Look like Lizard's Tail to me.
(
Saururus cernuus)
Quite invasive if let loose, basically a weed.
How big is it?
If it's over 2 feet tall, it could be Japanese Knotweed.
(
Fallopia japonica, syn. Polygonum cuspidatum, Reynoutria japonica)
Horribly invasive. Worse than a weed.
Both plants are in the same family.