Aquatic Weeds

• There are a number of different species of aquatic plant life that can be considered invasive weeds. Probably the most commonly encountered aquatic weed is algae; an outbreak of algae can threaten other plants and animals in the pond by competing with other species for sunlight and oxygen in the water. Other plants such as duckweed can grow so out of control that they cover the entire pond, blocking sunlight from the plants and animals in the water. Even plants that are considered highly beneficial to aquatic life can become weeds simply by growing too well.
  
  

Herbicide Recommendations

• You need to make herbicide choices in regard to the specific species that is causing problems in your pond, since herbicide formulations are always targeted at specific species or at a range of species. Some examples of commonly used aquatic herbicides include copper sulfate, copper chelates, diquat dibromide, endothall, fluridone, glyphosate, granular 2,4-D and others. Copper sulfate is the least expensive and most commonly used herbicide for algae and other aquatic weeds. Copper sulfate is not likely to damage other aquatic plants in the pond, so you don't have to worry particularly about herbicide injury to other pond plants.
  
  

Application Methods

• Always follow the manufacturer's printed application instructions meticulously. Copper sulfate is toxic to fish eggs; make sure you time the copper sulfate treatment so it will not coincide with fish spawning season. Fish egg toxicity is worse in highly alkaline waters, so it is often advisable to have your pond water acidity tested prior to beginning a copper sulfate treatment. Alkalinity also compromises the copper sulfate's effectiveness as an herbicide, underscoring the need for prior water testing. Copper sulfate is also a contact herbicide, so direct contact with the target vegetation is essential to successful weed control.
  
  

Other Considerations

• Just as is the case when battling land weeds, aquatic weed control is best achieved through an integrated approach to weed control that combines mechanical methods such as hand-pulling weeds and keeping a minimum depth of 3 feet throughout the pond, since shallower waters favor weed growth. Only use herbicides as they are needed. An integrated approach such as this one will keep your pond healthy and weed-free while minimizing your reliance on chemical herbicides.