How to Kill Water Weeds

Updated February 21, 2017

Water weeds come in all shapes, sizes and types. The best way to kill water weeds is to positively identify what type of weeds they are. Once identified, a specific aquatic herbicide is used to kill them. Timing is also the key. Aquatic herbicides must be introduced into a water-filled environment, like a reservoir or a pond, at the correct time of the season to prevent fish and other animals from being harmed. If the water reservoir is being used for irrigation, however, herbicides can be used throughout the season at the owner's discretion.

Apply a systemic herbicide to emergent vegetation. Some of the plants in this category are cattails, bulrushes and arrowhead. Apply the systemic herbicide when the seed heads on these plants are beginning to turn brown. Continued application must be maintained for several more weeks. Contact herbicides will control the plants, but will not kill the root structure. Only systemic herbicides will kill the weeds.

Apply a systemic herbicide to kill submerged plants in May or June. As a rule, once the water temperature has reached 15.6 degrees Celsius, the herbicide must no longer be used. For treatment in the summer, use a fluridone systemic herbicide. Using fluridone is a slow kill that extends from 45 to 90 days but can be used all summer long. Some common submerged weeds are naiads, pondweeds and coontail.

Apply 2,4-D herbicides and glyphosphate herbicides on floating weeds. 2,4-D herbicides are applied to floating weeds before the leaves reach the surface. Glyphosphate herbicides are applied just after the peak flowering bloom period has passed. Common examples of these plants are water lilies, water lotus and pennywart.

Apply fluridone, diquat and chelated copper herbicides to kill duckweed and watermeal. These weeds must be controlled as soon as they are noticed, or they will literally take over the water reservoir. Fluridone works slowly all year to keep duckweed and watermeal controlled. Diquat kills on contact, and adding chelated copper will strengthen the herbicide action of diquat. Pulling these weeds and/or introducing grass carp or koi into the water will also kill these weeds.

Apply copper algicide compounds to kill algae. Water temperatures must be at 15.6 degrees Celsius for at least one week before an algicide is introduced. May or June are the best months to use algicide.


Aquatic herbicides can be applied by any number of ways. Spraying, spreading granules and pouring from mixed buckets of water and herbicide are all common methods. Refer to the manufacturer's mixing and application directions for every aquatic herbicide used.

Things You'll Need

  • Systemic herbicides
  • Contact herbicides
  • Fluridone herbicide
  • 2,4-D herbicide
  • Glyphosate herbicides
  • Diquat
  • Copper algicide compounds
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About the Author

Dale Yalanovsky has been writing professionally since 1978. He has been published in "Woman's Day," "New Home Journal" and on many do-it-yourself websites. He specializes in do-it-yourself projects, household and auto maintenance and property management. Yalanovsky also writes a bimonthly column that provides home improvement advice.