TL;DR
Mildew on outdoor cushions almost always grows on the dirt, pollen, and body oil sitting on the fabric rather than on the fiber itself, which is why keeping cushions clean prevents it better than any treatment. The other half of the job is drying: foam cores hold water for days, and a cushion stored damp will smell from the inside out no matter how clean the cover is.
What Outdoor Fabric Is Made Of
Most quality outdoor upholstery is solution dyed acrylic or solution dyed olefin. Solution dyed means the color is added to the liquid polymer before the fiber is formed, so pigment runs all the way through the strand rather than sitting on the surface.
That single property explains why good outdoor fabric holds color in relentless sun and survives cleaning that would strip an indoor fabric.
Check the label or the manufacturer specification before choosing a cleaning method. The gap between these two categories is larger than the gap between most indoor fabrics. Performance fabric care covers the same distinction for indoor pieces.
Is That Mildew or Just Dirt?
Mildew appears as small black, grey, or pink speckles that do not brush off and often smell musty, while ordinary soil is a uniform dulling that lifts with brushing and rinsing. The important part is what mildew is growing on: it feeds on organic material sitting on the fabric -- pollen, dust, food, sunscreen, body oil -- rather than on synthetic fiber itself.
That is why cushions kept clean rarely mildew even outdoors, and why a neglected cushion mildews in one damp season.
Two limits worth stating plainly. Persistent mold in a structure -- walls, framing, a damp enclosure -- is a remediation issue and belongs with a mold remediation specialist rather than a furniture cleaning. And health questions about mold exposure belong with a physician. Mold and mildew basics covers the general household version.
The Basic Cleaning Method
For most solution dyed outdoor fabric:
Brush off dry debris first. A soft brush removes pollen, leaves, and loose soil. Doing this before water prevents turning debris into a stain.
Rinse with a hose at normal pressure. Skip the pressure washer -- high pressure drives water deep into the foam and can damage the weave and any water-repellent finish.
Wash with mild soap and water. A little dish soap in a bucket of lukewarm water, applied with a soft brush, handles most seasonal soil.
Rinse thoroughly. More than feels necessary. Soap residue left in outdoor fabric attracts dirt and provides exactly the organic material mildew needs.
Dry completely in open air, standing cushions on edge so both faces and the foam can breathe.
That covers routine maintenance. Doing it twice a season prevents most of what people call outdoor cushion problems.
Dealing With Mildew
Manufacturers of solution dyed acrylic outdoor fabric commonly publish a dilute bleach solution for mildew removal, on the basis that the pigment is locked in the fiber and will not strip. That guidance is specific to those fabrics and does not transfer to anything else.
Before considering it, three checks:
Confirm the fabric. Solution dyed acrylic or olefin, per the manufacturer. Surface-dyed polyester, cotton, or a blend will lose color.
Check the thread and the trim. The sewing thread, piping, and zipper tape may not share the fabric's resistance, and a bleached seam that fails is worse than a mildewed cushion.
Read the manufacturer's own instructions. Dilutions, dwell times, and approvals vary by brand, and the maker's document overrides general advice.
One absolute safety rule regardless of fabric: never combine bleach with ammonia, with vinegar or any acid, or with alcohol. Those combinations produce dangerous gases. Bleach solution goes in a bucket on its own, with nothing else added.
Rinse extremely thoroughly afterward. Residual bleach continues to work on fibers and thread over time.
For fabrics that cannot take bleach, use a dedicated mildew cleaner formulated for the material, or accept that heavily mildewed covers on cheap fabric are often better replaced than restored.
The Foam Is the Real Problem
Fabric dries in hours. Foam does not.
Standard open-cell polyurethane foam absorbs and holds water for days, and a cushion that feels dry on the surface can be saturated an inch in. That trapped water is what produces the musty smell people blame on the cover, and it is why cleaning a cushion sometimes makes it smell worse than before. Why foam cushions smell after cleaning covers the mechanism.
Three things help.
Stand cushions on edge in moving air rather than laying them flat. Air reaches both faces and gravity helps water drain.
Squeeze, do not wring. Press water out with flat pressure. Wringing tears foam internally.
Give it days, not hours. In humid weather a thick cushion can take two to three days to dry fully. A fan pointed at standing cushions cuts that considerably.
Better outdoor cushions use reticulated quick-dry foam, which has open cells designed to let water pass straight through. If you are replacing inserts, this is the upgrade worth paying for -- it changes the entire maintenance picture.
Zippered covers are worth using. Removing the cover lets the foam dry separately and far faster. Just check the label before machine washing, since many outdoor covers shrink or lose a water-repellent finish in a hot wash. How long upholstery takes to dry covers drying times generally.
Storage and Prevention
Most outdoor cushion damage happens in storage rather than in use.
Store completely dry. A cushion put away damp in autumn comes out in spring as a mildew problem. Store in a ventilated space rather than a sealed bin, and if a bin is unavoidable, make sure everything is bone dry first.
Use breathable covers rather than plastic sheeting. Plastic traps condensation against the fabric and creates the exact conditions mildew needs.
Bring cushions in during long wet stretches, or at least stand them on edge under cover. Boats and RVs face the same enclosed damp problem year round -- boat and RV upholstery covers that version.
And clean seasonally rather than reactively. Since mildew grows on accumulated soil, a rinse and wash twice a season is the whole prevention strategy.
FAQ
How do you get mildew out of outdoor cushions?
Brush off loose debris, then wash with a cleaner appropriate to the fabric. Manufacturers of solution dyed acrylic commonly publish a dilute bleach solution for this, but that guidance applies only to those fabrics and only per the maker's own instructions. Rinse very thoroughly and dry completely.
Can you pressure wash outdoor furniture cushions?
Better not to. High pressure drives water deep into the foam, where it takes days to dry, and it can damage the weave and strip water-repellent finishes. A normal garden hose rinse with a soft brush cleans just as well without those risks.
Why do my outdoor cushions smell even after cleaning?
Almost always trapped water in the foam. The cover dries in hours and the core takes days, and a damp core develops odor from the inside out. Stand cushions on edge in moving air, use a fan, and allow two to three days in humid weather.
Can you machine wash outdoor cushion covers?
Sometimes, and only if the label permits it. Many outdoor covers shrink in hot water or lose a water-repellent finish in a machine wash, and a cover that shrinks may never fit the insert again. Cold water and air drying are the safest settings when washing is allowed.
Is bleach safe on outdoor fabric?
On solution dyed acrylic and olefin, many manufacturers publish a dilute bleach solution as acceptable, because the pigment is locked inside the fiber. On surface-dyed polyester, cotton, and blends it strips color. Check the fabric and the maker's instructions, and never combine bleach with ammonia, acids, or alcohol.
For a whole-room reset rather than a single treatment, see carpet cleaning services.
Fabric type, foam type, and how the cushions are stored decide most of what happens to outdoor furniture. Get a quote and we will look at what you have and tell you what it needs.