Rosate 36 & some patience is what you need, or Roundup 3000 if you can't get the Rosate. Use it in its strongest recommended form & spray on the leaves 1 or 2 times this year. It should start to die back pretty quickly & any new shoots that come through, and there will be some, wait until they are about 3 ft tall and with nice big leaves and then spray them. Do this a couple of times this summer & you'll notice a big difference next spring, you won't kill it all in one year but you should get 70-80% of it, and what is left will be very weak & spindly in comparison. Simply repeat again next spring / summer. I would think after 3 years it will be all but a distant memory. If it is growing amongst flowers I would suggest injecting the stems first, simply pierce a hole in the top of one of the sections of the stem, just above ground level & inject some diluted weedkiller into the stem, I used a couple of cheap, plastics syringes I bought off eBay. Another possibility is to wipe the weedkiller on the leaves but I don't have any experience of doing that so it would be a try it & see job. This stuff is nasty & if left alone will happily take over your gardens, but it can be stopped. Good luck
