Written by Tim Parsons Landscape Horticulturist Middlebury College Certified Horticulturist and Arborist
Poison parsnip, Pastinaca sativa, is actually Parsnip, the root vegetable. As a vegetable, it was popular in colonial times, as it matures under a very short season, and tastes best after the cold set in. Potatoes became more popular, and parsnip escaped cultivation, and has plagued us since. Related to carrots, they haunted my childhood, not only for an ingredient in pot roast, but an unfortunate nickname through grade school. I bet I would like them now, but I just can’t bring myself to try them. The poison comes from a chemical in the plant sap called psoralen, which reacts with the human skin to cause (ready for this?) phytophotodermatitis. In a nutshell, the sap reacts with UV sunlight, and causes mild to in some cases quite severe burns. I’ll speak from experience here-a mild case can cause red skin, like a steam burn from your tea kettle. I had full second degree burns on all exposed parts of my legs one year, blisters from shorts to socks. Oh yeah, I was just a poster child for horticulture that summer. The blisters healed after a couple of weeks, but the dark reddish brown blotches stayed for the better part of a year.
The sap is not an oil, unlike poison ivy, so it won’t linger on your clothes, pets, lawn mowers, etc. Being phytotoxic, one must come in contact with the sap in the sun in order to get the burn. When dealing with parsnip, I pick cloudy or even rainy days. It is also safe to brush up against, unlike poison ivy, as the plant does not seem to exude sap from unbroken leaves or stems.
The plant is a biennial-a rosette of leaves the first year, and flower stalks the second. (Parsnip in the store is a one year root). I find this makes eradication a little difficult. Biennials are hell-bent on flowering in their second year-if they don’t, what’s the point to life? So cut down too early, and they form many smaller flowers, and therefore more seeds, than left untouched. The best time to mow seems to be right after they flower, but before they have set seed, you can fool mother nature that way.
It thrives on roadsides, and other poor growing locations, because the rosettes in the first year are poor competitors, and can’t keep up with a healthy stand of vegetation, such as grass. This is actually the best control method, growing better plants to choke out the parsnip. Other control methods go all medieval, by digging, ripping, pulling, repeated mowing, or just plain cursing the plant out of exisitence. Seeds of parsnip are viable in the soil for up to four years, so vigilance is required. A little herbicide does wonders, if you are so inclined.
This text originally appeared on the Middlebury College blog network on June 23, 2010. It has been republished with the author’s permission.
The Invasive Plants Association of Wisconson states the following advice for mowing poison parsnip on its wild parsnip fact sheet.
“Wild parsnip can become abundant along irregularly mowed roadsides as inconsistent mowing seems to facilitate seed dispersal. A single mowing late into the growing season (mid July thru August) will result in high seed dispersal as seeds have matured and are transported by mowing equipment. Because of this, mowing should be done prior to seed formation (June) with follow-up mowing throughout the summer to avoid flowering and seeding out (timely mowing). Mowing can however also stress other plant species that have the potential to be good competitors against parsnip.”