Wild Parsnip, Pastinaca sativa, is an invasive, non-native plant growing abundantly in portions of the Arboretum and in some of the un-mowed areas on campus such as around Lyman Lakes. While Arboretum and Grounds staff are working to control this plant, it is important that Arb users be aware of possible health risks. The juice from wild parsnip causes skin to become ultra sensitive to sunlight, a phenomena known as "pytophotodermititis". Skin sensitization happens within ten minutes, and lasts 6-8 hours. Once sensitized and exposed to the sun a severe rash develops including blistering and discoloration.
Avoiding the plant altogether is best, but if you are exposed to the juice of this plant you should wash the area immediately and avoid sun exposure for 6-8 hours by covering up the exposed area or staying indoors.
Wearing long sleeved pants, socks and a long-sleeved shirt will protect you if you are unsure of your plant identification skills. Staying on the trails will also protect you since you need direct contact with the sap of the plant to cause the rash.
More information is available from the Minnesota Department of Natural Resources web site. For photos of this plant in all stages see this site from Virginia Tech. For a comprehensive article about the plant, see this resource from the Extension Service of the University of Wisconsin.