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Side by side
What tells them apart
This is the real confusion: both have arrow-shaped leaves and grow in the same damp places. Arum has a thick glossy leaf, often with pale veins, and burns the mouth instantly. Sorrel is thin, matt and lemony. If it burns, spit it out and do not swallow.
Confusion that kills
How to tell it
An arrowhead leaf with the two basal lobes pointing downwards. A spike of reddish flowers in summer. An immediate, clean sour taste.
How to tell it
A thick glossy arrowhead leaf, often with strongly marked pale veins. The flower is a greenish spike inside a sheath, and in summer only the spike of red berries remains.