Pool covers accumulate mildew on their underside, particularly during winter when the cover stays in place for weeks without being removed or dried. When the cover is dragged across a patio and then through a back door, or stored indoors temporarily, the mildew transfers directly to carpet fibres. The result is dark grey or black speckled staining accompanied by a persistent earthy, musty odour.
Why pool cover mildew stains carpet
Mildew is a surface fungal growth that develops on any material kept damp and out of sunlight. Pool covers provide ideal conditions — constant moisture on the pool-facing side, with limited air circulation. The mildew colony deposits dark pigment into the cover material, and when that surface presses against carpet, the pigment and live spores transfer into the pile. Unlike a simple dirt stain, a mildew stain may continue to grow if the carpet remains damp after contact.
What you need
- White cloths or paper towels
- Vacuum cleaner
- White vinegar
- Baking soda
- Dishwashing liquid
- Hydrogen peroxide 3% (for synthetic carpet)
- Spray bottle
- Stiff-bristled brush
- Fan or dehumidifier for drying
Step-by-step removal
- Vacuum the affected area thoroughly to lift loose mildew spores and debris before applying any liquid. Empty the vacuum outside to avoid spreading spores indoors.
- Mix equal parts white vinegar and water in a spray bottle. Spray the stained area and allow it to sit for five minutes. Vinegar is a mild acid that kills surface mildew and neutralises the musty odour.
- Blot with a clean white cloth, working from the outside of the stain toward the centre. Do not scrub — scrubbing drives mildew deeper into the pile.
- Apply a small amount of dishwashing liquid mixed with warm water (one teaspoon to two cups water). Blot again to lift the remaining pigment.
- For persistent dark staining on synthetic carpet, apply diluted hydrogen peroxide (one part 3% peroxide to four parts water). Test in a hidden area first. Allow to sit for three minutes, then blot clean.
- Sprinkle baking soda generously over the treated area and leave for several hours to absorb any remaining moisture and odour. Vacuum the baking soda away completely once dry.
- Direct a fan or run a dehumidifier to dry the carpet thoroughly. Residual moisture after mildew treatment encourages regrowth.
Prevent spore spread
After vacuuming the affected area, remove and empty the vacuum bag or canister outdoors. Do not shake the cloth used for blotting indoors. Mildew spores are lightweight and airborne — minimising indoor handling reduces the risk of spreading the growth to other parts of the carpet or home.
Tackling the musty smell after stain removal
Even after the visible stain is gone, musty odour can persist if mildew spores remain in the carpet backing or underlay. Baking soda left overnight draws moisture and odour from deeper layers. If the smell persists after two treatments, the underlay beneath the carpet may have been affected and may need professional inspection. Running a dehumidifier in the room for 24–48 hours after treatment helps prevent any remaining spores from establishing.
Preventing pool cover contact with carpet
The most effective prevention is to avoid bringing the pool cover indoors at all. Store it in a dedicated outdoor area — a pool equipment shed, under the deck, or folded on a waterproof mat on the patio. When handling the cover, fold it mildew-side inward to contain any surface growth. If the cover must come inside temporarily, lay it on a tarpaulin on a hard floor rather than placing it near carpet.
Mildew stain or odour not responding to home treatment?
We can connect you with local carpet cleaning specialists in the Sutherland Shire who handle mildew and biological stain removal.
Find a Matched Local SpecialistSutherland Shire Cleaners is a local matching and referral service. We connect you with local providers — we do not supply cleaning services directly.
Frequently asked
The pool cover was only on the carpet for a few minutes. Can that leave a stain?
Yes. Mildew pigment transfers on contact — even brief contact with a heavily contaminated underside can leave visible dark speckling. The longer the contact, the more extensive the transfer, but a short contact time is not a reliable indicator that no transfer occurred. Check the carpet closely after any pool cover contact, especially in summer when mildew growth accelerates.
Is pool cover mildew the same as black mould?
Mildew and black mould are both fungal growths, but mildew is generally a surface growth that is easier to remove, while black mould (Stachybotrys) penetrates materials more deeply. Pool cover mildew is typically surface mildew rather than true black mould, but both should be treated promptly and the affected area dried thoroughly to prevent establishment. If you see extensive black discolouration that does not respond to surface treatment, professional assessment is advisable.
Can I use bleach to kill the mildew in the carpet?
Bleach is not appropriate for carpet mildew treatment. It can strip colour from carpet dye, damage fibres, and create a harsh chemical residue. White vinegar kills surface mildew effectively and is safe for most carpet types. Hydrogen peroxide (diluted, tested first) is a safer alternative for persistent staining on synthetic carpet. Neither creates the bleached patch that household bleach causes.
The musty smell is still there after three days. What should I do?
Persistent odour after surface treatment usually indicates mildew has penetrated the carpet backing or underlay. Baking soda treatments and thorough drying can help, but if the smell remains after several days of drying, professional steam cleaning with an anti-microbial treatment is the most reliable next step. A professional clean reaches the backing and underlay layers that home treatment cannot.