If you have a freshwater pond with fish, it is time to get that phytoplankton bloom going.
This is the most important annual event in pond management. Start fertilizing at the right time and do it the right way, and one avoids a plethora of problems that either are expensive to correct or wholly uncorrectable — at least until next year. Good pond management can quadruple the pond’s fish productivity.
Start fertilizing your pond when the surface-water temperature hits and holds 60 degrees Fahrenheit. The cold, cloudy weather has slowed down the usual water warm-up. A floating pond thermometer can be purchased for less than $12 and can save you twice its cost if it keeps you from applying just one bag of fertilizer too soon. Repetition aids comprehension, so here is the how-to on fertilizing your pond once it hits and holds 60 degrees from one of my columns last December:
As soon as the surface water temperature consistently holds 60 degrees you start fertilizing the pond with 40 pounds of a 20-20-5 granular fertilizer per surface acre of pond. We are trying to get the phytoplankton population to bloom early in the year in the top 18 inches of the pond water. Don’t spread the fertilizer across the pond surface because the granules will just sink to the bottom and be lost.
Open the flat side of a bag of 20-20-5 granular fertilizer by cutting an “H” into the top flat surface of the bag and gently lowering it into the edge of the pond just deep enough to cover the bag. Pull the two center panels open and there you have it!
Keep adding fertilizer the same way every two weeks until the proper color develops. Space the bags around the perimeter of the pond to get as even a supply of fertilizer into the top 18 inches of water.
The proper color is reached when a shiny object lowered 18 inches into the pond cannot be seen. If the proper color has not developed within six weeks, have the pond checked for water hardness.
Water that is too soft will not support the phytoplankton. The phytoplankton do a number of good things for your pond. They produce most of the oxygen the fish breathe, are the start of the food chain in the lake, and shade the water. Cooler water holds more oxygen than warmer water, which supports more fish. The shaded water does not allow enough light for immersed weeds to grow.
Phytoplankton also tends to out-compete the filamentous algae. Properly started, weed management in a pond easily can be done with triploid (grass) carp instead of expensive herbicide chemistry. Keep fertilizing throughout the summer to maintain the right color. If shiny objects disappear at a 12-inch depth, stop fertilizing until the shiny object is again visible deeper than 18 inches.
None of this takes much time. It’s easy to do and pays back big dividends in fresh fish for your table. Proper pond fertilization greatly reduces algae and immersed weed problems. Those weeds that do occur can be controlled by adding triploid grass carp to the pond. They eat the weeds for years, or until that blue heron swoops in and spots them.
Even with predation, grass carp offer major benefits over using chemical herbicides. First, have you priced pond herbicides lately? The point of the fish pond is the enjoyment of fishing plus a less-expensive protein source for your diet that you produce on your land. If you compare the cost of pond herbicides and other chemicals as well as application equipment to a few bags of fertilizer, the choice should be obvious.
This also is like the tide — it waits for no man. The time to move on this is now.

Gardner is an ag and natural-resources agent for the University of Georgia’s Glynn County Extension.