Small white dots in houseplant soil are most commonly either perlite (a volcanic mineral added to improve drainage and aeration, entirely harmless and desirable), white mineral salt deposits from hard tap water, or occasionally the pupae or eggs of fungus gnats or other soil-dwelling organisms.
Understanding which you have determines whether any action is needed.
Perlite: Harmless and Beneficial
If your compost contains perlite (a volcanic glass heated until it expands into lightweight white particles), you will see white dots throughout the compost mix and on the surface. Perlite is added to improve drainage and aeration and is a positive sign that the compost has been formulated for good drainage. It is completely harmless and should not be removed or treated. Many good houseplant composts, including our Simply Houseplant Potting Mix, include perlite as a component. Our perlite is the same material sold separately for amending composts.
Mineral Salt Deposits from Hard Water
In hard water areas (much of England, particularly London and the south-east), watering with tap water gradually deposits calcium and magnesium salts in the compost and on the pot surface. These appear as a white crust on the compost surface, white residue on the inside of terracotta pots, or whitish deposits on the soil surface around the pot base. This is cosmetic rather than immediately harmful, but over time significant salt accumulation can raise the electrical conductivity of the compost to levels that affect root function.
To address salt build-up, flush the compost thoroughly with plain water several times a year to dissolve and drain the accumulated salts. If the crust is heavy, scraping it off the surface before flushing helps. Using filtered, rainwater, or distilled water for watering prevents further accumulation. See our guide on whether tap water is safe for houseplants.
Fungus Gnat Larvae and Pupae
If the white dots are just below the compost surface and are small (1 to 3 mm), oval, and slightly translucent, they may be fungus gnat pupae. These are the resting stage before adult gnats emerge. The larvae (white, thread-like worms with dark heads) are the main soil-dwelling stage and are sometimes visible when the compost surface is disturbed. A separate and reliable sign of fungus gnats is seeing small dark flies emerging from the compost when you water or move the pot.
Fungus gnat larvae are controlled by allowing the top few centimetres of compost to dry out between waterings and treating with a biological drench containing Bacillus thuringiensis israelensis (Bti) if the infestation persists. See our full guide on getting rid of fungus gnats. A topdressing of shells over the compost surface discourages adult gnats from laying eggs.
Mould and Fungal Growth
White fluffy or powdery patches on the compost surface are typically saprophytic fungi (not harmful to the plant) growing on organic matter in the compost. This is most common in compost that stays moist for extended periods and in poorly-ventilated positions. The mould is breaking down organic matter in the compost, which is a normal biological process, but it indicates the compost is staying too wet. Improving ventilation, reducing watering frequency, and allowing the surface to dry out between waterings typically resolves it without treatment. It isn't harmful to the plant in small amounts.