Spot application of a glyphosate herbicide to individual purple loosestrife plants is the recommended treatment where hand pulling is not feasible. Glyphosate is available under the trade names Roundup and Rodeo, products manufactured by Monsanto. Only Rodeo is registered for use over open water. By law herbicides may only be applied according to label directions. Glyphosate application is most effective when plants have just begun flowering.
Timing is crucial, because seed set can occur if plants are in mid-late flower.
Where feasible, the flower heads should be cut, bagged, and removed from the site before application to prevent seed set. Another option is to apply glyphosate twice during the growing season. Foliage should be sprayed as described above, once when flowering has just started and a second time 2-3 weeks later. With this procedure control is likely more effective, because plants are not allowed to set seed and those missed because they were not flowering the first time are treated the second time. These plants, left unharmed, will be important in recolonizing the site after the loosestrife has been controlled.