Right now, it looks like half the county is underwater. So of course, everyone’s tanks are full to overflowing. Before this last round of rain, I was getting a good many questions about pond weeds and in particular “pond scum”. A few typical names people use to refer to various aquatic weeds are pond scum, grass and moss. These pretty well describe many aquatic weeds, however just like pasture weeds, when choosing a management plan to control them you must know their true identity.
There are four aquatic weed classes: algae and other plankton, floating plants, submerged and emergent plants. Most of the calls we have received so far this year have been algae, so we will focus on those for now. Two of the more prevalent types of algae this year have been filamentous and planktonic. Both of these algae have the appearance of what you would call 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 will start growing on the bottom in shallow water or on structure. It will often then float to the top forming large mats. Planktonic algae are made-up of floating microscopic plants. They are responsible for turning pond water shades of green, blue-green, brown or variations in between.
While filamentous algae have little value and are a nuisance to tank owners, planktonic algae blooms are considered desirable as the beginning of the pond food chain. In fact, fertilization programs are often used to promote algae blooms to thereby support a larger fish population.
Planktonic algae are desirable for shading the pond bottom (in areas over 2 feet deep). This shading suppresses the establishment of rooted aquatic plants. However, to much planktonic algae can cause oxygen depletions and fish kills.
Filamentous algae are the algae that most often requires control. Copper Sulfate and cooper complexes are the least expensive and provide excellent control of filamentous algae when you do not have hard water. In hard water situations the copper sulfate binds with the calcium and precipitates out of solution, thus becoming ineffective. If you do use cooper sulfate or a copper complex, you must put it into a solution prior to use. You can try mechanical control by seining or chopping, but the plant can come back from the fragments that will be left. Grass carp are ineffective on this type of aquatic weed. Tilapia do feed on the algae but will die off when water temperatures drop below 55 degrees. There are several herbicide options that can contain one of the following active ingredients: diquat, alkylamine salts of endothall, flumioxazin, sodium carbonate peroxyhydrate all of these will provide good control. When using any of the herbicides listed above, including cooper sulfate, you must have direct contact with the algae for good results.
With any herbicides, always read and follow the label directions.