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Aquatic Vegetation Identification and Control

 

By Kenneth Williams

Introduction

Excess aquatic vegetation is the most common problem observed in ponds. Over abundant aquatic plant growth frustrates the angler, can be a cause of stunted bluegill ponds and other fisheries management problems. Excessive aquatic vegetation causes water stagnation and decaying plants contribute to oxygen depletion and associated fish kills.

Because of problems associated with aquatic plants many pond owners aim at complete water weed eradication. When it is achieved, the results are often disappointing because pond fish production declines. Fish and wildlife need some aquatic vegetation to use as food, cover and nesting sites. Many of the insects and other small organisms that are the food source for fish live in the aquatic plant community. When the plants are completely removed the pond food chain can be disrupted and fish production is reduced.

Also, the cover aquatic plants provide prevent largemouth bass and other predatory fish from decimating forage fish population. With no weed cover, bluegill and other forage fish numbers are reduced to levels that can not support healthy bass populations. The result is many 12 inch or smaller bass in poor condition, and few small bluegill.

Ducks and other water fowl also are dependent on aquatic plants for food or they feed upon organisms that inhabit these plants.

Aquatic plants also help stabilize pond banks and the pond bottom.

What is excessive aquatic plant growth?

It is apparent that some balance must be maintained between complete eradication and unchecked plant growth. Most pond owners should try to maintain aquatic plant coverage at about 15-20 percent of pond surface area. This amount of vegetation can provide food for waterfowl and organisms in the aquatic food chain as well as cover for forage fish.

Causes of excessive aquatic plant growth

It is of little use to spend time and money killing noxious aquatic plants without first reducing or eliminating the root causes of the problem. Aquatic plants need the following conditions to become troublesome: Light, Shallow water and Excessive nutrients entering the pond.

Light

Light is required for photosynthesis. Aquatic plants can not grow at depths that are inadequately lighted. Light penetration can be limited by water turbidity due to clay (muddy ponds) or by phytoplankton (microscopic floating plant) growth. In some ponds, dyes can be used to reduce light penetration and control aquatic vegetation.

Shallow water

Shallow water is related to the problem of light penetration. Large areas of clear shallow water provide ideal habitat for most species of aquatic plants. Ponds become shallow as they age due to several factors.

Eroding pond banks and watersheds can deposit sediments into the pond that can greatly reduce depth and the productive life of a pond. Heavy rains can wash large quantities of mud into ponds with unprotected water sheds. Pond water sheds should be completely grassed or forested to prevent erosion.

Livestock also reduce pond depth by breaking down pond banks. Livestock trails are a common source of erosion that can eventually ruin ponds. Pond banks should have a 3:1 slope, that is for each 3 feet measured out into the pond, water depth should drop 1 foot. A 3:1 slope is steep enough to reduce rooted aquatic plants to a narrow band around the pond.

Ponds can become shallow over time due to accumulation of leaves from neighboring trees or from a build up of decaying aquatic vegetation. Old shallow ponds should be renovated by draining and reclaiming depth and bank contours.

Excessive nutrients

Excessive nutrients enter the pond from the watershed, or are deposited directly into the pond by livestock or water fowl. Animal waste is most often the cause of excessive aquatic plant.

growth. In rural areas livestock waste or crop fertilizers are of more concern. In urban areas excess or inappropriately applied nitrogen and phosphorous lawn and garden fertilizers are often the main nutrient problem source. Septic tank leakage also can significantly contribute to excessive nutrient levels in many areas.

It does little good to try to control aquatic vegetation until nutrient sources are reduced or removed. The vegetation will re-grow quickly or be replaced by other species.

The most serious aquatic vegetation problems occur in ponds that are clear, shallow and receive a high nutrient load from the water shed.

Repair erosion prone areas of the water shed and reduce nutrient levels entering the pond as a first step in reducing aquatic vegetation.

Pond and watershed nutrient management should be the first step in rooted aquatic plant control. Use a 100 foot buffer strip of grass or other vegetation around the pond. Reduce or redirect runoff water containing animal waste or fertilizer.

Pond treatments

In general it is best to treat aquatic vegetation problems in spring and early summer while plants are growing rapidly and water temperature has reached at least 65o F. Herbicides are most effective at this time.

Grass carp can be stocked from early spring through fall, however, they are more likely to survive if stocked in spring or summer.

