European water clover
Marsilea quadrifolia

European water clover (Marsilea quadrifolia) is an invasive plant. It is a native of Europe and Asia and was first detected in Connecticut in 1862.

M. quadrifolia is an aquatic fern that grows from creeping rhizomes anchored in the muddy bottoms of shallow ponds and lakes. It poses a realistic nuisance threat to ecosystems as it may crowd out native wetland plants by forming dense stands.
Summaries of State Plant Protection Laws and Regulations
Connecticut   Oklahoma