Fungus Gnats: Causes, Diagnosis & Fixes
Fungus gnats are small, dark flies that hover around houseplant soil. While adults are mostly a nuisance, larvae can damage roots in large numbers.
Quick diagnosis
Fungus gnats are controlled primarily by managing soil moisture. Let the top layer dry out, use sticky traps, and the population will decline.
考えられる原因
Consistently Moist Soil
Why: Fungus gnat larvae feed on organic matter and fungi in moist soil. Wet conditions are essential for their lifecycle.
Signs to check: Soil surface stays moist, tiny black flies around the plant, especially when disturbed.
Rich Organic Potting Mix
Why: Soil high in peat or decomposing organic matter provides abundant food for larvae.
Signs to check: Dense, dark potting mix, flies are more numerous in pots with organic-rich soil.
Poor Drainage
Why: Pots that retain excess water keep soil moist longer, creating ideal breeding conditions.
Signs to check: Water takes a long time to drain, pot lacks drainage holes, saucer holds standing water.
確認ポイント
- 1Check if the soil surface stays moist between waterings
- 2Count the severity — a few gnats or swarms?
- 3Look for tiny white larvae in the top inch of soil
- 4Check all nearby potted plants, as gnats can infest multiple pots
対処法
- Let the top 1-2 inches of soil dry out completely between waterings
- Place yellow sticky traps near affected plants to catch adult flies
- Apply a layer of sand or diatomaceous earth on the soil surface to deter egg-laying
- Water from the bottom by placing the pot in a tray of water
- For severe cases, drench soil with a dilute hydrogen peroxide solution (1 part 3% H2O2 to 4 parts water)
専門家に相談するタイミング
- Seedlings or young plants are wilting despite moist soil (root damage from larvae)
- Gnat population is growing despite drying out the soil
- Multiple plants throughout the home are affected
ポイント
Fungus gnats are controlled primarily by managing soil moisture. Let the top layer dry out, use sticky traps, and the population will decline.
Frequently asked questions
What causes fungus gnats in houseplants?
Fungus gnats are small, dark flies that hover around houseplant soil. While adults are mostly a nuisance, larvae can damage roots in large numbers. Common causes include consistently moist soil, rich organic potting mix, poor drainage.
How do I fix fungus gnats?
Start by identifying the likely cause instead of applying every remedy at once. Let the top 1-2 inches of soil dry out completely between waterings Place yellow sticky traps near affected plants to catch adult flies Apply a layer of sand or diatomaceous earth on the soil surface to deter egg-laying
Track the problem over time
アプリで回復を追跡
進捗を記録し、リマインダーを設定し、AI による診断を受けましょう。
Sources and review
Content last reviewed: September 4, 2026
教育目的で公開されている園芸資料をまとめたコンテンツです。情報源の組織は透明性のためにのみ記載されており、推薦を意味するものではありません。