Why Do I Have Gnats in My Plants?

By Julie Miller
Why Do I Have Gnats in My Plants?

You have reset your thermostat for the fall, are indulging in your first pumpkin spiced latte, and begin to conjure up memories of sitting around a blazing fire in your cozy flannel pajamas and warm fuzzy slippers. As you hum a favorite song, you begin to waltz around your home. As your hand grazes the top of your beloved peace lily, you see a little gray storm of insects shoot up and then resettle themselves. The music fades. Images of the fire vanish. You suddenly forget about your snuggly pajamas. The quick realization comes: You have a gnat issue in your home.

What Are Fungus Gnats?

Adult fungus gnats are small flying insects of about 1/16 to 1/8 of an inch in size. They can be confused with fruit flies. Because of the minute size of gnats, they may be mistaken for baby gnats. They’re not thought; gnats are just really tiny. While they’re about the same size as fruit flies, the appearance of a gnat is more like that of a mosquito—with delicate bodies, slender legs and long antenna. They tend to hop, as they are weak fliers, flying in short bursts with an erratic pattern when they must. 

Their larvae have shiny black heads and elongated, whitish-to-clear legless bodies. Fully grown larvae may grow up to ¼ of an inch in length. 

What Do Fungus Gnats Eat?

These minuscule non-biting insects will infest soil, potting mix, and other sources of organic decomposition. The larvae primarily feed on fungi and organic matter in the soil, though they will also blithely munch away on grass clippings, compost, and root hairs. It’s pretty rare for them to cause considerable damage to your plants, unless the larvae population is quite high (and out of control), in which case they will indeed feast on and damage roots and stunt a plant’s growth, particularly for seedlings and young plants. The adults, in fact, do not feed on or damage plants at all. They are just a cosmetic nuisance. 

How Do I Know If I Have Fungus Gnats?

Soiled plants

The most obvious clue is observing these small black flies crawling around on the top soil of your indoor plants. But, if you're having to get on a step stool to water your plants, you may not notice them at first. Other symptoms of a gnat infestation are sudden wilting, loss of vigor, poor growth, and yellowing of your plants.

A sure test to figure out whether or not you have these plant flies and their larvae is the potato wedge test. Insert ¼-inch slices of raw potato into your plant, making sure to partially bury the pieces under the soil. Larvae will migrate to the potato and begin feeding. After a few days, pull the potato wedges out, inspecting the soil directly underneath the potato too. If you have a larvae issue, the potato won’t lie.

Why Do I Gave Fungus Gnats in My House?

Fungus gnats thrive in moisture.

Moisture in Indoor Plants

Indoor plants

If you own plants and are just getting started with figuring out how often to water them, beware of these little weak-winged mongrels. They are very attracted to moisture (which promotes fungal growth) and will gleefully find their way to your overwatered plants. So, to all plant-owners near and far, be sure to use the finger test when watering your plants. Test: Stick your forefinger into the soil up to your first knuckle. If the soil is dry in the first inch to inch-and-half, water it deeply. If it’s not, wait a few days. This test is best for plants in medium-sized pots.

Moisture in Indoor Plants (in the Winter)

Something that I learned this week was that your indoor watering habits should alter with the change of seasons. Decreased day lengths and cooler temperatures slow a plant's growth and water usage. This makes sense, especially if you have a garden or lawn that you’re watering, as you begin to water it less as it gets closer to fall. So, it’s important to adjust your watering practices with the seasons and water less in the fall and winter. A failure to do so could result in over-moist plants which could attract moisture-happy gnats.

Indoor plants that fungus gnats are likely to attack are spider plants, inch plants, African violets, and peace lilies. I, personally, have dealt with them in my peace lily.

Moisture in Outdoor Plants

If you bring your geraniums and other outdoor flowering plants into your home as the outdoor temperatures cool, you may begin to notice their presence in the late fall. You’re actually (and accidentally) bringing them in yourself! Before you pick up those heavy ceramic or clay pots, wiggle your fingers around in the soil first to ensure that none of these hiding vermin are present. Look for their larvae too.

