Chop rhubarb into 1" pieces. Place in a pot with 6 cups of water, lavender and mint. Stir to combine. Bring to a boil. Reduce to a simmer and cook for 1 hour. Strain.
Add sugar. Stir to combine. Chill in refrigerator.
Notes
For additional flavor, you can leave rhubarb to soak overnight after it has simmered and strain and chill it in the morning.