Hmm...it looks like a member of the Bistort family(Polygonaceae), but I can't place it any closer than that.

I dont think the leaves are right for Polygonaceae. Could this be from the Taraxacum Dandelion family?

It looks like a type of Dock (Rumex crispus) to me - the stock is the flowering part that spreads their seeds. In the background it looks like some Pineapple weed and some Jacobs Ladder, though it is difficult to give a definitive answer from a pic.

Just dig it out - it should have a big tap root, so get it all out if you can. Its definitely a weed, but not a serious one like Knotweed.