Answer: Based on your description I am not certain what is on your plants. Any pollen from the viburnums themselves would be produced when they bloom in the spring, and only then. At this time of year, any pollen would have to be from some other plant nearby. Or, perhaps it is a dust that has blown onto them from elsewhere -- this would be consistent with the sandy texture. You might be able to hose it off, or prune shortly after a rain. Just to find out what it is, you might want to check with your local county extension and see if they can help you identify it. I'm sorry I can't be more specific for you.
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