Yellow skunk cabbage : Lysichiton americanus

From the internet:
This North American relative of the arum lily is an asset in any bog garden. It is a native of peat bogs and marshes and, given an acid soil with plenty of wet humus, it will settle down after a season or two and produce its startling flower-head in mid-spring. The flowers themselves are tiny and clustered on a cylindrical spike or spadix. This is surrounded by a beautiful and conspicuous bright yellow spathe, which is gradually overtaken by the developing huge leaves. Plants may also be grown in shallow water at the edge of the pond.