I can get rid of it permanently. The soil is quite dry with clay deep down and it seems to prefer the semi shade although it looks like it will grow anywhere.