More ideas.
Duckweed: Growing duckweed in polluted water will purify it. Hearthlings could put the plant in water barrels and after some time it becomes Pure Water. This could be a halfway point between regular water and Spring Water. Or maybe give it unique uses, like cleaning wounds.
If Jorb wants to make cisterns annoying, water in them could eventually become Stagnant Water and one must add duckweed to change it back. Stagnant Water can still be used, but lowers quality of recipes its in. Additionally a new critter, mosquito larva, could spawn in Stagnant Water. Mosquito larva can be used as bait or chicken feed.
Fringed Lilly also known as Floating Heart: When picked it gives leaves and/or flower. Leaves can used as salad greens. Flowers can also be eaten or studied.
Arrowhead: Although most are native to the americas, some are from Europe. It has a starchy tuber, similar to a potato. Hearthlings could finally make french fries!