I've only used it for medicinal tea, so can't speak to processing as a spice.
My mentor told me harvesting two weeks after a rain is ideal, for the reasons J Davis mentions. He also taught me to always ask any plant for permission before I harvest. And listen to the response! Sounds simple, but makes a huge difference, I find.
I always taste a few berries before I harvest as I've found wide variations in how tart and astringent they are from plant to plant as well as in different locations.
Try to get to it early. The bugs also love sumac and later on, there will be a lot of frass (bug poo) in between the berry clusters. It's very hard to remove given all the little hairs. Doesn't detract terribly, but not awesome.
This probably goes without saying, but avoid harvesting near heavily traveled roads. All those little hairs are a great filter for airborne pollutants.
I have dried it both hanging inside as well as on a drying rack covered with a light towel in my car. I liked the drying rack better since again, all the little hairs are a dust magnet. Sumac lasts a long time dried and in a jar.
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