Origin
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There are generally 2 types of sorrel: garden sorrel with light-green leaves.
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But there's also a wild sorrel that grows (in Europe), usually in hedges or fields that aren't mowed.
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You'll easily recognize it by the top of the plant, which has the shape of tiny red "petals", grouped together in stems.
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These are the leaves we use, darker green, small at the top of the stem...
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...and larger at the bottom.
If you're in any doubt as to whether it's sorrel or not, the easiest way is to taste a leaf, and you'll soon know whether it's sorrel or not.