Mould on outdoor textiles
Why it appears and how to get rid of it
Mould does not grow on the fibre. It grows on what settles on the fibre. That is the good news: on acrylic and polyester weaves the growth almost always sits on top and can be lifted off. This page explains how to tell mould from mildew stains, which cleaning agent your fabric tolerates and what keeps it dry in storage.
Mould needs three things, and the fibre is not one of them
Once you know what mould lives on, it is clear why it appears on outdoor fabrics and why it can usually be removed from them without a trace.
Moisture is the trigger
Nothing grows without a surface that stays damp. That is why mould almost always appears where water cannot escape: under a tightly sealed cover, inside a rolled-up cushion cover, in a seat pad that was put away wet. Moisture at the moment of storing is by far the most common cause.
The food is the soiling, not the fabric
Acrylic and polyester are indigestible to mould, they offer no nutrients. What feeds mould lies on top of them: pollen, dust, petals, skin flakes, bird droppings, sun cream, drink residue. A clean fabric develops mould far more slowly than a dirty one, even when both are equally damp.
Acrylic is a naturally mould resistant fabric
Our acrylic fabrics are water repellent, stain resistant and mould resistant without needing an additive for it. That is not the same as mould proof: the growth settles on the soiling and can therefore still appear. It then sits on the surface rather than inside the fibre.
Warmth and still air speed it up
A closed garden storage box in summer is the worst combination: warm, damp, no air exchange. That is why our waterproof covers get ventilation openings, and why a cover that seals completely is often worse than one that can breathe. More on that on What is waterproof?.
Mould, mildew stains or just dirt
Three appearances are often confused but need different treatment. Telling them apart decides whether a soap solution is enough or more is required.
| Appearance | How to recognise it | What to do |
|---|---|---|
| Mould | fluffy or speckled growth, usually grey, black or greenish, often a musty smell | brush off dry, then clean wet, choosing the agent by fabric type |
| Mildew stains | flat brownish or yellowish discolouration with no growth, the remains of dried-out mould or of damp storage | treat like mould, usually fades only after several passes |
| Soiling | pollen, dust, petal juice or bird droppings, no smell, brushes off | a washing-up liquid solution is enough, no bleach needed |
| Efflorescence | whitish crystalline film that wipes off with clean water | soap or salt residue, rinse thoroughly |
What your fabric tolerates, and what it does not
The difference lies in the dyeing. Solution dyed fabrics carry the colour in the fibre core, so a diluted bleach solution does not attack it. On conventionally dyed acrylic the colour sits on the outside, and there even a diluted bleach leads to fading or patches. If in doubt check the data sheet for how your fabric is dyed, or ask us.
For routine cleaning a washing-up liquid solution at one to twenty, that is one part detergent to twenty parts lukewarm water. For mould on solution dyed fabrics a bleach solution at the same one to twenty ratio. For stubborn growth soak the fabric in the bleach solution and wash it afterwards with the detergent solution. Always rinse thoroughly with clean water, otherwise residue stays behind and new dirt clings to it.
Waterproof versions with a backing laminate tolerate neither machine washing nor a tumble dryer. Treat stains only from the fabric side, never on the laminate, and stay below five per cent with bleach solutions. Pure cover fabrics such as Cover PVC are wiped down, not soaked.
Removing mould, step by step
Whether you are cleaning an awning, a seat pad or a cover, the method is the same. The order is what matters: dry first, then wet, then dry thoroughly.
Take it off dry first
Brush the growth off with a soft brush or vacuum it with the upholstery nozzle while the fabric is still dry. That removes most of it before water works it into the weave. Best done outdoors rather than in a living space.
Then clean wet
Apply the appropriate solution with a soft cloth, sponge or brush in circular motions and let it act briefly. Do not scrub - the fibre needs no force, the solution does the work.
Rinse thoroughly
Rinse with clean water until no more soap comes out. Any residue left behind is itself a nutrient and attracts dirt. On large pieces a garden hose without a pressure nozzle is the easiest way.
Air dry completely
Dry in a shaded, well ventilated spot, spread out rather than folded. Only once the fabric is dry all the way through does it go back on or into storage. A cover that is dry only on the surface brings the problem back reliably over winter.
Four habits that stop it happening
Removing mould is work, avoiding it is a matter of minutes. These four points cover the cases that reach our service desk most often.
Store dry and clean
Before winter storage brush or vacuum once thoroughly and let the fabric air dry for a day, even if it feels dry. Moisture at the moment of storing is the most common cause of storage mould. A second check before the season starts is worth the minute.
Let air reach the material
Covers need ground clearance and ventilation openings, cushions need space between them. Do not stack anything tightly into plastic film or an airtight bag - a breathable cover protects better than a sealed one. Our weatherproof cushions are made with openings for pressure equalisation for exactly this reason.
Do not let soiling overwinter
Pollen, petal juice and bird droppings are precisely the food mould needs. Removing them in autumn takes away the basis for growth in spring. One pass with the detergent solution is enough for that.
Refresh the water repellent finish
The finish wears off through sun, dirt and cleaning. Once water no longer beads but soaks in, the fabric stays damp for longer - and that is exactly what encourages growth. A fresh impregnation restores most of the water repellency.
Frequently Asked Questions
Sources
- Guideline on the prevention, assessment and remediation of mould growth in buildings, German Environment Agency (Umweltbundesamt), December 2017, updated April 2024. The reference for indoor mould and for when professionals should take over
- WHO Guidelines for Indoor Air Quality: Dampness and Mould, World Health Organization. The basis for the link between moisture and mould growth
- Solution dyed explained - why the dyeing method decides bleach tolerance
- All Germes fabrics with their technical figures
Last reviewed: 24.08.2026 · Reviewer: Germes-Team
Not sure what your cover tolerates?
Send us your order number or the fabric name. We will tell you whether bleach is allowed, whether the cover can go in the machine and which mixture suits your stain.