Filamentous Green Algae (Pond Scum)

Picture of a patch of filamentous green algae floating in a stream.
Scientific Name
Cladophora, Pithophora, and Spirogyra spp., and others
Includes families in divisions Chlorophyta and Charophyta

Filamentous green algae forms green, cottony masses that are free-floating or attached to rocks, debris, or other plants. It consists of fine, green filaments that have no leaves, roots, stems, or flowers. They often form dense mats. On warm, sunny days, they commonly float when bubbles, generated by the plant or created by its decay, get trapped in the mats and make them buoyant. The three genera listed below are examples.

  • Cladophora feels cottony and can form balls that float when the core decays; magnification reveals long, slender cells and a branching habit.
  • Pithophora is sometimes called “horsehair algae” because of its coarse texture, which can feel like steel wool.
  • Spirogyra is bright green and slimy; magnification reveals the chlorophyll-bearing pigments are shaped as spirals; the filaments do not branch.

Note: Not all types of green algae are filamentous; they occur in an great variety of forms, including single cells and colonies.

Other Common Names
Where To Find
image of Filamentous Green Algae Moss Pond Scum distribution map


Green algae grow in practically any water that is capable of supporting life and receives good light.

In early spring and again in late fall, hairlike strands of filamentous green algae have a heyday in forest streams, when leafless trees permit light to reach the water.

When a body of water receives too many nutrients — such as decaying animals or fertilizer or manure runoff — green algae can grow (“bloom”) in quantities that unbalance the ecology of the lake, pond, or stream.

As a base of aquatic food chains, algae are tremendously important for healthy rivers, streams, ponds, and lakes, and all the human interests that involve water, including boating and fishing.

When filamentous algae gets out of hand, it can impede fishing and swimming and clog water intake screens.

Green algae is vital for healthy aquatic ecosystems. As the base for aquatic food chains, it feeds and shelters tiny animals, insects, and young fish.

Greenside darters spawn in late March to the end of April, earlier than most Missouri darters. Timing is everything: this species of fish typically attaches its eggs to strands of filamentous algae (Cladophora spp.), which has its big "bloom" in early spring before trees gain their leaves and shade forest streams, causing the algae to decline.

Algae used to be called “primitive,” but a more accurate term is “ancestral” for organisms that have not changed much for billions of years.

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Similar Species
About Wildflowers, Grasses and Other Nonwoody Plants in Missouri
A very simple way of thinking about the green world is to divide the vascular plants into two groups: woody and nonwoody (or herbaceous). But this is an artificial division; many plant families include some species that are woody and some that are not. The diversity of nonwoody vascular plants is staggering! Think of all the ferns, grasses, sedges, lilies, peas, sunflowers, nightshades, milkweeds, mustards, mints, and mallows — weeds and wildflowers — and many more!