In a small saucepan, heat sugar and water until sugar dissolves.
Add tarragon, aniseed, lemon zest, baking soda, and a pinch of kosher salt. Boil 1 minute, then let cool 5 minutes. Transfer to blender with 1/2 cup ice and blend until smooth. Refrigerate at least 1 hour, then strain.
Stir in lemon juice. Refrigerate until ready to serve.
When ready to serve, fill each glass with ice, add 2 ounces of tarragon sugar syrup. Top off with seltzer water.
Adapted from Milk Street magazine.