What part of the rhubarb plant is poisonous if eaten?

The poisonous part of a rhubarb plant is its leaves. Rhubarb’s large leaves contain high levels of oxalic acid and other potentially toxic compounds, including anthrone glycosides, so they are considered inedible. The familiar red or green stalk is the part normally cooked and eaten.

Oxalic acid is a nephrotoxin: in substantial amounts, oxalates can bind calcium and contribute to kidney injury or kidney stones. Rhubarb leaf poisoning has been reported throughout history. During World War I, British advice mistakenly promoted rhubarb leaves as a food source, and similar misinformation resurfaced during World War II, causing further poisonings.

The edible stalk is technically a petiole—the leaf stalk—not a stem. It contains much less oxalic acid than the leaf blade and is usually cooked with sugar in pies, sauces, and crumbles. However, damaged rhubarb exposed to severe cold may develop higher levels of oxalic acid, so such stalks should not be eaten.

Rhubarb also has a medicinal history: roots of several Rheum species were used in traditional Chinese medicine as laxatives. That fact does not make the leaves safe, and cooking them with baking soda can increase danger by creating more soluble oxalates.