Even though no one on planet earth enjoys mildew, the good thing is it can be removed with a little effort on your part. If it happens to grow on your garden urn, try out this solution. To begin, you need to create a solution that will destroy the mildew. A good one would be to mix one quart of all-purpose bleach, 2/3 cup of trisodium phosphate and 1/3 cup of mild dish detergent together into three quarts of warm water.
Now that you have your solution, drag your garden hose so it is in an accessible area for you to use and also get a stiff-bristled nylon brush. All you are trying to do is remove the mildew so dip this brush into your solution and begin to scrub away. With the hose, rinse off area right after you cleaned it to see if it did the trick. Rinse and repeat until all the mildew is gone and then allow the urn to air dry.