Syrup- In small sauce pan, heat water, sugar and lemon zest to boiling over medium-high heat, stirring occasionally until sugar dissolves. Remove from heat; stir in rosemary sprig. Cover and let stand for 10 hour at room temp and refrigerate 1 hour or until chilled.
Lemonade- Strain rosemary mixture into large pitcher. Stir in cold water, ice and lemon juice. Serve in tall glasses over ice.