We use Polaris, which is an Imazapyr product. Mix it with a good surfactant (Activate+ is what we use). Its systemic and good for control over a long period of time. (Usually one spray job will keep it down for the year.) Also good for most other emergent vegetation as well. Just make sure you're not going to get any rain within 24 hours of spraying, if it gets watered down it isn't effective.