These dangerous carrots cause photo-dermatitis. Cow parsnip, wild parsnip, and giant hogweed contain an allergen that is activated by sunlight to cause rash, blisters, or other skin irritations in susceptible people.
Cow Parsnip (Heracleum lanatum) is a native plant that can reach eight feet tall. Like the hemlocks, its flowers are white, but cow parsnip flowers are larger, growing to 10 inches across. The large flowers were obvious as we rode past them on the motorcycle.
Wild Parnsip(Pastinaca sativa) has large yellow flowers that are about five inches across. It is a stout plant that grows to five feet tall with grooves along its stem. I know a few people, including my sister, whom are very allergic to this plant.
Giant hogweed (Heracleum mantegazzianum) is the worst of the three that cause blisters in the sun. This plant lives up to its name as a giant. It grows 15 feet tall with five foot leaves and 2 ½ foot white umbel flowers. Its massive size makes it desirable to some home gardeners, but it is a public hazard that causes severe skin irritation in susceptible people. A USDA pamphlet says this plant's sap produces painful, burning blisters that later develop dark scars that may persist for years. Fortunately, it has only been found in an isolated location in northwest Illinois, and those plants have been eradicated. Giant hogweed is a federal noxious weed, making it unlawful to propagate, sell, or transport this plant in the United States.
Finally, not all carrots are dangerous. Queen Anne's Lace (Daucus carota) is also called wild carrot due to its large, edible taproot. It grows four feet tall with lacy four inch flowers. This plant is the ancestor of our cultivated carrot, though its roots are white instead of orange.
Other edible plants in this family grown for their roots are parsnips and bulb fennel. Anise, caraway, celery, chervil, cicely, coriander (cilantro), cumin, dill, fennel, and parsley are grown for their edible leaves and seeds.