Combine sugar, water and rosemary in a saucepan and bring to a simmer over medium-low heat. Let rosemary mixture simmer for at least 20 minutes and up to 2 hours on low heat, stirring occasionally. Remove rosemary mixture from the heat and let it cool for 30 minutes before straining through a wire-mesh strainer, discarding the rosemary sprigs. Pour the syrup into an airtight container like a Mason jar or Grolsch beer bottle and garnish with a label and a sprig of rosemary. The syrup can be stored in an airtight container for up to 1 month.
To make other-flavored syrups, replace the rosemary with other herbs, fruit juices or spices by using the “cover the surface of the water rule” for solids and ¾ of a cup to 1 cup of fresh juice for liquids. Some other flavored syrups that we love are lemon, lavender, ginger, rose and peppermint.