Make the rhubarb syrup: put all the ingredients into a medium pot and bring to the boil. Reduce the heat to a simmer and cook for 20 minutes or until the fruit is very soft. Turn the heat off and cool in the pot.
Strain through a fine sieve into a measuring jug. Leave the fruit to drain for a few hours and then use a funnel to transfer to a bottle or other suitable container. Keep in the fridge for up to four weeks.
Make the cocktail. Put all the ingredients into a cocktail shaker. Add plenty of ice and shake hard for 30 seconds.
Strain into two rocks glasses filled with crushed ice. Garnish with a rose petal.
To make this into a Rhubarb Gin Fizz pour into a tall glasses filled with ice and top with soda water. Divide between two champagne flutes and top up with chilled Prosecco to make Rhubarb French 75.