Premise: Pontederia cordata, common name pickerelweed (USA) or pickerel weed (UK), is a monocotyledonous aquatic plant native to the American continent. It grows in a variety of wetlands, including pond and lake margins across an extremely large range from eastern Canada south to Argentina. A few examples include northern rivers, the Everglades and Louisiana.
Hypothesis: Pickerel weed was introduced to the UK.