This is an 'American skunk cabbage'. (Not the most flattering name!) There were lots of them growing in the boggy ground beside the stream that runs through Harlow Carr gardens. They were an evil, neon yellow and, whilst sculpturally arresting, they had something of a malevolent look, I thought. Apparently they are very invasive and cannot now be bought or planted. The plants at Harlow Carr are carefully controlled to prevent their spread.
The other yellow bog plant below - a marsh marigold - is altogether more benign looking, and good for attracting bees and butterflies.