You say it should be divided ever so often. I wish that was my problem, instead it has taken over a large area in my flower garden. Started digging it up but it is impossible to get all the roots, they go deep and spread. what can I do to open up my garden again?
Yes, in some regions it can become invasive or aggressive.
Glyphosate herbicides are very effective for killing loosestrife. Glyphosate is available under the trade name Roundup. Glyphosate is nonselective; however, selective application techniques allow it to be used effectively with minimum damage to desirable plants. It is taken up through the leaves or young stems and will kill any plant that it is applied to. Therefore, treat only the loosestrife plants and avoid contact with valuable plants.