In a wide pot, cook rhubarb, sugar, salt, and 2 1/2 cups water over medium heat, stirring often and mashing with a spoon if needed, until rhubarb turns to mush.
Pour mixture through a strainer into a bowl and rub with the back of a ladle to push through as much of the rhubarb as possible.
Transfer to a shallow pan and freeze until firm, stirring and scraping with a fork every hour if possible, about 4 hours total. Scrape up granita with fork and spoon into bowls.
Servings 0
* Percent Daily Values are based on a 2,000 calorie diet. Your daily value may be higher or lower depending on your calorie needs.