Remedies for fungus gnats on philodendrons |
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Jessica Walliser

Question: I was given a Philodedron scandens oxycardium. The original plant was around 20 years old, was divided, and I was given 1/3 of it. The plant is in a container and weighs about 150 lbs. It is outside on a screen in porch that gets the morning sun. It really likes where it is. The plant had only 3 leaves when I got it. Now it has 10 leaves, and more are about to come out. The problem is, I have gnats living in the soil and do not know what to do? I can’t bring it in the house in the fall until I get rid of them. I’ve tried Dr. Earth, but that is only for flying gnats, I believe. I would appreciate any suggestions.

Answer: When a cloud of tiny, black flies rises from the pot of a houseplant, it’s a clear sign of trouble – and it’s downright annoying. That cloud may consist of hundreds of adult fungus gnats and, while individually they are barely noticeable, in large numbers they are hard to miss. Fungus gnats become problematic when their populations reach conspicuous levels. Their gregarious nature makes them a classic example of a nuisance pest.

There are two primary genera of fungus gnats here in Pennsylvania, Bradysia and Lycoriella. But, regardless of which group you have, managing them is the same.

Mature gnats measure a mere 1/8th of an inch and live for one to two weeks. During this time, females lay eggs in the soil. The resulting translucent, minute larvae feed largely on the assorted fungi growing in the potting soil, though they can also feed on fine roots and plant debris. In a few weeks they pupate into adults within the soil and the cycle continues, with several generations occurring together at any given time.

Overwatering often leads to a fungus gnat issue. Constantly damp soils promote fungal growth, which serves as an excellent food source for the larvae. Simply cutting down on watering has solved many a fungus gnat issue. Water infested houseplants deeply — but not frequently — and only when the soil is dry. Be sure the pot itself has good drainage and the saucer underneath doesn’t contain standing water.

If changing your watering routine doesn’t clear up the problem, repot the plant with new, sterile potting soil, gently removing as much of the old soil as possible without decimating the roots. This might be difficult, though, since your plant is so large.

You can also trap adult fungus gnats on yellow or blue sticky cards placed an inch or two above the soil surface.

Lastly, if none of the above options work, there are two products you can purchase to help solve your problem. GnatNix is a gravel-like substance that’s spread over the surface of the potting soil. Adult gnats can’t access the soil to lay their eggs, eventually eliminating the problem. A second choice is to use a soil drench derived from beneficial nematodes of the species Steinernema feltiae or an organic pesticide soil drench based on Bti (Bacillus thuringiensis var. israelensis). The nematodes are microscopic organisms that burrow into the soil and eat the gnat larvae. The second is a bio-pesticide made from a bacterium that kills the gnat larvae. Both of these products are available from various online sources and may be found at some local nurseries.

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