I did a rhubarb berliner weiss last year. I chopped the stalks (don't add the leaves) into about 3 inch pieces, added a little water, and brought up close to a boil and then back down. Pitched the whole sloppy mess into the carboy. It was about 1lb/gallon. I got just a hint of color, some extra tartness, some rhubarb flavor, and a hint of vegetal greenness (more pleasant than it sounds).
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