Pond Weeds

I usually get a number of calls about all this “junk” on or in our tanks and ponds with the question, “what can I do about it?” Unfortunately there isn’t much we can do about it this time of year but maybe an explanation about the categories of pond weeds and some control measures might be helpful.
The first group of weeds are the algae including plankton which makes the green color in water, filamentous algae or pond scum and branched algae which includes chara or muskgrass that looks like underwater hay. Pond scum is probably the number one problem in tanks and causes the most aggravation. Pond scum usually begins growing near the bottom or edges of a pond and later floats to the surface where it then looks like a mass of wet, green wool. This type algae is best controlled by pond fertilization back in February. Since it starts at the bottom if we can encourage the growth of plankton, which is a good algae, these will shade out the pond scum and keep it from growing. There is nothing worse than a clear pond because the plankton are part of the food chain which eventually feeds fish. Chemically we control algae easily with copper or copper complex chemicals.
A second group of weeds are the floating plants. Duckweed is one that we have in abundance in our area and it is a small, floating plant, green in color and about ½ inch across with usually 3 leaves and below the leaves you can see a root. Diquat is a good, relatively inexpensive chemical control or you can rake this plant off the surface.
The third group we see a lot here are submersed plants. These plants are rooted to the bottom but generally don’t have plant parts above the water surface. The most common submersed weed is bushy pondweed which resembles coastal hay growing underwater. Another similar weed is coontail and it too can fill up a pond in short order. Diquat, endothall and floridone are all chemical controls or you may want to check into the triploid grass carp which do an excellent job of long term control of these problem weeds.
The last category includes our emersed weeds which includes all shoreline, marginal and shallow water plants with plant parts that extending above the water line. These include many species but most commonly we are dealing with cattails, willow, rushes, buttonbush, water primrose and frogbit. Most of the emersed weeds are easily controlled with glyphosate products which we commonly call Roundup although Roundup itself is not labeled for aquatic weed control. There are several name brand products that do contain glyphosate and are labeled for aquatics. Another excellent product for emersed weeds is 2,4-D.
Now that last question I usually get when talking about weed control in ponds is, “will it hurt my fish?” The chemicals themselves are harmless to fish but the dead vegetation they leave behind may not be. A lot of decaying vegetation will suck a lot of oxygen from the water and could leave your fish starving for a breath. It is best to control pond weeds a little at a time so that you don’t set yourself up for a problem one morning watching all your fish floating on the surface.