in The Forager’s Harvest by samuel thayer (really good book by the way) he says:
-wapato can be harvested beginning in late summer although they will not be fully grown until the plants begin to die back
-he prefers to harvest in late sept and early october (he lives in wisconsin)
-in areas where the water doesn’t freeze you can harvest all winter
-when he was first harvesting it he waded into the water and dug at the bottom with shovel and the wapato floated to the top
-later he used a method recommended in Stalking the Wild Asparagus, which was to put on waders and rake the mud with a potato hook
-now he uses a technique recommended by john kallas, it’s a “stomping” technique: on a warm fall day wear some old shoes, get in the water and start stomping around, occasionally flailing and kicking your legs. this loosens the mud and allows the wapato to float up. also, moving so much keeps you warmer than if you were just standing there.