White mould on houseplant soil is often surface fungal growth encouraged by moist conditions, but a white patch may instead be mineral residue or a pest. Check its texture, location and the plant's condition before treating it. University of Maryland Extension says surface fungi are not usually detrimental to plants, although a thick mat can interfere with water entering the compost.[8]
Is the white material mould, minerals or mealybugs?
Look in good light and take a close photograph. Do not sniff the compost or put your face close to the surface. Use the following clues to decide what to investigate, not to claim a certain identification.
Does it look like a fuzzy surface covering?
White to grey fungal growth can spread across moist potting media. Green surface growth may be algae, and the two can occur together, according to Maryland Extension.[8] Note whether it is limited to old bark or fallen leaves, or forms a broader layer.
Photograph the edge of the patch as well as the whole pot. A view showing where the white material stops is often more useful than a blurred close-up of its centre.
Is it a chalky or crystalline crust?
A pale coating on the compost, pot rim or outside of the container may be mineral or fertiliser salts. Maryland's mineral-deposit guidance describes these as white crystalline coatings or crusts and links them with hard water and fertiliser residues.[9] That needs a different response from a fungal film.
Check your watering and feeding history. A crust alone does not tell you which dissolved substance is present, and appearance is not a reliable water-quality test.
Are there cottony clusters on the plant itself?
Inspect leaf joints, stems, new growth and sheltered crevices. Mealybugs are wax-covered insects, and cottony egg masses or sticky honeydew can be clues. Some species feed on roots, so not finding insects on a leaf does not settle every case. University of California IPM explains these distinguishing features.[10]
If insects seem likely, separate the plant from neighbouring pots while checking. Use our mealybug identification guide rather than treating every white deposit as soil mould.
What should you do about a thin fungal layer?
First remove fallen leaves and other loose debris. Check that the inner pot has drainage holes and is not standing in water inside a cover pot.
For a surface layer, gently lift away the affected loose compost without scraping roots or the stem base. Maryland Extension recommends removing the upper layer if a mat develops, or breaking the mat up to restore water infiltration.[8] Work carefully rather than stirring the entire root ball.
Improve spacing so neighbouring plants are not packed tightly together. Let the surface become less persistently wet where the species permits, but do not deliberately dry a moisture-loving plant to wilting. Illinois Extension stresses that watering needs differ between species and growing conditions.[19]
Check moisture below the surface before the next watering. A dry-looking top is not a complete account of the root zone. When water is needed, water properly, allow drainage and empty the saucer.[19] Our potting mix guide explains why compost choice matters.
What if the white layer is a mineral crust?
Review fertiliser use against the product directions and consider the water source. Maryland Extension recommends flushing accumulated salts through a freely draining pot with low-mineral water, and replacing compost in severe cases.[9]
Do not keep flushing an already waterlogged, declining plant simply because the surface looks white. Assess drainage and roots first. Nor should you add vinegar, bleach or an improvised pesticide mixture to guess at the problem.
A photo cannot measure salt concentration. If crusting repeatedly returns despite sensible watering and feeding, seek advice about your water and compost rather than escalating random treatments.
When is white growth a reason for closer investigation?
A healthy-looking plant with a small surface patch is different from one that is collapsing. Persistent wetness, mushy roots or a soft stem base warrant a broader assessment. The RHS troubleshooting guide identifies these among signs associated with overwatering and recommends improving growing conditions rather than routine fungicide use on houseplants.[2]
Tiny flies are a separate clue worth checking, not proof that all the white material is a pest. See our fungus gnat guide if insects are present.
For help identifying the plant, try Green Sage's identification aid. Use it alongside clear photos and the care label; an image result cannot confirm a fungus, mineral deposit or pest diagnosis.
Sources
[2] How to Help a Poorly Houseplant | RHS Advice
[8] Algae and Fungal Growth on the Soil of Indoor Plants | University of Maryland Extension
[9] Mineral and Fertilizer Salt Deposits on Indoor Plants | University of Maryland Extension
[10] Mealybugs / Home and Landscape / UC Statewide IPM Program (UC IPM)