Aquatic Weed Control

AQUATIC WEED CONTROL

Carole A. Lembi, Aquatic Weed Specialist,
Purdue University

Importance of Controlling Aquatic Weeds

Aquatic plants are natural and important components of the aquatic environment. Microscopic plants (algae) form the base of the aquatic food chain. Larger algae and plants provide habitat for fish and food organisms, and all plants produce oxygen as they photosynthesize during the daylight hours. However, excessive growths of these plants can have a detrimental effect on a body of water and its inhabitants. Many ponds, lakes, and drainage ditches are shallow and nutrient-enriched and provide ideal conditions for abundant aquatic weed growth. Some of the problems caused by aquatic weeds are as follows:

Recreational activities such as swimming, fishing, and boating can be impaired and even prevented.
Aquatic weeds greatly detract from the aesthetic appeal of a body of water.
Excessive growths can lead to fish stunting and overpopulation. This occurs because the production of too much habitat prevents effective feeding of small fish by larger fish.
Aquatic weeds can cause fish kills. This usually occurs because oxygen is taken out of the water. For example, plants at night respire (take up oxygen) without producing it. Oxygen depletion also occurs when plants die and decompose. Photosynthetic production of oxygen ceases, and the bacteria, which break down the plant material, use oxygen in their own respiration. Fish kills in summer are frequently caused by die-offs of algae blooms. Fish kills in winter occur when snow accumulates on ice cover. Light is blocked thus preventing photosynthesis by any living plants or algae. Decomposition of plants that died in the fall causes further oxygen depletion. Fish kills can also be caused by insecticide runoff, ammonia runoff from feedlots, and diseases.
Aquatic weed growth provides quiet water areas ideal for mosquito breeding.
Certain algae can impart foul tastes and odors to the water.
Weeds impede water flow in drainage ditches, irrigation canals, and culverts and cause water to back up.
Deposition of weeds as well as sediment and debris can cause the gradual filling in of bodies of water. How do we go about controlling these plants? The five basic control approaches are preventive, mechanical, biological, habitat alteration, and chemical method of control.

Control Begins with Prevention

Many aquatic weeds or their seeds are carried into a pond by wind birds, fish introduction, fishermen, etc. These weeds infest a pond only if the water conditions are just right. This usually means that nutrients are entering the pond from runoff or stream inflow. To help prevent serious weed infestations you can do the following things:

Do not fertilize your pond. Most Midwestern waters are sufficiently rich in plankton and other food organisms to support large fish.
Maintain a good sod and grass cover around your pond. This will help prevent runoff and erosion. Do not fertilize the turf directly around the pond.
Do not allow livestock access to a pond except under conditions of extreme heat. If the water is used for livestock, fence the pond. Water the animals from a stock tank below and outside the fence. Animals will increase turbidity and fertility and tear down the banks.
Check septic tanks for possible leakage or seepage into the pond. Locate new septic drainage fields so that the nutrient-rich effluent will not reach your pond.
Do not permit runoff from chicken coops, feedlots. etc., to enter your pond. If this kind of runoff is occurring upstream from your pond, you should check with your county board of health to see if anything can be done about it.

All of these measures will help prevent weed growth, particularly in a new pond. In older ponds these measures will probably aid in reducing infestations of floating plants such as algae and duckweed.

Mechanical and Biological Control Methods

Even with preventive measures, many ponds still have severe waterweed infestations. Hand-pulling the weeds or dredging the pond are possible methods of control. But often the infestation is so severe that these methods are impractical or uneconomical. Motor-driven underwater weed cutters are available and can be used for the control of such plants as waterlilies and watermilfoil. Underwater weed cutting must be done continuously during the summer and usually represents a long term financial investment. For more information on weed cutting equipment, write to the following companies:

Altosar of America, Inc.

16764 Newton Road

East Lansing, Ml 48823

Aquamarine Corp.

Box 61 6

Waukesha, Wl 53186

Mudcat Div.

P.O. Box 16247

St. Louis Park. MN 55416

Biological controls for aquatic vegetation have received considerable publicity. Several species of fish are herbivorous in that their principal diet is aquatic vegetation. One such species, the grass carp (also known as the white amur or Chinese carp), is being tested in various parts of the United States and is legal in several states. However, it is illegal to introduce these fish to the ponds, lakes, and streams of many states. Check with your local or state fisheries department for regulations regarding the grass carp.

