Mould or algae on the soil surface
A white, grey, or green film on the surface of potting mix usually means the surface is staying damp with poor air circulation. On its own it is often harmless to an established plant, but it is a reliable warning that the mix is wetter than it should be — and that is the problem to fix.
Do not confuse surface mould with root rot: surface film alone, with firm roots and healthy leaves, is a moisture warning, not a diagnosis of rot. Check the roots before assuming the worst.
General beginner information only — not a substitute for advice from your local agricultural extension service. For edible plants, use only a product that is labelled for that particular edible crop, for indoor use, and follow any harvest interval on its label; try non-chemical steps first, and never use a product that is not labelled for indoor use on that plant.
Checklist to diagnose
- Is the film only on the surface, or does the mix smell sour and stay wet at depth?
- Is the pot draining freely, or standing in water?
- Is air circulation poor — a crowded corner, a closed room, no airflow?
- Are roots firm when you lift the plant, or soft and dark?
- Is there debris or dead leaves sitting on the surface?
Rescue steps
- Scrape off the surface film and any debris, and let the top of the mix dry before watering again.
- Improve air circulation around the plant and make sure the pot drains freely; empty the saucer every time.
- Water from the base or at soil level, and avoid keeping the surface constantly damp.
- If roots are soft and dark or the plant wilts in wet mix, follow the root rot page — surface mould was the warning, not the whole problem.
- Do not spray the surface with household chemicals to 'kill' the mould; fixing moisture and airflow removes the conditions it needs.
When to give up
- Surface mould alone is not a reason to discard a plant. Discard only if the roots underneath are rotted or the plant has no healthy growth left.
Plants on this site
This page gives general indoor advice: it applies to indoor-grown plants generally and is not a claim about any particular plant on this site.