Mechanical Methods of Control 



Submersed plants can be removed mechanically with an under- 

 water mower, such as illustrated in Figure 5, which employs 3 

 cutter bars operated from a boat which may use an outboard motor, 

 air motor, or other means of forward propulsion. This mowing 

 device gives good results along resort beaches and is occasionally 

 used in small lakes and fish ponds, but the benefits are usually 

 temporary. 



The Ziemsen submarine weed-cutting saw (Figure 6) has long 

 been in use for hand removal of weeds , 



Control with Fertilizers 



Fertilization has the advantage over chemical weed poisons, 

 because its addition increases productivity of the pond. 



In this preferred method of weed control, the propagation and 

 growth of one group of plants is fostered over the growth of 

 another. 



Microscopic green or blue-green plants (algae) , encouraged by 

 the addition of fertilizers, multiply in the open water where they 

 drift about, thereby shutting out light which might be available 

 for the coarse plants on the bottom. With proper fertilization 

 methods, at least in soft water, available nitrogen (N) , phosphorus 

 (P) , potash (K) , and other elements needed by the microscopic 

 plants, may be furnished them on a schedule more favorable to their 

 growth than it is to the coarse plants. This method of weed control 

 for new farm ponds was first suggested by Swingle and Smith (1939) , 

 who have also recommended fertilization for control of weeds in 

 ponds where they have already become a nuisance. 



For new ponds, the fertilization method is relatively simple: 

 as soon as the pond is filled with water, about 100 pounds per acre 

 of inorganic fertilizer of the combinations listed below, are added 

 at weekly intervals until the water becomes green or brownish from 

 the growth of microscopic plants. This usually requires two to 

 four applications, after which the fertilizer is added at about 

 monthly intervals, or when needed to make the water more turbid. 

 The criterion of "when needed" is transparency to a depth of 12 

 to 18 inches beneath the surface. When an object such as a white 

 dish or the hand can be seen at depths greater than about 18 inches, 

 it is time to add more fertilizer. For the production of the 

 microscopic plants in new ponds to prevent growth of coarse weeds, 

 fertilization only during the warm months of the year is necessary. 

 If the new pond is completed late in the fall, fertilization should 

 begin when the weather begins to warm in the spring. The following 

 fertilizer formulas are recommended for new or old ponds: 



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