Posted: 06/09/2012 at 14:59
Roundup should be able to kill this off, not sure if it is available in France. You need a weed killer containing Glyphosate which will be taken up by the plants and kill the roots. It only works when the plant is actively growing and may take a couple of goes if the foliage is dense. I would try cutting down all the foliage and then in spring next year gong over the new growth of weed killer.