Rodeo is registered for use over open water and is the most commonly used herbicide to control purple loosestrife. Glyphosate is nonselective and should be applied carefully. Excessive application, allowing the herbicide to drip, can kill desirable plants under the loosestrife. Application to the tops of plants alone can be effective and limit exposure of nontarget species. Herbicide treatment should be conducted as early as possible during the manufacturer's recommended time of application in order to kill the plants and prevent seed production.
Application is most effective when plants have just begun flowering.
Timing is important because seed set can occur if plants are in mid- to late flower. Where possible, the flower heads should be cut, bagged, and removed from the site prior to application to prevent seed set. Another option, which may be more effective, is to apply glyphosate twice during the growing season. The plants should be sprayed as described above when flowering has just started and a second time two to three weeks later. Application of ghyphosate from a vehicle-mounted sprayer is usually necessary in areas with extensive stands of purple loosestrife.