Least Duckweed is native to northern and southern America. It is thought that its introduction to Britain was a mistake that occurred when it was brought over as a contaminant on aquatic horticultural plants. It was most likely transported between waterways on clothing or equipment or by animals. It has been recorded at Eythrope Lake and Chalgrove Brooke in the River Thame catchment. Unfortunately it is now the most common species in some areas and is likely to sill be spreading out further. You can identify it by its leaves that are 1-2.5mm long. Our picture here can be deceiving as it is actually pale green-grey in colour with almost see-through leaves.