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SRAC Publication No. 361

February 2001 VI PR Revision

AQUATIC WEED MANAGEMENT

Michael P. Masser,1 Tim R. Murphy2 and James L. Shelton2

Managers can quickly and eco- The selected must be with herbicides during the hot nomically control problem weeds labeled for use with food fish. It is summer months is risky, because in commercial fish ponds with important to note any water-use at this time of year dissolved oxy- aquatic herbicides. However, her- restrictions that may prevent the gen tend to be bicides are just one method of application of a herbicide in a par- lower and weed biomass higher. managing aquatic weeds. There ticular situation on a specific body Treating only one-fourth to one- are also: 1) preventive methods of water (Table 2). Restrictions on third of the total surface area of a such as proper pond site selection secondary water uses (i.e., swim- pond at one time can minimize and construction, fertilization and ming, livestock watering, and irri- the risk of depleting dissolved periodic draw-downs; 2) biologi- gation) also must be considered . However, some herbi- cal methods such as grass carp before a herbicide is applied. cides cannot be used for partial (Ctenopharyngodon idella); and 3) pond treatments. During the sum- mechanical methods such as cut- Application timing mer, even partial treatments may ting, seining and raking. Using a be risky in some ponds. combination of methods, within a The best time to apply herbicide is comprehensive plan, is the most in the spring when water temper- o Application methods cost effective and environmentally ature is between 70 and 80 F. At safe way to manage aquatic this time of the year, weeds are The herbicide formulation and the weeds. SRAC Publication No. 360, small and easier to control than weed species determine the appli- Aquatic Weed Management - Control during the summer, and levels of cation method. Many herbicides Methods, contains additional infor- dissolved oxygen in the water are can be applied directly from the mation on the various methods usually higher. container, while others must be used to control undesirable weeds Aquatic herbicides are not toxic to diluted with water first. in fish ponds. Once undesirable fish when applied according to To treat large areas you will need weeds are eliminated, applying label directions. However, after a mechanical sprayer or spreader fertilizer periodically will stimu- aquatic weeds are killed the and a power boat to ensure ade- late planktonic algal blooms that decomposition process consumes quate distribution of the chemical. suppress the growth of sub- oxygen and can reduce the Sprayable herbicide formulations merged weeds. amount of dissolved oxygen in can be applied with hand-held or the water. If large quantities of mechanical pressurized sprayers Herbicide selection aquatic weeds are killed, their or with a boat bailer. Injecting the decomposition can reduce the dis- chemical near the outboard motor The effectiveness of herbicides solved oxygen to propwash will help it disperse. varies (Table 1). The first critical such a low level that fish die. It is Hand-operated or mechanical step in selecting an appropriate important to observe fish closely rotary spreaders can be used to herbicide is identifying the weed. for 1 week after treatment. Have apply granular or pelleted formu- emergency aeration equipment lations. Soluble crystals, such as 1Texas A&M University handy and aerate the pond if fish copper sulfate, can be dissolved in 2University of Georgia seem stressed. Treating a pond water and sprayed over the pond; or, the required amount can be Acre-foot treatments: Aquatic herbicides placed in burlap bags and An acre-foot of water equals 1 dragged behind a boat, or sus- The herbicides discussed in this surface acre of water that is 1 foot pended in the water near an aera- section are labeled for use in com- deep. The number of acre-feet of tor, until the herbicide dissolves. mercial fish production ponds. water can be found by multiply- Before using any herbicide, read If herbicide will be applied to ing the number of surface acres and understand the label. emergent weed foliage, adding a times the average water depth. surfactant to the chemical may The amount of herbicide needed Copper Sulfate help it wet and penetrate the for an acre-foot treatment is deter- (Various trade names) foliage. Use only registered aquat- mined by the following formula: ic surfactants and follow product Copper sulfate, often called “blue F = A x D x R label directions. Surfactants are stone,” is primarily used to con- not recommended when treating F = Amount of formulated trol algae. It is a contact herbicide. submerged weeds. herbicide product However, it does not control algae A = Area of the water surface such as Pithophora. Copper can Herbicide dosage in acres interfere with gill functions and, if improperly used, can be toxic to Aquatic herbicides must be D = Average depth of water in feet fish and zooplankton. Trout and applied at labeled rates. Appli- R = Recommended rate of product koi are particularly sensitive to cation rates were developed from per acre-foot copper. However, most fish kills extensive research and provide after copper sulfate treatment are effective, yet safe, weed control. PPMW treatments: related to a massive algae kill and Applying an excessive rate of a the subsequent depletion of dis- herbicide does not provide better Some treatment rates