5 Reasons Why Your Indoor Plants are Attracting Fungus Gnats
Fungus gnats are a nuisance pest that has the potential to cause stress both for houseplants and residents. We normally don’t welcome flying insects indoors, though the good news is that they don’t usually cause severe damage to houseplants (and the tiny creatures don’t bite). But, they cause stress when conditions are right and in large numbers.
To avoid annoyance and potential costs to plant health, there are preventative measures that keep your houseplants from attracting fungus gnats. If you’re already experiencing the winged invaders, we’ll cover how to deter them.
What Are Fungus Gnats?
Fungus gnats (Family Sciaridae) are delicate winged flies with dark, narrow bodies and long legs. Their small wings enable them to fly only short distances (meaning limited travel, which is good). The adult flies don’t damage plants except to lay eggs, which produce root-feeding larvae. The pale worms with dark heads reach ¼ inch long.
Fungus gnat larvae first feed on decomposing organic matter in soils. This may be fungi, algae, mold, compost, peat, and other matter naturally decaying within the soil structure. After organic matter, they’ll feed on roots, causing an inability to absorb moisture or nutrients to support upper growth. With an outbreak of larvae, roots decline, either through their feeding or tunneling, and leafy growth shows stress.
The whole life cycle of a fungus gnat is about one month. They hatch and feed for a few weeks before becoming adult flies for about one week. So, much of their time is spent feeding beneath the soil surface.
Signs of fungus gnat stress include yellowing, dropping leaves, weak stems, and overall lack of vigor. These indicators are easy to mistake for other problems (over/underwatering, other pests and diseases) until the adult flies are visible. You’ll see movement on the soil surface as the flies crawl and flutter with disturbance.
In addition to potential root damage is their capacity to spread disease, these gnats are disease vectors that distribute spores and pathogens among roots.
Overwatering
Too much moisture is the top cause of fungus gnats in houseplants. They thrive in damp conditions. Adult females easily lay eggs in the crevices of moist soil, and as the larvae hatch, they burrow to feed on organic matter.
Many of our indoor specimens do best with consistent moisture, but oversaturated conditions lead to pest and disease problems. Pests flock to plants experiencing weakness from overwatering. Fungal issues like root rot occur as roots sit in waterlogged soils.
Not Adjusting Seasonally
As indoor selections move from actively growing in the warm season to an inactive or dormant period in winter, reduce watering sessions to accommodate. Roots absorb less water and nutrients in the “off” season. While they need it to thrive, the length of time between watering increases as soils stay moist longer. Plant processes slow, and evaporation decreases.
Let the soil surface dry slightly between watering. Depending on the species, they’ll tolerate drying slightly between sessions. Water when the soil is dry to the depth of an inch. Use the surface texture, rather than calendar days, to gauge when to water.
If you’re dealing with a fungus gnat outbreak, apply BT (Bacillus thuringiensis) or mosquito bits to the soil to impede the larval stage. The biological controls render them unable to feed, killing off the population.
With a history of the problem, consider adding a layer of coarse sand or gravel to the soil surface. The dry layer prevents adult females from laying eggs in the cracks of damp soils. And, if larvae hatch, it’s challenging for them to reach underlying organic matter. Bottom-watering or self-watering pots may be helpful in minimizing saturation at the surface.
Poor or Aged Potting Soil
With regular water and nutrient uptake, the potting mix degrades over time. It becomes compact with less aeration for roots. With less airflow and access for developing roots, soils offer less drainage. They hold moisture longer as a result. Nutrition, too, depletes.
With aged or low-quality potting soil, the lack of drainage leads to oversaturated situations. Even if you aren’t inadvertently overwatering, the moisture retention of the soil creates the same soggy conditions, becoming a magnet for houseplant fungus gnats.
Some fast-growing selections need more frequent repotting, while others benefit from being slightly root-bound in a container. Aim to repot at least every two or three years to maintain a healthy potting mix.
Use a high-quality houseplant potting formula, or make your own for a light, fluffy, well-draining blend.
Too Large a Pot
It’s easy to get ambitious as we envision the full-sized glory of our indoor jungle, but starting with a pot that’s too big spells growing problems. Roots absorb water and nutrients to the depth they can reach. Surrounding them with excess soil becomes a mode of holding moisture with no outlet. The soil volume remains soggy for a prolonged period, creating the ideal habitat for algal growth, fungus gnats, and root rot.
When repotting, choose a pot one or two sizes larger than the existing container. This gives extra space for root growth while preventing excess soil mass and moisture retention. Ensure containers have good drainage holes, and don’t let them sit in a saucer or tray of water much beyond watering. This contributes to the absorption of excess moisture.
If using a tray of pebbles and water to increase humidity around your indoor collection, make sure the pebbles elevate the pot. It shouldn’t sit in the water.
Rich Organic Amendments
While nutrient-rich soils create healthy growing foundations, they’re also happy feeding grounds for fungus gnats in houseplants. Houseplants do well with high-quality formulas tailored to container culture. Many do well with occasional balanced organic fertilizers during the active growing season.
When adding amendments, it helps to know what attracts these pests. Non-sterile compost invites the unwelcome guests. As compost is the perfect host with its decaying organic matter, an amendment that hasn’t reached the right temperature for sterilization can bring in pests.
Peat, too, is decayed plant matter and a basis for many potting and garden soil blends. Fungus gnats enjoy a hospitable environment to feed. Peat is also a non-renewable resource, and there are other reasons to explore alternative, peat-free products.
Coffee grounds make a convenient nutritive resource for some houseplants. With a history of fungus gnats, avoid applying them. Damp coffee grounds are another rich site for flies and larvae.
Rely on sterile potting mixes and compost amendments for houseplants. Pest and pathogen introduction are less likely with pasteurized compounds.
Bringing Them Home
Sometimes, pests and other unwanted issues arrive from the retailer on a new purchase. Or, maybe the plant is a share or a giveaway or not in the best shape when it comes to us.
Fungus gnats often appear on houseplants in the fall when they transition from being outside to overwintering indoors. An outdoor plant is a contender for pests. They may become noticeable once inside or occur because of the change in the growing environment.
Give new additions a quarantine period before introducing them to the group. Place them in an isolated spot, well-spaced from other indoor selections, to avoid transmitting any pests and diseases on arrival.
Whether for a few days or a few weeks, keeping them separate gives time to observe their condition. Yellow sticky traps attract the flies, and help indicate their presence. It also captures them in the adult stage.