Herbicide treatments

Herbicides considered in this article are based on 4 common active ingredients. Different brand names have somewhat different formulations. Active ingredients are:

Glyphosate - trade names include Rodeo, Pondmaster

2,4,-D - trade names include Navigate and WeedRhap

Fluridone - trade names include Sonar and Avast

Diquat - trade names include Diquat and Reward

Endothall - trade names Aquathol and Hydrothol

Triclopyr - trade name, Renovate

Many herbicides are not registered for use without an applicators license. Read all label warnings and all directions before purchase or use of herbicides. A listing in this article is not an endorsement of any particular herbicide.

Herbicides do not solve underlying causes of aquatic vegetation problems. Consequently, herbicide applications often must be repeated regularly to maintain vegetation control.

Aquatic plant identification

Plant identification is important when selecting appropriate control measures. Use the illustrations and accompanying text to identify plants. 

For most purposes the plants can be divided into five groups:

Algae

Emergent plants

Submerged rooted plants

Free-floating plants

Rooted plants with floating leaves

Some plants will fit into more than one of these categories.

Algae

Algae are the most important plants in the pond. Microscopic algae called phytoplankton are responsible for the green color of many ponds. These plants produce most of the oxygen that aquatic organisms use. Heavy blooms of phytoplankton (secchi disk reading of less than 15 inches) can look and smell noxious. Heavy phytoplankton blooms may die out suddenly. When this occurs, oxygen in the water is consumed as the dead material decays. If oxygen levels drop low enough most of the fish in the pond will be killed.

Filamentous algae

Often called "moss", filamentous algae are long and hair-like. They reach nuisance levels in many ponds that contain high nutrient levels. Filamentous algae grow on the pond bottom in late winter through early summer.  As water temperatures warm, oxygen and other gases collect under the filamentous mat and cause it to float to the surface. Filamentous algae also grow on most plants and other objects in the pond. There are two major kinds of filamentous algae found in Oklahoma ponds and they often can be found in the same pond. Spirogyra is somewhat more common in the early part of the year and can be identified by its dark green color and slimy texture. Pithophora is usually more prevalent in summer. It can be identified by its yellow green color and coarse, hair-like texture. Too much filamentous algae interferes with angling and swimming. It can also affect the taste of drinking water.

Macrophytic algae

Macrophytic algae are commonly mistaken for rooted aquatic plants because they appear to be rooted to the pond bottom and have stem and leaf-like structures. Chara, also called stonewort is the most common of these algae. It can be identified by its musky smell ( somewhat skunk like) and by the coarse texture of the plant which is attributable to large amounts of calcium carbonate deposited on the plant surface. Click on the plant name for illustration and description. Stonewort - Chara sp.

Algae control

Begin by reducing nutrient runoff into the pond from animal waste, feedlots, fertilizers and septic systems. If the pond is shallow and the bottom is covered in a deep muck, often found in old ponds, it may be best to renovate the pond by draining the water and removing the sediments. Sediment removed from the pond should be placed downstream from the pond watershed. Before the pond is allowed to refill, make sure no exposed soil will be allowed to wash into the pond.

Mechanical control of algae

In most instances it is not practical to control algae by mechanical means. It is possible to clear small areas with rakes, seines or by hand; however, it takes a large amount of time and labor. Also, algae will quickly repopulate the area negating the effort.

Biological control

Grass carp will not control planktonic algae.

Grass carp can control filamentous algae if the pond is stocked at a density of 40-60 small (8 inch or smaller) grass carp per acre. This is not practical or advisable for most pond owners. Gras carp can effectively control macrophytic algae such as Chara at stocking densities of 4-6 fish per pond surface acre.

Chemical control

Algae can be controlled with most chelated or copper based herbicides. Examples of commonly used herbicides approved for algae control include: Copper Sulfate, Algae Pro, Cutrine Plus, K-Tea, Clearigate and Captain

Copper compounds can be toxic to fish when applied at higher than directed rates. These products are more toxic in ponds with alkalinity levels below 50 mg/l. Koi carp, ornamental fish and trout are more sensitive to copper compounds than many other species. Test alkalinity and treat the pond based on this test if you have valuable pond fish (See LU fact sheet "Water Quality In Sportfish Ponds" for alkalinity testing information.). Treat only 1/4 of aquatic vegetation per application to avoid oxygen related fish kills. Repeat the treatment at 2 week intervals until the desired quantity of vegetation is killed.

