Make sure you use the proper surfactant with the Rodeo or your results will be less than optimum. Cidekick II is a commonly used aquatic surfactant. Make the mixture on the strong side of the recommended dosage which usually has a low and high suggested rate. Please keep us advised as to your progress so others can benefit. As with many herbicides timing and proper application are important to success.
Rodeo Label wants the non-ionic surfactant to be 80% active ingredient. Normal concentration range is 1 oz (0.7%) to 2 oz 1.5% of Rodeo per gallon of spray. I would use at least 2 oz per gallon for spatterdock after they have flowered or at end of flowering. Retreat remaining green leaves after 3 weeks.
Label:
http://www.domyownpestcontrol.com/msds/Rodeo%20Herbicide%20(2.5%20gal)%20Specimen%20Label.pdf
CideKick II
http://pentairaes.com/cide-kick-ii-surfactant.html
