Posted: 05/06/2013 at 21:31
Chinese lanterns; I spent 3 years trying to get them to grow, and then they tried to take over. It took days to dig them out and filled 3 sacks with roots.
Nigella, it self seeds like a maniac, took years to get rid of it.
Welsh or Icelandic or Californian poppies; I don't know which, I allow it but am ruthless about pulling them out if they are getting too invasive.
Cowslips, they get dug up after flowering, I only allow a few to remain.