Combine the rhubarb, sugar, and water in a pot and bring to a simmer. Turn down the heat and let simmer on low for 10 to 15 minutes. Strain the syrup through a wire mesh strainer and transfer to a clean jar. Add the rosewater.
Let cool completely before using. The syrup will keep for 2 to 3 weeks in the fridge. This syrup should make enough for around 10 cocktails.
Sparkling Rhubarb Rose Cocktail
In a cocktail shaker add the gin, elderflower liqueur, rhubarb rose simple syrup, lemon juice, and rhubarb bitters. Add ice, then cover and shake for 10 to 15 seconds. Strain into a coupe glass. Top with the sparkling wine. Garnish with fresh flowers and enjoy!