You may have parsnips on your patch that are past their best. Take care when clearing then away as sap from parsnip leaves can react with sunshine and cause the skin to be badly burned and blistered.
Parsnips are related to the giant hogweed which is known for its danger to skin. Parsnips are not the only plants to have potentially dangerous sap, among many others, carrot and celery sap can cause a similar problem.
Care should also be taken when strimming grass in sunny weather - if legs are not covered grass sap is likely to splatter your skin and again react with the sunshine to cause burning.
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If in doubt always wear gloves and have skin covered when cutting plants to expose their sap.