Duck weed has three species. It is a tiny 1 to 3 leafed or lobed green plant with a single tiny root hair that drops down from the middle of the 'leafbase'. The whole single plant will be about 1/2 the size of your smallest finger nail. For a pretty good picture and cause see this:
http://www.lakerestoration.com/t-duckweed-control.aspxIf you have duckweed this is another good sign the pond needs to be dredged and deepened. The pond is telling you it is going toward swamp conditions. It only grows in very nutrient over enriched calm water bodies. Where I live, I see it most often in drainage ditches that receive septic waste as the nutrient source. It is characteristic of very old ponds that have become nutrient over enriched from annual deposits of nutrients and organic materials.