are given as solved oxygen. weed control but does increase the the final concentration of the cost of the treatment and may chemical in the water body on a The effectiveness and safety of increase the risk of injury to fish part per million by copper sulfate are determined by and other organisms. Applying (ppmw) basis. The amount of pH, alkalinity, hardness, water less than the recommended rate herbicide needed for a ppmw , and several other usually results in poor weed con- treatment is determined by the environmental factors. In water trol. following formula: with an alkalinity ≤ 50 ppm, the rate of copper sulfate needed to Some herbicides, such as those for F= (A x D x CF x ECC) ÷ I control algae can be toxic to fish. control of emergent plants, are F = Amount of formulated Copper treatment at water alka- applied on the basis of the area to herbicide product linities of ≤ 20 ppm is extremely be treated. Others, such as those risky and should be avoided. In used to control certain submerged A = Area of the water surface in acres high alkalinity (≥ 250 ppm) water, weeds, are applied on the basis of copper sulfate quickly precipitates the volume of water to be treated. D = Average depth of the water out and is not effective for algae Read the label instructions care- in feet control. The toxicity of copper sul- fully, because mistakes in calculat- CF = 2.72 pounds/acre-foot (the fate to fish increases as water tem- ing treatment rates can be costly conversion factor—CF— perature increases. Avoid copper and dangerous. For information when total water volume is sulfate applications during hot on calculating the area and vol- expressed on an acre-foot summer months. ume of ponds see SRAC basis; 2.72 pounds of a Publication No. 103, Calculating For additional information on herbicide per acre-foot of treating with copper, see SRAC Area and Volume of Ponds and water is equal to 1 ppmw) Tanks. Publication No. 410, Calculating ECC = Effective chemical concen- Treatments for Ponds and Tanks. Surface acre treatments: tration of the herbicide’s active ingredient that is Chelated Copper The amount of herbicide needed needed in the water to (Cutrine®, Komeen®, K-Tea®, for a surface acre treatment is control the weed others) determined by the following for- I = The total amount of active mula: Copper that is held in an organic ingredient divided by the total complex is known as chelated F = A x R amount of active and inert copper. Chelated coppers are used ingredients. F = Amount of formulated to control planktonic and filamen- herbicide product. For products, I = pounds of tous algae. Chelated copper for- A = Area of the water surface active ingredient ÷ 1 gallon mulations do not readily precipi- in acres For dry products, I = percent tate in high alkalinity waters, but stay in and remain active R = Recommended rate of product active ingredient ÷ 100% longer than copper sulfate. per surface acre Chelated coppers are sometimes mixed with other aquatic herbi- endothall. Hydrothol® is more should be used with cides (e.g., diquat) to better con- toxic to fish and aquatic inverte- (Rodeo® formulations only). If trol algae as well as certain brates, so Aquathol® is generally rain falls within 6 hours of appli- species of submerged plants (see used in commercial ponds. cation, the effectiveness of labels). Chelated copper is less Hydrothol® controls algae (fila- glyphosate will be reduced. corrosive to application equip- mentous and stoneworts) and ment than copper sulfate. It is also many submerged weeds. 2, 4-D slightly less toxic to fish. Aquathol® controls many sub- (Various trade names) However, in water with low alka- merged weeds but is not effective 2,4-D is a translocated herbicide linity (≤ 20 ppm), or in water with for filamentous and macro-algae that is available as a granular or an alkalinity of ≤ 50 ppm that con- control. Both Aquathol® and liquid formulation. Granular 2,4-D tains trout, it is extremely risky to Hydrothol® are contact herbicides controls submerged weeds such use chelated copper, particularly and may be used for spot or par- as coontail (Ceratophyllum demer- during the hot summer months. tial pond treatments. sum) and emergent weeds such as Diquat Fluridone water lily (Nymphaea spp.) and water shield (Brasenia schreberi). ® ® ® (Reward , Weedtrine-D ) ( ) Liquid formulations of 2,4-D are Diquat is a contact herbicide that Fluridone controls most sub- used to control floating weeds can be sprayed on or injected into merged and emergent weeds and such as water hyacinth (Eichhornia water to control submerged is available as a liquid or pelleted crassipes) and several emergent weeds and filamentous algae; or, formulation. Liquid formulations weeds. 