Which biennial weed with yellow umbel flowers causes severe blistering if skin contacts its sap?

The biennial weed that causes severe blistering upon skin contact with its sap is wild parsnip. This plant is notable for its yellow umbel flowers, which are characteristic of the Apiaceae family, to which it belongs. When the sap of wild parsnip comes into contact with skin, particularly in the presence of sunlight, it can lead to a condition known as phytophotodermatitis, resulting in painful blisters and skin irritation.

Understanding this aspect of wild parsnip is critical, especially for those working in right-of-way management and herbicide application, where contact with various plant species can occur. This knowledge aids in not only safe handling but also in effective management and control methods concerning this invasive species. The other choices, while notable plants in their own right, do not possess the same properties regarding skin reactions.