Emergent plants

Emergent plants may be rooted in the pond bottom or on the shoreline. Their leaves and flower parts extend above the water surface. Excessive emergent plant growth can block pond access and render bank fishing nearly impossible. Emergent plants grow rapidly and are difficult to control. If possible, physically remove the plants at the first sign of an infestation.

Common examples are illustrated.  Click on the plant name for illustration and description. 

bulrush Scirpus sp.                  cattail Typha sp.

spikerush Eleocharis sp            burreed Sparganium sp.

sedges Cyperus sp.                  smart weed Polygonum sp,

water primrose Ludwigia sp.     arrowheads Sagitarria sp.

pickerelweed Pontederia codata      willows Salix sp.

Mechanical control of emergent vegetation

Cattail can be controlled by cutting unwanted vegetation. Control is most effective when plants are cut in late spring or early summer (June through mid-July), before seed heads turn brown and mature. Timing is important because cattail roots consist of thick starchy rhizomes that store food over the summer. Rhizomes spread in the pond soil and new cattails emerge along the length of the root. If plants are cut after energy stored in the rhizome has been expended in producing new shoot growth and seed heads; the plant is more likely to be killed. It can require timed cutting for 2-3 consecutive years before cattail is controlled. Cutting cattail in the fall is rarely an effective means of control.

Emergent vegetation can be dug or raked from the pond bottom by hand. This is a laborious and time consuming procedure, however, it can be easily accomplished with a back hoe or track hoe. As cattails are removed the equipment operator can steepen pond bottom to the recommended 3:1 slope and help prevent future emergent plant problems.

Biological control

Grass carp are not effective for control of emergent aquatic vegetation. They may consume some new growth but will not eat mature plants.

Chemical control

The following herbicide have proven excellent or good in controlling emergent plants.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Free Floating Plants

Free floating plants are small, usually about the diameter of a pencil eraser or smaller and not rooted in the soil. Common examples are illustrated.  Click on the plant name for illustration and description. 

Duckweed Lemnea sp.       Watermeal Wolffia sp.     Water fern Azolla caroliniana

Low numbers of these plants do not cause pond problems however, they can reproduce quickly and completely cover the water surface under favorable conditions. They are found at nuisance levels in ponds protected from wind and containing excessive nitrogen and phosphorous levels.

Research has been conducted to convert these fast growing plants into livestock feeds but their high water content has prevented development of economically useful rations.

Free floating plants shade ponds and reduce phytoplankton production. Phytoplankton form the base of the food chain and also release oxygen into the water as a byproduct of photosynthesis. Free floating plants can reduce pond productivity and contribute to oxygen depletion related fish kills.

Mechanical control

Reduce nutrient inputs into the pond as a first step in free floating plant control. Nutrient laden muck in old, shallow ponds may have to be removed to adequately reduce pond nutrient levels.

Free floating plants can be removed from small ponds by skimming the surface with small mesh netting or screen. This is only a temporary solution but may improve pond conditions for a brief period.

Biological control

Grass carp readily consume free floating plants however, they seldom provide adequate control due to the rapid reproductive rate of the plants.

Chemical control

The following herbicides are currently approved for use in duckweed and watermeal control: Weedtrine-D and Sonar

In ponds known to have severe duckweed or watermeal infestation, begin treatments as soon as plants begin to appear.

Treat no more than 1/4 t o1/3 of the pond at a time to prevent fish kills due to low dissolved oxygen caused by rotting dead vegetation. Repeat treatment at 1 week intervals.

Treat the pond when the water surface is calm and follow label directions. A non-ionic surfactant mixed with the herbicide is often recommended.

 

 

 

 

 

Submerged, Rooted Plants

Submerged rooted aquatic plants are often found in ponds with large areas of clear, shallow water. The plants form dense beds that restrict fishing. Large areas of these plants provide too much cover for bluegill and other forage fish. This situation results in crowded or stunted bluegill and a bass population characterized by poor growth and little reproduction. Decaying vegetation associated with large submerged aquatic plant beds contribute to oxygen induced fish kills.

Common examples are illustrated.  Click on the plant name for illustration and description. 

Pondweed Potamogeton sp.          Naiad Najas sp.

Coontail Ceratophyllum sp.            Milfoils Myriophyllum sp.

