Old nettles.

Old nettles aren't good for you. Young nettles are.

Some people, not many, get a lower back ache when drinking tea from old nettles (Urtica dioica). That's not old in the sense that the nettles have been in the jar for too long - it's old in the sense that the nettles have been growing on the plant for too long.

You'll find calcium carbonate crystals forming in older leaf.

These crystals can irritate the kidneys; if you drink old nettle tea and get a lower backache use your common sense and stop ingesting those nettles. Don't use them in stews, either - make them into a footbath.

You really do not want to irritate your kidneys to the point of pain.

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Related entry: Nettle greens - Nettle tea - Picking nettle seed