That green stuff all over our still waters is probably not algae like most people think. It’s a floating flowering plant without stems called duckweed (Lemna sp.). Its growth rate can be phenomenal. Under good conditions duckweed can double its biomass in less than 24 hours. This means it can cover an acre of water in less than a month and a half.
As one might suspect this aquatic invader is eaten by many species of ducks. The problem
is that the birds don’t really control it. They spread it to other waters. These floating plants cover the surface and eliminate light to submerged aquatic plants, causing their demise.
Anything that dies, whether it is plant or animal, must decompose. This generally requires oxygen, which in aquatic environments is less available than in terrestrial ones. The result is a decrease in dissolved oxygen of the water and that causes problems for many aquatic animals including fish.
But duckweed has many redeeming qualities. It helps purify water by extracting nutrients from it. It has the potential to remove fertilizer nutrients from hog and other animal waste. When skimmed off lagoons and processed it has great feed value for fish and livestock. It’s high in protein and highly digestible.
It’s probably not a great human food source as high oxalate content could pose a problem for kidney stone formers. Small quantities are fine in survival situations as it’s not poisonous and is easily digestible. Bacterial contamination might concern me though. I don’t eat it.
Use of dried duckweed as a fertilizer has potential. It decomposes quickly and provides organic matter, nitrogen and other nutrients to the soil. Skimming it off the surface is a simple process, but removing all of it this way is impossible. Don’t expect to cure your duckweed problem simply by mechanical removal.
Duckweed research is ongoing. Its potential as a biofuel is well documented. Phenomenal growth rate make it better suited for the production of carbon-based fuels than any of our agricultural crops. Several sources claim gasoline produced from duckweed would be competitive with a $72-$100 per barrel crude oil price.
It tolerates a wide range of water conditions too. Duckweed can thrive at a pH range of 4.5 to 7.5. Fertilization is unnecessary. In fact, duckweed removes runoff fertilizer from our waterways, which cleans the water.
All this talk about livestock feed, fertilizer and biofuels mean nothing to you if you have a farm pond and don’t want it overrun with duckweed. There is hope. Several approved herbicides including Diquat will kill it. Proper pesticide certification is necessary, so if you don’t have it please hire a professional. He can also deal with algae, which could result from killing the duckweed.
One problem still remains. If duckweed wants to grow prolifically it means that the water has a high nutrient content. Duckweed may grow, but it doesn’t thrive in clean water. Fertilizer nutrients are coming from somewhere and eliminating the source is the only way to truly eliminate the problem.
Ted Manzer teaches agriculture at Northeastern High School.