Short answer
White fuzz on top of cannabis soil is usually saprophytic fungus: mycelium living on organic matter in a surface that never dries. It rarely hurts the plant directly. It does mean the top of the pot stays wet, so water less often and move air over the pots. Grower practice.
Why
Potting media are full of fungi. Unpasteurised coco, for example, carries fungal communities of its own [2]. Most of them break down organic matter, and they only grow a visible white mat when the surface stays damp and the air is still.
The wet surface is the real problem. Fungus gnats lay their eggs in moist, fungus-rich media, and their larvae feed on fungi and on roots [1].
A pot that never dries back also keeps the roots short of oxygen. Constantly wet roots favour Pythium and Fusarium root rot, which cause wilting and yellowing [3].
The numbers
| What you see | Likely cause | Action |
|---|---|---|
| Soft white fluff, grows back on wet spots | Decomposer fungi | Dry the surface, add airflow |
| Hard, crunchy white or tan crust | Salt build-up | Check runoff EC; flush the root zone |
| Fuzz plus a drooping plant, heavy pot | Overwatering, possible root rot | Check the roots |
| Fuzz plus small black flies | Fungus gnats | Dry the top 3–5 cm, use sticky traps |
What to do
- Lift the pot. If it is still heavy, wait until it is about 40–50% lighter before watering.
- Scrape off the fuzz and the top centimetre or so of mix. Bag it and take it out of the room.
- Let the top 3–5 cm dry between waterings, and water so the pot drains freely.
- Run a fan low across the pots so the surface dries.
- Topping the pot with a thin layer of dry, coarse material such as perlite keeps the surface drier.
- If the plant is wilting or yellowing as well, slide it out and check that the roots are white.
Common mistakes
- Spraying fungicide or peroxide on the soil without fixing how you water.
- Mistaking salt crust for mould.
- Watering a seedling in a huge pot every day.
- Leaving runoff sitting in the saucer.
Related
- Full article: Sanitation and disease prevention
- Answers: small white bugs in soil · how often to water
- Articles: drooping: over- vs underwatering · fungus gnats
- Terms: fungus gnat · salt build-up
Coco coir is a growing medium made from processed coconut husk, prized for holding water and air at once; it binds calcium and magnesium unless buffered.Small dark flies of the genus Bradysia (family Sciaridae) whose larvae live in wet growing media, feeding on fungi, organic matter and young roots.A genus of oomycete water moulds, such as Pythium myriotylum and P. aphanidermatum, that cause brown, slimy root rot, especially in warm hydroponic systems.A soil-borne fungal disease, chiefly Fusarium oxysporum, that blocks water-conducting tissue, causing one-sided yellowing, wilting and brown streaks inside the stem.Salt build-up is the accumulation of unused fertiliser and water salts in the growing medium, raising root-zone EC and causing osmotic stress and lockout-type symptoms.Runoff is the water or nutrient solution that drains out of the bottom of a container after irrigation; growers test it to see what is happening in the root zone.EC (electrical conductivity) measures how well a solution conducts electricity, a proxy for total dissolved salts; growers read it in mS/cm to gauge nutrient strength.Feeding only plain water for days or weeks before harvest to "clean" the plant; GenetixBay rates it a Myth that harms yield, unlike a root-zone salt flush.The root zone is the volume of medium or solution where a plant's roots live, including its water, air, temperature, pH, EC and microbes.Overwatering is watering so often that the medium stays saturated, starving roots of oxygen, which causes drooping, slow growth and root disease.Decay of roots by pathogens such as Pythium, Fusarium, Rhizoctonia or Phytophthora, usually triggered by low oxygen, heat and waterlogging around the roots.A coloured card coated in non-drying glue that catches flying insects, used mainly to monitor pest trends: yellow for gnats, whiteflies and aphids, blue for thrips.Perlite is a light, white, puffed volcanic glass added to growing mixes to increase drainage and air space; it is chemically inert and holds no nutrients.Hydrogen peroxide (H₂O₂) is an oxidising sanitiser used to clean hydroponic equipment and reservoirs; it breaks down quickly into water and oxygen and kills beneficial microbes too.A seedling is a young cannabis plant from emergence of the cotyledons until it has several sets of true leaves, typically the first 2-3 weeks.