In addition to geraniums, outdoor plants that fungus gnats are likely to attack are carnations, cyclamens, and poinsettias.

Moisture in New Plants

While fungus gnats may come indoors through the pots that you’re transferring inside, they can also hitchhike through newly purchased plants. Before purchasing, do a thorough plant inspection. Get your fingers in that soil and look for glossy larvae or hopping gnats. Even if you don’t immediately see anything, you may consider isolating the plant for a few weeks.

Do Fungus Gnats Reproduce Quickly?

Unfortunately, yes. Adult lives are short-lived, enjoying the fruits of life for around a week’s time. Within that week, though, the mamas get to work, laying some 200 eggs in your moisture-rich plants. Under optimal conditions (fungus gnats really like peat moss and temperatures of 65-75 F), fungus gnats can develop from egg to adult in three to four weeks and reproduce year round. Thus, overlapping generations of eggs, larvae, pupae, and adults are often present when living (and breeding) indoors.

Fungus gnats can go through multiple generations per year. Indoors, fungus gnats tend to have overlapping generations where eggs, larva, pupae and adults are all present at the same time.

How Do I Get Rid of Fungus Gnats?

Most of a fungus gnats life is spent as a larvae. Therefore, it makes the most sense to try and rid your potted plants of these immature life stages, rather than focus on the adult’s short-term life. To do so, I recommend the following natural methods:

  • Let your plants dry between waterings.
  • Ensure that your pots provide good drainage.
  • Remove any containers with an abundance of decaying plant matter, such as decayed bulbs and roots.
  • Toss out severely infested plants.
  • Catch the larvae with potato wedges. (See the potato wedge test above.)
  • Improve the drainage of the potting mix by increasing the proportion of perlite or sand in the mix.
  • Use Dr. Killigan’s Insect Buster to spread diatomaceous earth (DE) on the top soil of your plants.

Why Use Diatomaceous Earth to Get Rid of Fungus Gnats?

DE has microscopic shards of silica (a hard, nonreactive, colorless mineral found in the earth’s crust) that will rip into those tiny fungus gnat larvae as they unabashedly crawl through your plants. (In addition to gnats, it will also kill other common plant pests like mealy bugs, spider mites, or aphids.) But, it will not cause harm to your plants.

Using Dr. Killigan’s Insect Buster, the most effective tool for dispersing DE, spray a light powder over the topsoil of your dry potted plants. You don’t want the soil completely dried out, but topsoil that is dry to the touch will prove helpful.

Ensure that you purchase the food-grade version of DE, wear gloves and consider using a mask (to prevent yourself from inhaling these tiny particles), and be at rest, knowing that DE is completely safe to use in your home and around your children and pets.

How Do I Prevent Fungus Gnats?

Although the following are listed as prevention tips, they can also be used as getting-rid-of-fungus-gnats tips with the understanding that it may take several weeks of modified watering and use of sand/gravel to get fungus gnats in check.

Prevention is typically easier, keeps you from any unneeded stress, and allows your plants to continue thriving. Here are my recommendations:

  • Re-pot every so often, especially when the growing medium has "broken down" and is retaining too much moisture.
  • Avoid fertilizing with excessive amounts of manure, blood meal, or other similar organic materials.
  • Avoid using incompletely composted organic material in your potting soil, unless it is pasteurized first. This material is often infested with fungus gnats.
  • Consider watering your plants from the bottom. In doing so, you are providing moisture for the roots while keeping the soil surface dry. 
  • Cover the soil with ½- to one-inch layer or coarse sand or fine gravel. This will keep the surface drier and make the soil less attractive for egg-laying. 
  • Use Dr. Killigan’s Insect Buster. Add DE to every outdoor plant that you bring indoors through the winter and every new plant that you purchase.

Final Word on Why You May Have Gnats in Your Plants

Having gnats in your plants is a problem easily overcome. It’s such a relief to know that simply by changing your watering habits, you can eliminate your gnat issue. Have you had gnats? Were you able to successfully use these recommendations? We’d so enjoy hearing your story of triumph (or momentary failure). Share in the comments below.


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