Cut off any white pith from zest. Bring zest, juice, sugar, and cold water to a boil, stirring, then let stand, uncovered, off heat 15 minutes.
Discard zest. Blend syrup with tarragon in a blender 30 seconds. Strain through a sieve lined with a dampened paper towel into a glass measure and cool completely.
Pour boiling-hot water over tea in a heatproof pitcher and let steep 6 minutes, then discard tea bags. Cool tea completely.Stir syrup into tea and chill at least 1 hour and up to 6 (cover tightly).