Habitat Alteration Methods

Certain methods of manipulating or altering the aquatic environment can be effective in controlling aquatic weeds. One of the more successful methods is the drawdown technique in which water levels are lowered over the winter. Exposure of the sediments in the shallow areas of a lake or pond to alternate freezing and thawing action will kill the underground rhizomes of many aquatic weeds (the majority of aquatic weeds are perennial and come from rhizomes). This method has been quite successful for the control of Eurasian watermilfoil and waterlilies, although the degree of control depends somewhat upon the severity of the winter. There are several advantages to a winter drawdown in addition to weed control. As the sediment dries, it is compacted, thereby increasing the depth of shallow areas. Drawdown also concentrates the fish which increases the predation of the smaller fish by the larger ones. Fishing quality can often be improved following a drawdown.

Other types of habitat manipulation include riprapping shorelines and anchoring screens (e.g., Aquascreen) or black plastic sheets on the bottom sediments to prevent rooted plant establishment. Dyes such as Aquashade are used to inhibit light penetration through the water. This blue dye can be applied right out of the bottle along the shoreline. It mixes throughout the body of water within 24 hours. The dye intercepts light normally used for photosynthesis by underwater plants. The dye can only be effective if its concentration is maintained. Some general rules for using Aquashade are as follows:

Do not apply where water outflow will reduce Aquashade concentration.
Apply in March or April before weeds reach the water surface. Midsummer reapplication is usually necessary. It is effective only on rooted underwater plants growing at depths greater than 2 to 3 feet. Supplemental treatments of copper sulfate might be needed for algae control.
Do not use in muddy water.

Aeration has been publicized as another method of weed control. Although aeration is definitely beneficial for fish life and can help prevent fish kills, there is no evidence that aeration inhibits weed growth.

Chemical Control Methods

For most aquatic weed problems, properly-used herbicides control vegetation without harming the fish. Aquatic herbicides are effective and commonly used means of controlling aquatic vegetation.

All of the herbicides discussed in this publication are registered with the federal Environmental Protection Agency (EPA) and, when used in water as directed, generally pose no significant threat to the environment or public health. Most herbicides, however, ARE TOXIC IF TAKEN INTERNALLY; and direct contact with the chemical should be avoided. If herbicide comes in contact with the skin, it should be washed off immediately with water. If herbicide is accidentally swallowed, go to a physician immediately and consult the container label for first aid information.

What You Need to Know Before Using a Chemical

The most important considerations before buying and applying a herbicide for aquatic weed control are these:

Identity of the weed. This can save you a lot of money because certain chemicals will work only on certain weeds and not on others. Identification help can be obtained from your county Extension Service or a fisheries biologist. Always transport the plant in a plastic bag without extra water.
Restrictions on use of water treated with herbicides. Although most aquatic herbicides break down readily and rapidly in water and pose no threat to human or animal health, there are waiting periods on the use of water treated with most herbicides. These restrictions--usually on fishing, swimming, domestic use, livestock watering or irrigation--dictate which herbicides will be appropriate for your pond or lake. Always check the herbicide label for possible restrictions.
Dosage. Calculate carefully, and don't overdo it. Most aquatic herbicide labels give dosages on the basis of acre-feet (volume measurement). Acre-feet is calculated by multiplying the surface area by the average depth. For example, a pond with a surface acreage of l/2 acre and an average depth of 4 feet contains (4 feet x Ť acre) 2 acre-feet. The herbicide label can then be checked for the amount of chemical to apply per acre -foot.
Timing. Late spring is usually the best time to apply aquatic herbicides. The plants are young and actively growing and most susceptible to herbicides. Do not wait until July or August! If you wait until late summer to treat, you are running a serious risk of killing fish. By that time, the vegetation is usually extensive and thick. Also the water is warm and still. Killing all vegetation at once under these conditions could seriously deplete the water of its oxygen and cause a fish kill. If you must treat this late in the summer, treat only a portion of the weed growth at a time.
Temperature. Aquatic weeds are not affected by herbicides when the water is too cold. The water temperature should be in the 60's, preferably the upper 60's (in the area to be treated). These temperatures usually occur from late April to early June. This means that as soon as the plants are up and actively growing, and if the water temperature is right, the herbicide should be applied.
Retreatment. More than one treatment, (e.g., copper sulfate on algae) may be required for adequate control. Retreatment is usually required in succeeding years. Plants can regenerate each spring from seeds, spores, and underground rhizomes. These structures generally are not affected by aquatic herbicides. Exceptions include Dowpon, 2,4-D, and Banvel 720, chemicals which do translocate into underground rhizomes and kill them. However, new plants can sprout from seeds.

