This is the time of year when I start to get a lot of calls about something growing in ponds. Actually it is growing on the pond. This weed is Mosquitofern or Azolla.
Azolla is often confused with duckweed but it is actually a fern. This fern will often turn red because of the nitrogen fixing cynobacteria Anabaena azollae, that will colonize the upper lobes of azolla.
Azolla is considered a desirable native plant, but can become a nusiance when the population becomes high.
The best way to control Azolla is to limit the amount of nutrients, especially phosphorous, that gets into the water. Another way to control Azolla is to skim it off the water and use it as compost.
Chemical control is difficult because to control Azolla you must hit every plant and these plants are not connected.
Aquatic Weed Sites
Non-native, Invasive & Freshwater Plants
Department of Ecology, State of Washington
Excellent source for ID and cultural, mechnical and chemical management of aquatic weeds. Always check the label before making any pesticide applications.
Fact Sheet on Toxic Blue-green Algae
Dr. Carole Lembi
Department of Botany & Plant Pathology
General Aquatic Plant Management
Benefits, problems, ID, and management methods of aquatic plants
Best Management Practices for Aquatic Weeds
Aquatic Ecosystem Restoration Foundation