Wild parsnip is an invasive plant commonly found in roadsides, pastures, restored prairies and other areas with full sun and minimal disturbance. It is a biennial that forms a rosette with 'celery-like leaves' (Fig. 1.) and an umbel with yellow flowers (Fig. 2). It contains a class of chemicals (furocoumarins) that cause a skin rash known as phytophotodermatis, sometimes called parsnip burn. The response is caused by the chemical hypersensitizing skin to UV light, thus persons only develop the rash if they remain in the sunlight following exposure. The rash often is much more severe than that caused by poison ivy, and can cause permanent scarring.
I recently received a call from a client wondering whether the toxin in wild parsnip remains active in dead plants, similar to the toxin in poison ivy (urushiol). While I was unable to find any information on the half-life of the toxin, these compounds are present in seeds suggesting they are somewhat persistent. The highest concentrations of the toxins are found in leaves, thus the stems that remain following plant death probably contain relatively low concentrations. I've been scouting CRP fields for milkweed this month and have walked through some fairly dense patches of dead wild parsnip and have not had any reaction on my arms. I would conclude that dead wild parsnip plants pose much less risk of causing dermatitis than dead poison ivy stems which are known to contain toxic concentrations of urushiol for long periods.
But the purpose of this article is to report a fun fact I discovered while researching the question. Wild parsnip is a member of the Apiaceae family. This family contains several important crops (carrot, celery, dill, etc.) along with the weeds (wild parsnip, wild carrot, poison hemlock, etc.) Many members of the Apiaceae family produce furocoumarins, although most contain them at lower concentrations than wild parsnip. Celery contains the compounds, and people working in the crop can develop the rash. There is a documented case of a woman eating a significant amount of celery prior to going to a sun tanning booth. She developed severe phytophotodermatis over her body caused by the consumption of the furocoumarins present in the celery. As they say, watch what you eat.
Figure 1. Wild parsnip rosette. Figure 2. Wild parsnip umbel (flower head).