Aquatic Herbicide Formulations And Application Methods

Copper sulfate: granular crystals, diamond form, powder. The granular form is best applied by putting it in a burlap sack and towing it by boat around the pond until it is dissolved. The powder form is best used by dissolving it in water and spraying directly onto the algae mats and into the water. All copper-containing compounds, including the copper chelates, are highly corrosive to metals so that wooden, painted, and enameled or copper-lined containers might be needed for mixing and applying the herbicide. Sprayers should be thoroughly cleaned and rinsed out after every operation to prevent damage.
Cutrine Plus, AV-70, Algimycin, and other chelated copper compounds: liquid (Cutrine Plus is also available as a granule). Mix liquid with water in a container and spray or inject into infested area, or use it as a spot treatment.
Hydrothol and Aquathol: granular or liquid. Hydrothol liquid is recommended for use only by commercial applicators. It can cause fish kills and severe skin burns. Also use caution in handling Aquathol liquid, but it is much safer than Hydrothol liquid where fish are present. Spray or inject liquid-water mix into infested area, or use as spot treatments.
Aquazine: wettable powder. Make a slurry with water in a bucket and pour in area to be treated. Aquazine can only be used as a whole pond treatment. Uniform treatment is not necessary since the material will become distributed in the water over time.
Diquat: water-soluble liquid. Spray or inject liquid-water mix into infested area. The mix can be used as a spot treatment. Spray is used for duckweed control.
Sonar: liquid and pellet. Spray or inject liquid-water mix into infested area. Can be applied to entire surface area of ponds. In lakes and reservoirs should be applied to areas greater than 5 acres to prevent dilution. Takes 30-90 days to see results.
Aquakleen: granular. Distribute evenly over infested area.
Rodeo: liquid. Mix in water and spray on foliage. Always add surfactant.

Aquatic Weed Identification Guide

Aquatic weeds can be divided into two botanical groups; algae and flowering plants. Algae are usually very simple in structure, but some (for example, Chara) can resemble flowering plants. For effective chemical control, it is essential that you distinguish between algae and flowering plants. Algae

Microscopic algae form scums and/or color the water green or yellow-green. Sometimes they cause red, black, or oily streaks in the water. When in sufficient numbers to color the water they are called "blooms." Die-off of these algae can cause fish kills. Blooms usually occur where abundant nutrients are reaching the water. They should be treated with chemicals before they cause a noticeable color.

Filamentous algae (also known as moss) form floating, mat-like growths which usually begin around the edges and bottom of ponds in the early spring. This type of growth is probably the most common in lakes and ponds in the Midwest. Repeated chemical treatments during the summer season are often necessary for effective control.

Chara or stonewort usually grows in very hard water and is often calcified and brittle. The plant is rooted, and leaves are arranged along the stem in whorls. The plant is completely underwater and has a musky smell. It can be difficult to control once it has become established and has a heavy coating of calcium carbonate. Use contact herbicides when the plants are still young and not heavily calcified. Although this plant resembles some flowering plants, it is an alga.

AQUATIC HERBICIDE RECOMMENDATIONS AND USE RESTRICTIONS
Aquatic weed	Herbicide	Product rate¹	Restrictions ²
Algae (microscopic, filamentous, Chara)	Copper sulfate	2.7lb./A-ft.	Do not use in trout waters.
	Copper chelates (Cutrine Plus)	0.6-1.2 gal./A-ft.	Do not use in trout waters.
	Endothall (Hydrothol 191 )	1.1 pt./A-ft.	F = 3 days, L, D = 7 days.
	Simazine (Aquazine)	1 -3.4 lb./A-ft.	I, L, D = 12 months
Submersed plants (coontail, watermilfoil, pondweeds such as sago, curlyleaf, leafy)	Endothall (Aquathol K)	1.3 gal./A-ft.	S= 1 day; F= 3 days; I³, L D= 14 days.
	Diquat	1 -2 gal./SA	I, L, D = 14 days.
	Simazine (Aquazine)	3.4-6.8 lb./A-ft.	I, L, D= 12 months.
	Fluridone (Sonar)	1 qt. or 20 lb./SA	Do not apply within 1/4 mile of potable water intakes.
Free-floating plants (duckweed, watermeal)	Diquat	1 gal./SA	I, L, D= 14 days.
Free-floating plants (duckweed, watermeal)	Simazine (Aquazine)	3.4-6.8 lb./A-ft.	I, L, D= 12 months
Rooted-floating plants (waterlilies, spanerdock)	Glyphosate (Rodeo + surfactant)	6 pt./SA	Do not apply within 1/2 mile upstream of potable water intakes.
Rooted-floating plants (waterlilies, spanerdock)	2,4-D (Aquakleen)	200 lb./SA	Do not apply to waters for I, D, dairy animals.
Emergent plants (cattails, perennial grasses, and broadleaves)	Glyphosate (Rodeo+ surfactant)	4.5-7.5 pt./SA	Do not apply within 1/2 mile upstream of potable water intakes.
¹ SA = surface acre; A-ft.= acre-feet. ² F = fishing; I = irrigation; L= livestock; D= domestic use; S = swimming ³ Treated water may be used for sprinkling grass immediately. ⁴ State labels only; check with appropriate authority.