Bring 625 ml of water and sugar to a boil in a saucepan, stir until sugar is dissolved.
Add lavender and remove from heat, let cool under room temperature with lid closed. Pour through a strainer and remove lavender.
Pour tea into a glass pitcher, add 500 ml of water and lemon juice.
Stir well, add more sugar if desired, and let cool thoroughly in the refrigerator.
Just before serving, stir the lemonade again and add ice cubes to the pitcher. Pour into pre-chilled glasses and garnish each glass with a stem of lavender.