Along with Algae, this is the category of aquatic vegetation that most frequently causes pond owners problems. There are invasive species, such as Eurasian Watermilfoil, and many beneficial native species. Usually, the biggest problems arise when people kill off all the native aquatic vegetation, which just opens the door for invasive species and algae to proliferate. Always consider what will grow next before killing aquatic weeds. Generally speaking, we recommend spot-treating Submerged Aquatic Weeds to control them in areas where they interfere with your water use, and leaving them in place in the rest of the pond or lake to provide habitat and help keep the water clean. The only exception to this is with exotic species, like Eurasian Watermilfoil and Curly-leaf Pondweed, where we recommend total elimination followed by introduction of beneficial native species. Below is a listing of some common types along with control options:
Pondweed Family (Potamogeton spp.)
This is a common group of weeds with literally dozens of species. Most species are completely submersed, but some, like Floating-leaf Pondweed (Potamogeton natans) are submersed plants with floating leaves.
Curly-leaf Pondweed (Potamogeton crispus) is a notable species. It was introduced from Europe prior to 1814, and is now wide-spread. It grows rapidly early in the season and then dies back during midsummer. If treated early, it rarely grows back in the same year. Fortunately, there is a very good treatment option available. Sonar can be applied at very low doses (as low as 10 parts per billion!) that will kill the Curly-leaf Pondweed, but will not harm most native plants. Ideally, you should encourage the growth of native, more desirable vegetation, to discourage the reestablishment of the Curly-leaf Pondweed.
Generally, all Pondweeds should be treated early in the season, before the seed heads appear. Doing so can limit the regrowth of pondweeds later in the season and may, in some species, limit regrowth in subsequent years. To prevent growth of pondweeds, you can apply Pond Dye early in the year to prevent sunlight from reaching the bottom, where the growth begins. Triploid Grass Carp are a biological control consideration, but partial control is rarely possible. There are numerous chemical treatment options for pondweeds, but whenever possible it is best to spot treat these weeds (with the exception of Curly-leaf Pondweed), as many of them are beneficial for water quality, fish and wildlife. One of the simplest spot treatments is to sprinkle Aquathol Super K around areas you wish to clear. This product is very effective early and mid season and has only limited water use restrictions. Late in the season, when some species are more resistant to treatment, Hydrothol is a better treatment option. See the Product Selection Chart for alternative treatments.
Wild Celery or Eelgrass (Vallisneria americana)
Commonly found in the eastern half of the USA, Wild Celery grows rooted in mud with ribbon like leaves extending 1 to 3 feet vertically. This is a terrific species for aquatic habitat development, providing both food and shelter for a variety of aquatic organisms. Chemical control is difficult, but limited results can be achieved with Hydrothol.
Water Milfoil (Myriophyllum spp.)
Many beneficial native species occur throughout the USA. All have hollow stems with whorled, feather-like leaves along the entire length of the plant. Water Milfoil is entirely submerged, with the exception of a stalk of flowers. To best control Milfoil, spread Navigate over treatment areas when plants are small and before flowering occurs. You can prevent growth by applying Pond Dye early in the season. See the Product Selection Chart for alternative treatments.
Eurasian Watermilfoil (Myriophyllum spicatum)
This plant has been introduced to the USA and is a serious nuisance. It spreads rapidly by developing roots from each fragment that is separated from the original plant. Due to its dense growth, waters infested with Eurasian Watermilfoil can become useless. Triploid Grass Carp are a biological control option, but they may prefer to consume certain native species prior to Eurasian Watermilfoil. There is info at the link provided for which plant species the grass carp prefer. A new biological control option is currently being developed using the Milfoil Weevil (Euhrychiopsis lecontei), but the process is not yet perfected. Probably the best control option to date is Sonar, which can be applied at very low doses (as low as 10 parts per billion!). This treatment will kill the Eurasian Watermilfoil, but not harm most native plants. Plus, if applied early in the year, the treatment usually prevents the regrowth of this invasive weed for up to 3 years! Ideally, in the interim, you should encourage the growth of native, more desirable vegetation to discourage the reestablishment of the Eurasian Watermilfoil. If the water body to be treated is very large, the second best option is a granular application with Navigate applied early in the season. See the Product Selection Chart for alternative treatments.
Coontail (Ceratophyllum demersum)
Often confused with Water Milfoil, Coontail is distinguished by forking leaves rather than feather-like. It grows in dense patches that float to the surface late in the summer. These mats are best controlled with liquid sprays (refer to Product Selection Chart), but early season growth can be treated with Navigate. Coontail can also be prevented with Pond Dye. Triploid Grass Carp enjoy consuming the bushy growth of this plant, so it is a biological control option. However, partial control is usually not possible, and because this plant is particularly effective at taking up nutrients out of the water, there may be adverse results from its complete removal.
Water Buttercup (Ranunculus spp.)
This plant occurs throughout the USA, and has distinct white or yellow flowers that form at the surface from June through September. Tufts of thread-like leaves form alternately along the erect, submersed stem. Pond Dye is effective at preventing growth if applied early. This is a beneficial species, but if control is needed, the best option is to spray treatment area with Reward.
This free-floating plant is common throughout the USA. It has finely divided leaves that are scattered along the stem, and numerous bladder-like structures that make it float. Pond Dye is effective at preventing growth if applied early. The best control option is to spray treatment area with Reward. Triploid Grass Carp are very effective at controlling this plant, and should be considered if chemical control is not an option.
Elodea (Elodea canadensis)
This beneficial native plant is common in the Northern half of the USA. Elodea has broad oval leaves arranged in whorls around the stem. Spacing of the whorls generally increases from the tip on down the plant. Pond Dye is effective at preventing growth if applied early. The best control option is to spray treatment area with Reward. See the Product Selection Chart for alternative treatments.
Hydrilla (Hydrilla verticillata)
This is an exotic plant, very similar to Elodea. It is most common in the Southeastern USA, Arizona and California. Triploid Grass Carp are very effective at controlling this plant, and should be considered if chemical control is not an option. Refer to the Product Selection Chart for numerous chemical treatments.