As days shorten in late October, two problems spike right after you bring plants indoors: fungus gnats and root rot. Cooler rooms, reduced light, and habit carryover from summer watering create a perfect storm. Here’s how to diagnose fast, fix confidently, and prevent repeat flare‑ups through autumn.
Quick diagnosis checklist (2 minutes)
- ●Fungus gnats (adults): tiny gray-black flies hovering when you water or disturb soil; cluster on windows at dusk.
- ●Fungus gnat larvae: thin, translucent worms with black headcaps just below the soil surface; most active in constantly damp media.
- ●Early root rot: pot stays wet 7–10+ days; sour, swampy smell; lower leaves yellowing; roots tan-to-brown, mushy, with slipping outer sheaths.
- ●Healthy roots: firm and crisp, mostly white to cream with light tan tips.
- ●Risk triggers this month: overpotting, dense soil, poor airflow, and delayed shift to reduced watering.
Pro tip: Don’t rely solely on cheap moisture meters. Cross-check with finger test at 2–3 cm depth and pot weight.
Why autumn makes it worse
- ●Less light = slower use of water, so soil stays wet longer.
- ●Indoor air is calmer and more humid; fungus gnat life cycles speed up in consistently moist media.
- ●After pruning outdoor growth or moving plants, roots may be temporarily less active, raising rot risk in wet pots.
48-hour action plan
- ●Isolate and inspect
- ●Move affected plants to a bright, ventilated spot away from the collection. Do quick pest checks on neighbors.
- ●Dry the surface
- ●For gnats, let the top 2–3 cm of mix dry thoroughly before the next watering. Bottom‑water only if the core is dry.
- ●Trap adults now
- ●Place yellow sticky cards at soil height; replace weekly until catches drop to near zero.
- ●Target larvae
- ●Drench once with BTI (Bacillus thuringiensis israelensis) per label, or apply beneficial nematodes (Steinernema feltiae). These are safe for most houseplants and go straight after larvae.
- ●Check roots if symptoms suggest rot
- ●Slide the plant out. Trim brown, mushy roots with sterilized snips. Rinse away soured mix, then repot in fresh, airy substrate and a pot that fits the root mass.
Pro tip: If >30% of roots are removed, step down one pot size and reduce watering for 2–3 weeks while new roots form.
Best treatments, by problem
Fungus gnats
- ●Cultural control: Water only when the root zone is truly drying. Bottom-water occasionally; let excess drain. Add a 1–2 cm top dressing of horticultural sand or fine pumice to discourage egg laying.
- ●Biocontrol: BTI drenches every 7–10 days for 2–3 cycles; or apply S. feltiae nematodes per label in the evening. Keep mix slightly moist for 48 hours so agents contact larvae.
- ●Trapping: Yellow sticky cards near soil and windows. Vacuum resting adults daily for one week to reduce breeding.
Pro tip: Avoid sealing wet pots in decorative covers without drainage. If you love cachepots, elevate the nursery pot on spacers so the drain hole never sits in water.
Early to moderate root rot
- ●Unpot, rinse, and prune: Remove all mushy tissue back to firm, white roots. Disinfect tools between cuts.
- ●Repot: Use an airy, fast‑draining mix tailored to the plant (e.g., for aroids: chunky bark + perlite + peat/coir; for succulents: mineral-heavy with coarse grit). Choose terracotta or breathable pots when possible.
- ●Watering reset: After repotting, water once to settle the mix, then wait until the pot is light before watering again. In autumn, most indoor plants need reduced watering compared to summer.
- ●Optional chemistry: A phosphorous acid (phosphite) drench labeled for root rots can help protect recovering roots. Follow product directions exactly.
Pro tip: If the stem base is blackened and collapsing, take clean cuttings from healthy tissue immediately; discard the infected base and soil.
Prevention through late autumn
- ●Light and airflow: Move plants closer to bright windows; add a small fan on low to keep air moving without drafts.
- ●Reduced watering: Expect longer dry‑downs in October–November. Track pot weight and schedule; don’t water by calendar.
- ●Substrate tune‑up: Blend in extra perlite, pumice, or bark when mixes feel heavy. Top‑dress with sand/pumice for gnat deterrence.
- ●Potting discipline: Right‑size pots; avoid overpotting post‑pruning. Always empty saucers.
- ●Quarantine & pest checks: Keep new or recently outdoor plants separate for 10–14 days. Inspect soil surfaces, leaf axils, and drainage holes weekly.
One-week checklist after you bring plants indoors
- ●Day 1: Inspect, isolate if needed, set sticky cards, water only the truly dry.
- ●Day 2–3: Apply BTI or nematodes if gnats are present. Increase airflow.
- ●Day 4–5: Lift a suspect plant; trim rot, repot into airy mix; reduce watering.
- ●Day 6–7: Replace full sticky cards, log catches, reassess moisture and light.
Pro tip: Keep a simple log—watering dates, gnat catches per card, and any pruning. Patterns jump out fast and help you fine‑tune care.
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Ready to keep your collection healthy through autumn? Do a 2‑minute check, set your traps, and breathe easy.
