Duckweed is a group of tiny floating plants which can be seen covering still water in a carpet of green. The still waters found around the world, rich in dissolved salts, allow aquatic plants to grow and absorb enough salts for their needs.
Duckweed is a small, flowering plant found in still waters. They grow close together, resembling a carpet or field of green. Duckweed provides food for many insects and animals and helps remove pollutants from the water.
Some aquatic plants are anchored to the bed of the water via their roots. Still, others are free-floating and have the advantage that they can grow in areas where the water is too deep for bottom-rooted plants, thus avoiding competition for space that can often occur in shallow waters.
By floating on the water’s surface, they are also not competing for light with water weeds that are submerged or the many types of algae found in rich waters.
Some of the smallest aquatic flowering plants are duckweeds. They can grow where the water is too shallow for aquatics and can often be found at the edges of fields or filled wheel ruts. Duckweed can also be found in small pools such as the depression left by a cow’s hoof print.
Duckweed is often seen as a carpet of green over a stretch of still water. The still water will usually have a muddy bottom rich in decomposed organic matter. Duckweed has often been given some local names to scare children away from the depths of the water it is hiding. Jenny Green-teeth is a name given to duckweed in Britain. She is a pond-elf who lurks just out of view in duckweed-covered waters.
Structure
When looking at a carpet of duckweed, you will notice that it consists of innumerable small leaf-like structures called ‘platelets.’ Each has the properties of a stem and a leaf and has a different name. The platelets are called a thallus, with the plural being thalli. If you pick one up, you will notice they have one or more roots hanging below.
Flowers often grow on duckweed that is fully exposed to the sun. The flowers are tiny, consisting of a single flower just 1mm across. The flowers have one ovary and two male flowers, each with one or two stamens. Both the male and female parts are enclosed in a hooded sheath in a pocket on the thallus edge.
There are 38 species of duckweed. Here are some of the most common.
Giant duckweed
The largest species of duckweed is the giant duckweed. These can grow large, with each thallus up to 15mm long. They are green on top and a reddish-purple below. In autumn, giant duckweed produces purple thalli. These are called turions which fall away from the thallus to the bottom of the water.
They survive winter here with a much better chance than if they were floating at the top where the water is prone to ice over. Once spring comes, the turions become buoyant and rise to the surface, where they resume growth and produce green thalli.
Common duckweed
True to its name, common duckweed can be found in many still bodies of water worldwide. It can be seen covering the entire surface of the water, giving no room for other aquatic plants. The talli of the common duckweed is up to 4mm across with a single root. They flower in June and July with a green bloom.
Rootless duckweed
One of the smallest flowering plants, rootless duckweed, has a thallus that measures about 1mm. They resemble algae rather than flowering plants and are spread by a new thallus budding from the end of an old thallus.
Submerged duckweed
Some duckweed spends most or all of their time submerged. Ivy duckweed has a longer, elongated thallus than other species and tapers into a short stalk at the base. Two more thalli come up from the base at right angles to the original thalli. These are stunning to view as they appear in a three-dimensional pattern built up of interconnecting thalli.
Ivy duckweed is noted for its dark green colour, while its thalli are translucent.
When it is ready to flower, they appear similar to other species. They send up fertile thalli to float on the surface of the water. These are paler green, smaller, and oval than the submerged thalli and more similar in appearance to typical duckweed thalli.
Duckweed copycat
A plant similar to duckweed is a fern rather than a flowering plant. Azolla filiculoides or mosquito fern is common in North America. The fern floats on the surface of still water, forming carpets similar to duckweeds. It can multiply in long, hot summers and can overcrowd other aquatic plants to the point where it will push them out of the water.
Azolla filiculoides don’t only grow aquatically. It can also survive on damp sites, such as mud or wet moss. Although similar in appearance at first glance, it has a different structure, with two ranks of leaves.
