Rhubarb pie is a sweet dessert pie with a tart undertaste made primarily from rhubarb, a sour-tasting perennial plant that is actually a vegetable but has been reclassified in the United States as a fruit. Because of the plant's natural tartness, it has to be sweetened with a relatively large amount of sugar. Writing in her seminal 19-century cookbook, The American Frugal Housewife, Lydia Child cautioned: "These are dear pies, for they take an enormous amount of sugar."  Cooked rhubarb produces a large amount of liquid, so the pie is usually thickened with flour or cornstarch or a combination of the two. Salt, lemon juice, orange peel, butter, and cinammon are some of the seasonings that can be added to the pie.
Rhubarb pie is perhaps more commonly eaten where rhubarb is widely cultivated, in the British Isles and the Northeast of the United States. In the 19th century, rhubarb pie was so common in New England that the plant itself was sometimes called "the pie plant" and the pie was called "pie plant pie." Today a popular pie throughout the United States is strawberry-rhubarb, a combination of the primary ingredients.
- The American Heritage Cookbook and Illustrated History of American Eating & Drinking, American Heritage Publishing Co., New York, 1964, Vol. 2, page 584
The American Heritage Cookbook and Illustrated History of American Eating & Drinking, American Heritage Publishing Co., New York, 1964, Vol. 2, page 584