Posted: 14/08/2012 at 11:53
I had a couple of these plants last year and this year they covered an area of about 6ft by 4ft - they totally take over! To get rid of it firstly cut all the flower stems and ensure that none of the seeds drop onto the ground, then use Roundup ensuring you spray ever leaf (if you don't it will keep growing), it will start to die off. The top will begin to die off but don't be fooled as it needs time to get into the root system. Respray again in about a week, again ensuring you spray all the leaves. Then leave for another two weeks by which time the top will have completed died off and Roundup will have worked on the roots and it can then be dug up. 
