Duckweed (Lemna minor) is a tiny plant with a thread-like dangling root that floats upon the surface of still freshwater. Native to North America and Eurasia, this aquatic plant provides food to fish and waterfowl and can control algal blooms in waters that are too warm and in bright sunlight. It is a hardy plant, surviving in regions where winter temperatures dip as low as -40 degrees Fahrenheit and in warm summer regions where highs in the 90s are not uncommon. As long as fresh water is unpolluted, winds scarce and the water surface not fast-flowing, duckweed will quickly multiply in the growing season to create a delicate floating mat of green.
Acquire duckweed from garden centers or pet supply stores with aquarium supplies and live fish.
Float duckweed in a natural or artificial water feature that is filled with fresh water.
Ensure the water is still or very slow moving. A stream or waterfall that ripples the water surface is not conducive for duckweed's growth.
Locate duckweed in a sunny area of the garden or pond where it will receive at least four to six hours of direct sunlight. While tolerant of shadier spots, its growth will be slower and seafoam-green color not as intense if the indirect light is too dim.
Consider over-wintering a cup full of duckweed in a bowl of water indoors on the windowsill in areas where winters are very cold or if outdoor container water gardens are drained over the winter. Bright light exposure and cool water is all that is needed.