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Miss Floribunda: Duck[weed] and cover

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Posted on: August 4, 2020

Dear Miss Floribunda,


Years ago you wrote a column where you surprised me by recommending a water garden as a way to reduce the mosquito population. Your “Mr. Minnowhaven” had one, and it was stocked with minnows that lived on mosquito larvae. I did put in a very little pond, with plenty of  minnows, and can say that I don’t have a mosquito problem anymore. Sometimes I’ve added mosquito fish, but raccoons soon ate them. Although I’m very careful to use tightly sealed raccoon-proof garbage containers, one night I was awakened by splashing and looked out the window to see a raccoon fishing right in the middle of my pond! I put wiring over the pond, but raccoon hands removed it. 


Last weekend I visited someone who has amazing koi in her pond. She has no netting or wire over it. I asked her how she keeps raccoons away, and she pointed out the lacy network of tiny green leaves covering the water. She said it was “duckweed” and that it hid the koi from predators. 


I’d like to try this, but I’ve been warned that duckweed is invasive. Is it? If it is, is there something else that might work? I’ve heard that koi are “living jewels,” and I’d like to have some.


Coveting Koi on Kennedy Street


Dear Coveting Koi,


You may want to give both koi and duckweed more thought. You say you have a very little pond, and if they survive predators and find enough to eat, two-inch koi easily grow to 24 inches in two years. They don’t stop growing. In addition, they much prefer eating minnows and small fish to eating mosquito larvae. The rarer of these living jewels can be very expensive. While you can get a tiny gem in a pet store for just a fin ($5), if you look at online auctions, you’ll see Tiffany prices. And you would want a very controlled environment for such an investment. 


Have you considered goldfish (Carassius auratus)? They are cousins of koi (Cyprinus rubrofuscus), both being in the carp family (Cyprinidae), but goldfish actually do eat mosquito larvae, are very inexpensive, and they don’t get as enormous as koi. Given a hospitable natural environment, they will grow to 12 to 14 inches long. The world record for size was won by a goldfish in the Netherlands that reached 19 inches. By comparison, the largest koi ever recorded is 4 feet long, 90 pounds, and is still living in England — and continuing to grow. But regardless of size, even those feeder goldfish you can get for pennies come in many attractive patterns of gold, red, white and black. In general, be sure to research any ornamental fish that tempt you to make sure they are not going to eat the minnows. Minnows devour enormous amounts of mosquito larvae, are minimal maintenance, and are seemingly impervious to climate change. In addition, they eat algae — a serious threat to any pond.


Now, here’s where the smallness of your pond is an advantage: Even though native duckweed (Lemna valdiviana) and the nine native forms of the common duckweed (Lemna minor) can spread very rapidly, they aren’t difficult to control in a small pond. You can cull duckweed from time to time and use the excess to mulch your garden. If you get goldfish, they will help you out because it’s one of the plants they like most on their menu. I even know one pond gardener who complains that her goldfish often dangerously deplete her duckweed by August. 


For larger ponds, however, Dr. Greengenes recommends a water fern called Azolla. Among its common names, interestingly enough, are duckweed fern and mosquito fern. Not only does it weave a floating veil of beauty over a pond, but it suppresses mosquitoes, cleans the water and fixes nitrogen. She tells me it has been used in rice paddies in China for centuries to actually fertilize the rice. You might choose a North American variety, such as the beautiful Azolla caroliniana, also known as Eastern mosquito fern, fairy moss and water velvet. It can overwinter in our area and is not considered invasive. It has earned fame for its use in decontaminating waters polluted with heavy metal. Few fish find it palatable. 


Now have you considered the low-maintenance and lovely waterlily to hide fish and suppress algae? Submerged plants provide wonderful hiding places and the handsome horsewort does not need any substrate because it is rootless. It aerates the water and tastes good to goldfish. And because it provides such a cozy enclosure away from prying eyes and grasping jaws, it is popular for trysts at breeding time.


Consider installing artificial shelters, and if you can’t afford a stylish rocky formation, or even plastic or mesh shelters, PVC pipes offer an inexpensive and easy alternative. A PVC pipe well-anchored in the bottom of your pond will give fish a place to swim into when frightened. 


I’m sorry to report that at this time no date has been set for the next meeting of the Hyattsville Horticultural Society. Please watch our website,, for information.



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