The MM-Minis will last for years. But since the material allows water and air to pass through, as time goes by some grimy spots will appear. It's unavoidable: wet soil mix and the outdoors means nature does its thing.If you see green spots, they're most likely caused by algae. Algae love cold, wet weather and stagnant water.Luckily, the material is easy to clean:During the season you can easily remove the dirty spots with a sponge and some gall soap.At the end of the season, put your plantless grow bags in a dry place (like in the shed), to prevent unnecessary algae growth.At the start of a new season, you can wash dirty minis just fine.First, soak them for a few hours in soapy water using a mild laundry detergent and some gall soap. If you have algae stains, add a generous splash of vinegar as well: algae can't stand that. Then rinse the minis well.If they are still dirty, you can even put them in the washing machine: use a cold setting or max 30°C.