Hydrogen peroxide drench ratio for fungus gnats (chart)

The room was quiet except for the hum of a clip-fan and that tell-tale flicker at the soil surface. One pour, a soft fizz, and the gnats went silent. Hydrogen peroxide works—if you get the ratio and timing right.

“I overdid it the first time; once I switched to a gentle dilution, the roots stayed happy while the larvae didn’t.”

“The population bounced back when I skipped the second round—put it on a calendar.”

3% hydrogen peroxide, jug, and sticky trap beside indoor plants

Quick answer (use today)

Mix (3% H₂O₂): 1:4 (peroxide:water) → ~0.6% final solution; apply as a soil drench at one full watering volume.
Repeat: Repeat in 4–5 days (for the next larval wave). Most homes need 2–3 applications.
Safety: Pour into soil only—do not mist foliage. Pre-moisten very dry media so the drench spreads evenly.
Long-term control: Peroxide knocks larvae right now; BTI (Bacillus thuringiensis israelensis) breaks the ongoing cycle (use on a different day).

For a complete integrated approach (dry surface, airflow, traps, BTI): Fungus gnat control: complete home guide.

Why ratios matter (and when to choose BTI)

Hydrogen peroxide (H₂O₂) breaks down into water and oxygen in the pot, disrupting soft-bodied larvae. Too strong, and you can stress fine roots; too weak, and nothing changes. A 1:4 dilution from 3% stock (~0.6%) is a sweet spot widely recommended for home settings. As Cornell Cooperative Extension puts it: “Mix one part 3% hydrogen peroxide with four parts water and pour it over the soil.” Read the guidance: Cornell Cooperative Extension — Fungus Gnats.

BTI is biological and excellent for ongoing suppression. Use peroxide and BTI on separate days (peroxide can oxidize BTI).

Drench Ratio & Safety chart

Plant/media sensitivity	Examples	3% H₂O₂ : Water	Final %	Notes
Sensitive / seedlings	tiny starts, ferns, peperomia	1 : 10	≈ 0.27%	Start here; if weak, try 1:6 next time
Most houseplants	aroids, pothos, philodendron	1 : 4	≈ 0.6%	Baseline; drench evenly to slight runoff
Coarse mix / heavy infestation	large pots, very airy media	1 : 3	≈ 0.75%	Only in well-aerated mixes with strong roots

Application tips:

If the surface is bone-dry, lightly pre-water so the drench doesn’t tunnel.
Do not combine peroxide and BTI on the same day; use BTI the next day.
Soil only—no foliar spraying.

Diluted peroxide drench fizzing on potting mix surface

7-day protocol (simple & repeatable)

Day 0 (evening): Lightly pre-moisten the top layer → apply 1:4 drench (about 20–30% of pot volume; allow some runoff).

Day 1: Keep the surface dry; no foliar misting.

Day 4/5: Repeat the drench (second larval wave).

Day 7+: Switch to BTI (per label) and keep yellow sticky traps up for adults.

Ongoing: Favor bottom watering for 1–2 weeks to keep the surface drier.

Warm, humid rooms shorten the life cycle—lean toward the 4-day repeat. Cooler rooms: 5–6 days.

Diagnostics: is it working?

24–48 hours: Surface movement drops; adult flyers may still be present (trap them).
Days 4–7: Fewer flyers; fewer larvae visible when you gently rake the top layer.
Week 2: Little to no reinfestation in adjacent pots if you maintain dry surface + BTI.

“Once I set reminders for Day 0 and Day 4/5, the ‘oops I forgot’ relapse stopped—and so did the gnats.”

When NOT to drench

Ultra-delicate species (some ferns/carnivores): test 1:10 on a small section first.
Waterlogged, compact mixes: repot to an airy blend; rely on BTI + cultural fixes instead.
Freshly repotted roots: wait until root tips thicken; use BTI meanwhile.

For safe local humidity and airflow (without soaking pots), see Cloud Forest Plant Care & Humidity at Home.

Common mistakes (fast fixes)

Spraying leaves: Don’t—soil only.
Going stronger “for speed”: Usually just adds root stress; ~0.6% (1:4 from 3%) is the balanced middle.
One-and-done: The life cycle demands a second round (Day 4/5).
Same-day BTI: Peroxide can inactivate BTI—wait till the next day.

Sticky traps, gentle airflow, and a calendar marking repeat day

What I actually do in my apartment

I drench at 1:4 on Day 0 and Day 4, then water with BTI on Day 7. For two weeks I bottom-water so the surface crusts a bit, and I keep a clip-fan skimming past foliage to discourage adults.

Expert insight

Cornell Cooperative Extension recommends the gentle home-use dilution: “Mix one part 3% hydrogen peroxide with four parts water and pour it over the soil.” It balances root safety and larval knock-down; use BTI for sustained control: Cornell Cooperative Extension — Fungus Gnats.