2,4-D is available as an it can be sprayed on duckweed also control duckweed and water- ester or formulation. Amine (Lemna minor and Spirodela meal. Fluridone is a translocated formulations are slightly better for polyrhiza) or emergent vegetation. herbicide that slowly kills plants aquatic applications because they Repeated applications on surface over a 30- to 90-day period. Its are less toxic to fish, although the mats of algae (e.g., Pithophora) slow action generally prevents the granular ester form is safe to use. may be necessary. An approved depletion of dissolved oxygen. Only those formulations of 2,4-D non-ionic surfactant is required Fluridone is not effective as a that are labeled for aquaculture when diquat is used as a foliar spot treatment. The entire pond are legal to use in culture situa- application. Diquat binds tightly must be treated to control the tar- tions. to clay particles and is not effec- get weed. The information and suggestions tive in muddy water or on mud- included in this publication reflect coated weeds. Diquat quickly kills Glyphosate the opinions of Extension fisheries plants and should be used as a (Rodeo®) specialists based on field tests and partial pond treatment for dense treatment experience. Manage- vegetation. Glyphosate is a foliar-applied, ment suggestions are based on translocated herbicide used to research and are generally effec- Endothall control most shoreline vegetation tive. Conditions or circumstances (Aquathol®, Hydrothol®) and several emergent weeds such which are unforseen or unexpect- as spatterdock (Nuphar luteum) ed may lead to less than satisfac- Two salts of endothall are used for and alligatorweed (Alternanthera tory results even when best man- aquatic weed control. A dipotassi- philoxeroides). Glyphosate translo- agement practices are used. um salt (trade name Aquathol®) is cates from the treated foliage to Neither the Cooperative Extension available as a granular or liquid underground storage organs such Service nor the Southern Regional formulation. Hydrothol® is avail- as rhizomes. It is most effective Aquaculture Center assumes able as a liquid or granular formu- when applied during the weed’s responsibility for such occur- lation and is a mono-(N,N- flowering or fruiting stage. An rences. All risk shall be assumed dimethylalkylamine) salt of approved non-ionic surfactant by the applicator. Table 1. Treatment response of common aquatic plants to registered1 herbicides. Aquatic herbicides2 Copper and Aquathol¨ Aquatic group copper Reward¨ Hydrothol¨ Rodeo¨ Sonar¨ and vegetation complexes 2,4-D (diquat) (endothall) (glyphosate) (fluridone) Algae planktonic E P P G3 PP filamentous E P G G3 - P4 PP Chara/Nitella EPPG3 - P4 PP Floating plants duckweeds P F5 GPPE salvinia P G G G E water hyacinth P E E G P watermeal P F F G Submerged plants coontail P G E E P E elodea P E F P E fanwort P F G E P E naiads P F E E P E parrotfeather P E E E F E pondweeds P P G E P E Emergent plants alders P E F P E P arrowhead P E G G E E buttonbush P F F P G P cattails P F G P E F common reed P F F E F water lilies P E6 PGE frog’s-bit P E E pickerelweed P G G F P sedges and rushes P F F G P spike rush P G P G smartweed P E F E F southern watergrass P P E G water pennywort P G G G P water primrose P E F P E F willows P E F P E P 1 Registered as of 4/99 by the U.S. Environmental Protection Agency (EPA). 2 E = excellent control, G = good control, F = fair control, P = poor control, blank = unknown or no response 3 Hydrothol¨ formulations. 4 Aquathol formulations. 5 Liquid 2,4-D formulations. 6 Granular 2,4-D formulations. Table 2. Restrictions on the use of water after treatment with aquatic herbicides1 (number of days after treatment before use in private waters only). Human use Livestock Irrigation Common name Drinking Swimming Fish Watering Turf Crops copper sulfate2 00 0 0 00 copper complexes 0 0 0 0 0 0 2,4-D ✳✳ ✳ ✳ ✳✳ diquat 2-3 0 0 1-33 2-3 5 endothall4 7-25 1 3 7-25 7-25 7-25 glyphosate5 00 0 0 00 fluridone6 0 0 0 0 7-30 7-30 1 Aquatic vegetation control (particularly algae) can cause low dissolved oxygen, which can stress and/or kill fish. It is best to treat most aquatic vegetation early in the growing season, when the plant is rapidly growing. Treating small areas (e.g., one-fourth of the pond at a time) at 10- to 14-day intervals usually prevents serious oxygen depletion. 2 If water is for drinking, the elemental copper concentration should not exceed 1.0 ppm (i.e., 4.0 ppm copper sulfate). 3 Depends on formulation. Read the label. 4 Length of use restriction for endothall varies with the concentration used. Read the label. 5 Do not apply within 0.5 mile of a functioning potable water intake. 6 Do not apply within 0.25 mile of a functioning potable water intake. ✳ Water restrictions on 2,4-D vary with formulation, rate and time of year. Read the label.

Precaustions County Extension agents and arise from drift or movement of appropriate fisheries/aquaculture herbicide from his/her property to All aquatic herbicides must be specialists are advised of changes that of others. Always read and registered and labeled for use by as they occur. Please check with follow carefully the instructions the Environmental Protection your Extension Service if ques- on the container label. For addi- Agency and the Department of tions arise. tional information on aquatic Agriculture. The status of herbi- vegetation information and man- cide label clearances is subject to The applicator is always responsi- ble for the effects of herbicide agement see the following Web change and may have changed site: http://wildthings.tamu.edu/ since this publication was printed. residues on livestock and crops, as well as problems that could aquaplant. The information given herein is for educational purposes only. Reference to commercial products or trade names is made with the under- standing that no discrimination is intended and no endorsement by the Southern Regional Aquaculture Center or the Cooperative Extension Service is implied.

The work reported in this publication was supported in part by the Southern Regional Aquaculture Center through Grant No. 98-38500-5865 from the United States Department of Agriculture, Cooperative State Research, Education, and Extension Service.