Broadleaf weed killer like 2,4-D or highly translocative Amitrol-T, would be way more effective than round up, not sure if any of those products are registered for use in your area. If one was to use a herbicide, it is always better to cut it down and just spot hit the shoots that come back, using the least amount of product as posible while it is actively growing. There may be some IPM measures of contol you could look into first. Just a suggestion, jim.