Glen - I think you'll find that RODEO (or any of the other aquatic glyphosate formulations) will be ineffective on watershield. Granular 2,4-D (AQUA-KLEEN or NAVIGATE) or SONAR (applied early season to static ponds) are the only two treatments that I know of that will control this particular specie. Glyphosate doesn't seem to move (translocated) very well in this plant. Also, the slimy coating on the submerged stem and underside of the leaf provide it with protection from other herbicide and biological options. Good luck.
