Wild Parsnip

Photo | Bob Embleton


Wild Parsnip
Jerry Doll

Wild parsnip, a member of the Umbelliferae family (also known as the Apiaceae family or in common terms, the parsley or carrot family), is among the rapidly increasing weeds in many areas of Wisconsin. As wild parsnip has spread, so has the realization that human exposure often leads to serious burns and blisters on the arms and legs. Being able to readily identify wild parsnip and early detection of infested areas will minimize inadvertent and excessive exposure to this plant and the often painful results that follow.

During much of July, wild parsnip is one of the dominant yellow-flowered weeds in many roadsides and other right-of-ways, fence rows, prairie restorations, CRP sites and poorly managed pastures. Plants are most abundant in sites dominated by perennial grasses that are mowed once or twice annually.

The species is native to Eurasia and may have been introduced as a vegetable as plants have long, thick, white to yellowish taproots that are edible. True parsnip plants have larger roots than wild parsnip. The entire plant has a parsnip odor. Cattle will not eat wild parsnip but deer may feed on it and birds and small mammals eat the seeds.

Description. The first true leaves seedlings produce are small and ovate while the latter leaves are arranged in spiral fashion, forming the typical rosette in the first year of growth. Fully developed rosettes have 10 to 15 pinnately compound leaves with broad ovate to oblong leaflets that reach. After reaching the minimum size, plants produce a stout, smooth, hollow, grooved flower stalk that may reach 5 feet in height. Leaves on the stem are alternate and have 2 to 5 pairs of opposite, sharply toothed leaflets that may be somewhat mitten-shaped and a terminal leaflet that may be diamond-shaped. The petioles are broad and the base of each leaf stalk completely encircles the stem. The top most stem leaves are reduced to narrow bracts and the flowering branches arise from their axils. Flowers have five small petals that are clustered in umbels that are 4 to 8 inches in diameter. Seeds (mericarps) are flattened on one side to round and have five ribs.

Life Cycle and Growth. Plants require a cold period (winter) after germinating and reaching the rosette stage before they flower, but not all vernalized plants flower the next summer. After a plant flowers, it dies. This life cycle is called a monocarpic perennial. Most plants flower in the second, third or fourth season after germination (Baskin and Baskin, 1979). When a given plant flowers is determined by the size of the rosette. Plants that reach the critical size by the end of the growing season will be vernalized during the winter and flower the following spring. The critical size was found to be the root crown diameter; all plants with root crowns 10 mm (0.25 inch) or greater in diameter in the fall flowered the next year. Some plants with root crown diameters of 5 to 10 mm also flowered while others this size did not. Those below 5 mm in diameter did not flower.

Plants form primary umbels on the top of the main stem, with secondary and tertiary umbels produced as side shoots from the main stalk. Seeds from tertiary umbels weigh about 50% less than those of the primary umbel. Secondary umbels mature 10 to 14 days later than the main umbel and tertiary umbels about 1-0 to 14 days after the secondary umbels.

Most seeds are dispersed from the parent plant by the end of September and germination occurs from this time through November and again in the spring in Iowa (Hendrix and Trapp, 1989). Wild parsnip has a long germination period, but the optimum time for germination is in the early spring and that is when most germination occurs in Kentucky (Baskin and Baskin, 1979). Most fall germinated seedlings die during winter. Wild parsnip seedlings are among the first plants to greenup early in the spring.

Why the explosion of wild parsnip in Wisconsin? Only Mother Nature knows for sure. Birds and small mammals eat the seed and they may be spreading the problem from site to site. There is no doubt that the delay in mowing roadsides until mid summer as an official roadside management policy of the state opens the door for this plant to complete its life cycle and produce ripe seeds well before any mowing is done.

Also, when roadsides and pastures are mowed in late July and August, parsnip seeds probably move as hitchhikers on the mowers. Mowing also creates a much more favorable environment for parsnip seeds to germinate than if the sites were left undisturbed. Relatively mild winters may enhance survival of wild parsnip plants that germinate and become established in the fall.

Health concerns. This section is adapted from the excellent article on wild parsnip burns by David Eagan (1999). Wild parsnip is of concern because humans develop a severe skin irritation from contact with its leaves. Plants have chemicals called psoralens (more precisely, furocoumarins) that cause phyto-photodermatitis: an interaction between plants (photo) and light (photo) that induce skin (derm) inflammation (itis).

Once the furocoumarins are absorbed by the skin, they are energized by uv light on both sunny and cloudy days. They then bind to DNA and cell membranes, destroying cells and skin. Parsnip burns usually occur in streaks and elongated spots, reflecting where a damaged leaf or stem moved across the skin before exposure to sunlight.

Wild parsnip burns differ from the rash caused by poison ivy in several aspects. First, everyone is sensitive to wild parsnip and you do not need to be sensitized by a prior exposure to develop burns or blisters. You can brush against wild parsnip plants and not be affected. Parsnip is only dangerous when the plant sap from broken leaves or stems gets on your skin. Lastly, the wild parsnip’s “burn” is usually less irritating that poison ivy’s “itch.” The worst of the burning pain caused by wild parsnip is usually over within a couple of days while the rash and itch of poison ivy can last a long time.

In cases of mild exposure to wild parsnip, affected areas turn red and fell sunburned. In severe cases, the skin first turns red and then blisters form. The arms, legs, torso, face, and neck are most vulnerable and affected areas may feel like they have been scalded. Blisters form a day or two after sun exposure and soon after the blisters rupture and the skin starts healing. But for many people the ordeal is not over as dark red or brownish “scars” remain in the burned areas for several months to years. Animals can also get parsnip burns if they have little hair and lightly pigmented skin, characteristics that allow the chemical and sunlight to reach the skin.

The burning sensation can be relieved by covering the affected areas with a cool, wet cloth. Try to delay blisters from rupturing as long as possible as blisters protect the skin by keeping it moist and clean while the areas heal. For those cases with extensive blistering, consult a doctor.

Tips to avoid exposure include wearing gloves, long pants, and long-sleeved shirts. Planning control activities for the early evening will minimize sunlight and thus activation of the blistering process. If you are exposed to the plant juice, wash the contaminated areas thoroughly as soon as possible.

Control. Plants can be controlled mechanically by cutting them just below the soil surface. Mowing will not kill wild parsnip plants but will reduce seed production.

If herbicides are needed and practical, a timely application of 2,4-D and/or dicamba to plants in the rosette growth stage (early fall or late April to mid-May) should control all treated plants. Careful applications of glyphosate will also kill wild parsnip plants. By the time plants are bolting or flowering, mowing is the best alternative.

Many asked why wild parsnip hasn’t been declared a noxious weed. Good question, but given the area and density of infestation, this weed has clearly moved beyond the scope of an eradication program. Towns and counties that do have little if any wild parsnip may want to give this serious consideration as a weed to add to their local noxious weed list. We would certainly see less wild parsnip if 2,4-D were applied when this biennial weed was in the rosette stage. It is doubtful that county highway departments will do this unless it is made a local noxious weed but certainly privately owned pastures and CRP sites could benefit greatly from a timely early fall 2,4-D treatment.

There are no planned biological control programs in Wisconsin but the parsnip webworm often destroys many wild parsnip inflorescences.

July 2001