How to battle pond algae

Spring time brings warm weather and loads of calls about Pond Scum.  Filamentous algae are single algae cells that form long visible chains, threads, or filaments. These filaments intertwine forming a mat that resembles wet wool. Filamentous algae start growing along the bottom in shallow water or attached to structures in the water.  The algae then traps air and floats to the surface forming large mats, which are commonly referred to as “Pond Scrums.” There are many species of filamentous algae and often more than one species will be present at the same time in the pond.  To control filamentous algae, a homeowner will probably need to consider a multi-system approach from deepening pond banks to adding fertilizers or pond dyes along with grass carp and possibly some herbicides.      

Since these algae grow in shallow water, deepening the pond’s edges is one of the mechanical controls sometimes recommended.  Filamentous algae can also be raked or seined from the pond.             

Fertilization to produce a phytoplankton or algal “bloom” can prevent the establishment of filamentous algae if started early enough in the spring. Fertilization also produces a strong food chain to the pond fish.

Non-toxic dyes or colorants prevent or reduce aquatic plant growth by limiting sunlight penetration, similar to fertilization.  However, dyes do not enhance the natural food chain and may suppress the natural food chain of the pond.

Grass carp will seldom control aquatic vegetation the first year they are stocked. They will consume filamentous algae but it’s not one of their preferred foods. Therefore, they will usually consume other types of submerged vegetation before they consume filamentous algae. Grass carp stocking rates that will control filamentous algae are usually between 10 and 20 per surface acre.

Copper sulfate or “blue stone” is probably the most commonly used algal treatment because of its availability and low cost. Another product that is recommended is Cutrine Plus.  All copper compounds can be toxic to fish if used above labeled rates and can be toxic in soft or acidic waters even at label rates. Please contact your local Extension Office with any questions regarding pond weeds.