Single-celled plants suspended in water. They may aggregate into small colonies that are visible to the naked eye, but are usually microscopic and evident only as a green (or sometimes brownish) coloration of water. Abundant phytoplankton—and the resulting green water—is not necessarily a bad condition: it can even be beneficial when it prevents other more problematic plants from growing in the green, turbid water. Excessively abundant phytoplankton, however , can be toxic to fish and cause foul odors and unsightly coatings of blue-green scum on the water surface. The best way to reduce phytoplankton is by reducing nutrient input levels (whether from fertilizer or other sources) into the water.

Simple plants whose cells attach end-to-end to form long filaments that intertwine to form thick, unsightly mats. Like with phytoplankton, the best way to reduce their abundance is by decreasing nutrient inputs. However, other methods can be effective and often are necessary, including spot treating algae mats with algaecides, introducing weed-eating fish such as triploid grass carp, and adding specially manufactured pond dyes.

The key is to prevent the problem from becoming too severe. Physical removal of excess filamentous algae to reduce levels can increase the efficiency and safety of algicidal or fish treatments.

Aquatic plants that root in the pond bottom while the body is suspended in the water. Some are useful for aquascaping. However, in small ponds they can grow to objectionable levels. Since partial or selective control of submersed plants is difficult, it’s easier to eliminate all submersed plants in a pond. Contact herbicides or systemic herbicides can be effective, depending on water conditions. If water flow is too great, herbicides may be ineffective. Triploid grass carp is effective at 2 to 20 fish per acre; stocking rates depend on target weeds and their density as well as other regional factors. Using grass carp for weed control can be unpredictable due to fish loss from escape or predation by wading birds or alligators, and can also impact emersed vegetation. Adding or removing fish as necessary and the judicious use of herbicides will enable better management.

Plants that float on the water’s surface. The most common problem plants are duckweed, giant duckweed, and watermeal. Selective methods can help control duckweeds but requires frequent applications. Triploid grass carp are also effective but high stocking rates are necessary. With proper conditions (minimal water exchange and no concern over sensitive non-target plants), you can achieve long-term control of floating plants (including watermeal) with Flouridone.

Plants that root in the pond bottom and extend above, or float upon, the water’s surface. Many are important for aquascaping and wetland mitigation. Some of the more popular ones include arrowheads, spike rushes, bulrushes, soft rush, and water lilies. While aquascapes can be attractive and enhance landscapes, they do require more maintenance and expense than plant-free ponds since it is necessary to manage the desired plants as well as control the weedy species. Common immersed aquatic weeds include torpedo grass, alligator weed, and pennywort. These invasive species can be controlled using selective methods as to preserve Florida native vegetation.