Looks like Sago to me... I use 5:1 Chelated Copper (Pondweed Defense) to Diquat (Reward). It does the trick for most pondweeds (Widgeongrass included). So either way, this mix should get you there. Just follow the label on the copper to get your amount needed, then add Reward at 1 part for every 5 parts copper.
