"Well, since the fluxweed has got to be picked at the full moon and
the lacewings have got to be stewed for twenty-one days ... I'd say
it'd be ready in about a month, if we can get all the ingredients."
-- Hermione Granger (CS10)
A mundane plant used in potions.
- An ingredient in Polyjuice potion, for which it must be picked at the full moon (CS10).
The author wrote on Pottermore that she chose the plant as a Polyjuice Potion ingredient because the people involved are in a state of flux or change: "the mutability of the body as it changed into another (Pm).
Isanthus brachiatus, a member of the mint family also known as "false pennyroyal" (pennyroyal being another member of the mint family). Fluxweed is native to the eastern United States and is a threatened or endangered species in several areas (PLANTS).
Ingredient used in Polyjuice Potion.
"flux" = "change"
When Hermione is explaining how long it will take to make the potion, it is implied that she and Harry and Ron will need to pick the fluxweed themselves. It would seem, then, that this North American plant must be kept in the greenhouses or else grow somewhere else on the Hogwarts grounds.