Lemna, or common duckweed is a very small light green free-floating aglae. Lemna has 1 to 3 leaves, or fronds. A single root (or root-hair) protrudes from each frond. It tend to grow in dense colonies in quiet water, undisturbed by wave action. Often more than one species of Lemna will be associated together in these colonies. They can be aggressive invaders of ponds and are often found mixed in with mosquito fern or watermeal. If colonies cover the surface of the water, then oxygen depletions and fish kills can occur. These plants should be controlled before they cover the entire surface of the pond.