how to control duckweed?

Discussion in 'Water Features' started by lilmarvin4064, May 31, 2007.

  1. lilmarvin4064

    lilmarvin4064 LawnSite Senior Member
    Posts: 757

    a friend of mine has a small pond, about 1500 sq ' , 3' deep. He wants spray something that will kill the duckweed. I was thinking about an aquatic glyphosate product like Rodeo or Aquaneat. I didn't see "duckweed" on the label.

    The main problem is that he has horses that drink from the pond and that makes me a little nervous. After reading the label on the product I am still a little confused as to if spraying would be the best choice. Horses have to wait 48 hours before drinking the water??? If this is ok, what is a good aquatic surfactant to use?
  2. Victor

    Victor LawnSite Bronze Member
    Posts: 1,637

    Instead of using an aquatic herbicide to kill the Duckweed, I'd recommend increasing the plantings in the pond. Does he have room in the pond for aquatic plants? If he does, they would compete with the Duckweed for Nitrates (the reason the Duckweed is there in the first place). If he step up his water-changes, that will help also. What he wants to accomplish here, is to make it so the Duckweed won't have enough food to live on. If he can accomplish that, say goodbye to the Duckweed. If he doesn't address the Nitrate problem in his pond, the Duckweed will come back (whether he sprays it, or not).
  3. CypressAg83

    CypressAg83 LawnSite Member
    Posts: 2

    I'm new to the site, and ran across your old post while browsing.
    In case your friend is still battling duckweed, SONAR Herbicide and REWARD Herbicide are both viable options - and the only registered herbicides that are truly effective on duckweed (IMO).
    SONAR is a whole-pond treatment, which involves dosing the entire volume of water within the pond (injection-style, not via surface treatment). REWARD is a topical/surface treatment (applied only to the pond's infested areas).
    However, REWARD has a water-use restriction of 1-day for livestock; whereas SONAR will have a 30-day water-use restriction for irrigation (if he irrigates from the pond), yet no livestock restrictions.
    REWARD has very little residual activity (measured in hours). So, repetitive treatments may be necessary as new duckweed plants emerge from seeds (lying on the bottom of the pond). SONAR, at the opposite extreme, has a long period of activity (90-120 days, possibly longer), and usually requires only one treatment per season.
    Lastly, SONAR is ONLY an option if the pond's water is static (i.e. not prone to discharging water following rainfall events). Actually, after reviewing the size of the pond, only a minute amount of SONAR would be needed to treat the entire pond (3/4 oz of SONAR would approximate a 90 ppb concentration - which is the upper-limit of the labeled rate).
  4. Fishwhiz

    Fishwhiz LawnSite Member
    from Oregon
    Posts: 112

    I believe the trick here is to avoid treating and treating and treating forever while also avoiding a pond that becomes choked out with weeds. A little aeration may go a long ways in this case.

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