Mechanical control

In old silted in ponds or ponds with extensive areas of shallow water, the best control method is to drain the pond and deepen and contour the pond bottom.

Aquatic dyes can effectively control submerged plants in ponds that do not have water flowing through them regularly. Dyes color water green, blue or black and prevent light penetration necessary for plant growth.

Dye treatments should begin in early spring before plant growth becomes abundant.

Dyes must be used cautiously in ponds with heavy submerged plant growth. Plants will begin to die as the dye blocks needed light. Dead vegetation uses up dissolved oxygen as it decays and may reduce oxygen to levels low enough to cause a fish kill.

Small areas can be cleared by raking or pulling the vegetation. However, regrowth is rapid and other methods are recommended for long term control.

Biological control

Grass carp can control effectively most submerged rooted aquatic vegetation. Stocking density depends on extent of plant coverage. (See fact sheet titled "Controlling Vegetation With Grass Carp"). Usually 4-8 grass carp per acre are enough to provide adequate control within 2-3 years. Grass carp should be restocked if plant growth increases significantly. Life span in most ponds is about 8 years but it has been documented that grass carp can live for up to 15 years in some ponds.

Chemical control

Herbicides approved for control of submersed aquatic plants:

 

 

 

 

 

 

 

 

 

 

Rooted Plants With Floating Leaves

Many of the rooted plants with floating leaves have beautiful flowers and are purposely planted by pond owners. The plants are allowed to spread and may reach nuisance levels, particularly in shallow or clear ponds.

Common examples are illustrated.  Click on the plant name for illustration and description. 

American lotus Nelumbo lutea          Water lily Nymphaea sp.

Spatterdock Nuphar sp.                    Watershield Brasenia schreberi

 

 

 

 

 

 

Identifying plants in the lily family  -  American lotus has large round leaves 18_25 inches in diameter. Water lily has circular leaves, 6_8 inches in diameter and notched to the center. Spatterdock leaves are circular to heart shaped, notched and 6_10 inches long. Leaf veins radiate to edge of leaf from a lateral, central vein. Watershield leaves are oval, about 4 inches in diameter and without a notch in the leaf. Click on plat species names given above for an illustration and more detailed description.

Mechanical control

These plants can be removed by digging, however rhizomes spread over large areas and they all must be removed to eradicate the plant.

Consistent repeated efforts will control the plants.

Cutting is another effective means of control for the floating leaved plants. Cutting is most effective if done just before flower buds open in early summer. Remove cuttings from the pond; or alternatively, cut only 1/4 to1/3 of the pond at 1-2 week intervals to prevent low dissolved oxygen conditions that can result in fish kills.

When possible, water level drawdown can be used to control floating leaf plants. Drying or freezing the plants requires at least 4 weeks or more of exposure. Winter is the best time to draw down the pond both because freezing weather helps kill the plants, low dissolved oxygen conditions are less likely to occur and because the pond can be refilled with spring rains. Leave 6-8 feet of water in the pond to safely overwinter fish.

In many ponds draw-down is not practical because of livestock watering needs, lack of drains, drought conditions or because the pond was too shallow to begin with.

Biological control

Grass carp do not effectively control the floating leaf plants.

Chemical control

Rodeo or similar glyphosate herbicides can control floating leaf plants if precautions are followed. Currently, Rodeo is labeled for use only for American lotus, however, it has been found to be effective on water lily and water shield.The following measures must be taken for this herbicide to be effective:

 

 

 

 

 

 

 

1. Use with a surfactant to help the herbicide "stick" to the plant.

2. Application must be made in calm weather to prevent the herbicide from being washed off the leaves.

3. Rodeo will work best if applied during bright sunlight.

4. The herbicide must completely wet the leaves and must dry on the plant for 2-6 hours.  So apply in the morning after dew evaporates.

5. Wind spray or boat wakes can wash the herbicide off the leaves and render the treatment ineffective. Avoid splashing water over the leaves during and after application.

6. Follow manufacturer label directions.

 

 

 

 

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Copyright© 2000 Langston University • Agricultural Research and Extension Programs 
P.O. Box 730 • Langston, OK 73050 • Phone 405.466.3836


Copyright© 2000 Langston University • Agricultural Research and Extension Programs 
P.O. Box 730 • Langston, OK 73050 • Phone 405.466.3836