Eliminating Undesirable Aquatic Plants

By Dave LaBomascus, Stoney Creek Fisheries, Grant, MI Originally published in Passion for Water, Spring 2008 - Revised Summer 2014

Summertime is here with plant life in full bloom! Plant enthusiasts scurry to tend and coax their flower beds, gardens and green landscaping into dense, lush growths to be admired by all.

Dense, “lush” plant growth in backyard ponds and water gardens!?!?! Well, that’s another story. Tending and coaxing normally becomes pulling, ripping, raking and treating. Granted that many water garden fans passionately grow beautiful, often potted, aquatic plants alongside and within their water features, but most pond owners battle for control of their water against nuisance aquatic plants throughout the growing season. In this article, we’ll talk about two aquatic plants that are very different from one another, yet are nearly equal in stubbornness towards their eradication: Cattails & Duckweed.

 

Cattail (Typha spp.)

Everyone can recognize the cattail, which is one of the most common emergent weeds throughout the United States. Emergent aquatic plants grow above the water in shallow areas of pools, lakes, ditches and rivers. Many emergent plants are not dependant upon standing water, requiring only saturated soil to thrive. Cattails will grow in moisture conditions ranging from saturated soil to water up to four feet deep. Left unattended, cattails can thickly populate up to 100% of a pond’s littoral zone (shore margin).

CONTROL:

If cattails are not desired at your pond, be aware that full control is difficult, but not impossible. Literature describing control of cattails can be confusing as there are differing recommendations. Treatment in spring vs. summer vs. fall? Cutting stems vs. not cutting stems? Cutting back the growth before treatment … after treatment? It certainly can be bewildering. The key to full control of most aquatic plants is to kill the root system. Failure to accomplish this normally results with the quick return of the undesirable plant.

One of the best aquatic plant herbicides to eradicate cattails is “Shore-Klear” (53.8% isopropylamine salt of glyphosphate as active ingredient). Shore-Klear is available in one quart containers. “AquaNeat” (same active ingredient) is available in 2.5 gallon containers for large pond and lake owners. There are basically no restrictions for Shore-Klear usage. Check the label for information concerning distances allowed from potable water intake. It’s always a good idea to also check with your state’s Conservation or Natural Resources department for their restrictions. Shore-Klear is a non-selective herbicide so it will harm your lawn and other desirable plants, so be careful … spray only on a windless day to avoid drift.

Aquatic herbicide applicators recommend the following steps for controlling cattail:

  • Do not treat in the spring.
  • Treat in early fall when the cattail “fruit” is fully grown, but has not yet gone to seed. Everyone is familiar with cattail seeds blowing in the wind by the billions.
  • Treat in the morning of a clear, calm, sunny day (maximum plant activity!).
  • In a simple two or three gallon hand sprayer, or a larger backpack sprayer, mix Shore-Klear with a wetting agent (surfactant) like Cygnet Plus. They are safe and fully biodegradable. Surfactants break down the waxy coating on the foliage and helps the chemical “stick” better.
  • Application rate: Three ounces of Shore-Klear plus two ounces of surfactant per gallon of water.
  • Spray directly onto foliage.
  • Be patient … cattail is a difficult plant and eradication takes time … sometimes two or three fall seasons are required for full control.
  • Always consult the label on the product for exact application instructions and use restriction information. 
  • Be sure to consult local authorities for rules and permit requirements for use of aquatic herbicides.

In the meantime, and since you should wait until early Fall to treat anyway ... pull up a few handfuls of young cattail root this Summer … the roots have a flavor similar to celery!

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Duckweed

Duckweed (Lemnaceae spp.)

A pond covered with duckweed makes for one poor swimming hole. This tiny aquatic plant seems to get into everything... whether it’s your pond filter or your swimming suit. Duckweeds are members of the family containing the world’s smallest flowering plants. Duckweed ranges in size from two to eight millimeters and is often mistaken for an algae. The plant floats on the surface of the water and reproduces very quickly by means of fragmentation. Duckweed may or may not have a “root” extending from the underside, but is never found actively rooted in soil. To the best of my knowledge, duckweed received its name due to the fact that it is a favorite food source for puddle ducks, such as the mallard. Do not confuse duckweed with Watermeal (Wolffia spp.) which is smaller (1.0-1.5 mm) and looks like green cornmeal or grits floating at the surface.

CONTROL:

Duckweed can be even more difficult to rid from your pond than cattails. The best control is with Sonar (41.7% fluridone as active ingredient). Restrictions include: 14 to 30 days for irrigation of turf, forage and food crops. Again, consult the manufacturer’s label and your pertinent state agencies. Sonar is a broad spectrum herbicide and will kill duckweed, Elodea, milfoil, coontail and other aquatic plants. Sonar is absorbed by the plant and inhibits the weed’s ability to make food … it “starves” the plant. Due to this fact, time is again required. Initial results can be seen in ten days. A complete kill may not be achieved for 30 to 90 days. Sonar has a long lasting residual effect, with one treatment lasting more than one season. It is not recommended to use Sonar if your pond has an outlet, as aquatic vegetation downstream may be harmed. Also, due to the residual effect and regardless of the stated restrictions for irrigation, you may not want to use your pond’s water for irrigation at all. Another product for excellent duckweed control is Clipper, a fast acting alternative to Sonar. Clipper is fast and effective, breaking down quickly in the water to leave no accumilation in the sediment. 

  • As with all aquatic plant treatments, treat in the morning on a clear, sunny day for maximum effectiveness.
  • Apply as early in the season as possible. Do not wait until your pond is covered!
  • The treatment rate is one quart Sonar per surface acre of water. Sonar is available in pint, quart and gallon containers.
  • Treatment is simple … simply pour the required amount into the pond. Add one half of the treatment (one pint per acre) first and then repeat with the second half in about two weeks.
  • TIP: A pond aerator will help restrict duckweed to the perimeter of your pond!
  • Always consult the label on the product for exact application instructions and use restriction information. 
  • Be sure to consult local authorities for rules and permit requirements for use of aquatic herbicides.

Another treatment option is Reward (37.3% diquat dibromide as active ingredient). Application is via a spray mix of Reward - plus surfactant - plus water. Full effectiveness will require three to six applications. Restrictions include one to three days drinking and one to five days for irrigation.

So take care of that annoying duckweed … and enjoy your swim!

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