Prep time: 90 minutes. Yields about 3 cups.

Ingredients

• 2 cups (400g) sugar
• 2 cups (480mL) water
• 1 teaspoon (6g) citric acid, or the juice of one whole lemon
• 5 cups (225g) roughly chopped rhubarb, skin on (about 3 large stalks)

Directions

• In a large saucepan, combine ingredients and bring to a simmer.
• Cook over low heat until the rhubarb has fallen apart, about 1 hour.
• Take care not to reduce the liquid by boiling.
• Strain the syrup through a fine-mesh strainer into a resealable container.
• Let cool and refrigerate until ready to use.

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Is rhubarb poisonous?

Sort of. From Wikipedia:

Rhubarb leaves contain poisonous substances, including oxalic acid, a nephrotoxin. The long term consumption of oxalic acid leads to kidney stone formation in humans. Humans have been poisoned after ingesting the leaves, a particular problem during World War I when the leaves were mistakenly recommended as a food source in Britain. The toxic rhubarb leaves have been used in flavouring extracts, after the oxalic acid is removed by treatment with precipitated chalk (i.e., calcium carbonate).

The LD50 (median lethal dose) for pure oxalic acid in rats is about 375 mg/kg body weight, or about 25 grams for a 65-kilogram (143 lb) human. Other sources give a much higher oral LDLo (lowest published lethal dose) of 600 mg/kg. While the oxalic acid content of rhubarb leaves can vary, a typical value is about 0.5%, meaning a 65 kg adult would need to eat 4 to 8 kg (9 to 18 lbs) to obtain a lethal dose, depending on which lethal dose is assumed. Cooking the leaves with baking soda can make them more poisonous by producing soluble oxalates. The leaves are believed to also contain an additional, unidentified toxin, which might be an anthraquinone glycoside (also known as senna glycosides).

The takeaway? Eat rhubarb stalks, not the leaves.