In a small pot over medium heat, whisk 3½ cups (800 grams) cold water, the sugar, and the hibiscus flowers. Bring to a boil, stirring occasionally until the sugar dissolves. Turn off the heat and stir in the ginger. Cover the pot and refrigerate until completely cooled, about 1 hour.

When you are ready to churn the sorbet, set a fine mesh strainer over the bowl of an ice cream maker. Strain the hibiscus mixture, pressing on the solids to extract as much liquid as possible, then discard the solids. Churn the mixture according to the manufacturer’s directions until the sorbet is thick and frozen, with the consistency of soft serve, about 30 minutes. Transfer to an airtight metal or plastic container and freeze until set, at least 3–4 hours.

To serve, dip an ice cream scoop or spoon in hot water, or temper the sorbet in the refrigerator for 20 minutes before scooping. Serve immediately.