Cow parsnip, Heracleum maximum, is a big plant with a big name. This 4- to 8-foot-tall native plant of damp meadows and fields, streamsides, ditches, and low, open woods is named for Heracles or Hercules, the mythic Greek hero of superhuman strength. Even the species name, maximum, hints at its size. Few herbaceous plants are as robust.
A biennial or short-lived perennial in the carrot family, cow parsnip has alternate, divided leaves up to 2 feet across, with smaller, undivided leaves higher on the stem. The leaflets are irregularly lobed, coarsely toothed, and hairy. The petioles of lower and middle leaves are 3 to 10 inches long with sheaths where they meet the stem. Stems are stout, hollow, ridged, and hairy.
|Lower leaves of cow parsnip are divided into three lobed, toothed leaflets. This leaf blade (left) is about 18 inches long. Petioles are long with sheaths where they meet the stem (right).|
Cow parsnip flowers from late May into early July. Small, white, 5-petaled flowers are in flat-topped, compound umbels, clusters that resemble a collection of little umbrellas. (See Flower Parts for Plant ID to learn more about types of flowers and flower clusters, called inflorescences.) The flowers are pollinated by honeybees and many kinds of native bees and flies, the variety so great that some consider cow parsnip a pollinator magnet.
|Cow parsnip flowers are arranged in compound umbels. Each spoke of the umbel extends to another, smaller umbel, called an umbellet. The tiny, white flowers have five petals.|
Fruits are flattened, up to 1/3 inch long and 1/4 inch wide, and brown with dark, vertical lines when they mature in late summer or early fall. They are winged on their edges and so can be dispersed a limited distance by wind. Eventually the fruits split apart and release two seeds. Cow parsnip reproduces only by seed.
|These immature fruits of cow parsnip will turn brown when they mature in late summer.|
Although the plant is native here and in much of North America, in some states it’s introduced and invasive. In North Carolina, for example, cow parsnip is described as aggressive and insidious and is classified as a noxious weed.
Cow parsnip is rarely considered an ecological problem here, but it should be handled with care. The leaves and stems contain furanocoumarins, compounds that sensitize skin to ultraviolet light. If skin is exposed to plant sap and then sunlight, blistering rashes may result. The condition, called phytophotodermatitis, can take weeks to heal. Always wear gloves, long sleeves and pants when handling this plant, and wash off and cover skin if exposed to the sap. The sap can also damage the eyes, so it’s best to wear eye protection if the plant will be disturbed.
Watch Out for a Look-alike
Giant hogweed (Heracleum mantegazzianum), a similar but much larger plant, is introduced and invasive. So far, this 10- to 15-foot-tall plant hasn’t been found in Minnesota, but it is present in Wisconsin, and it’s known to be especially hazardous. The Minnesota Department of Agriculture includes giant hogweed on its list of noxious weeds that should be eradicated if found. Report the plant’s location using Report a Pest or EDDMapS and handle it using extreme caution.
Invasive Plants in the Carrot Family
Cow parsnip isn’t one of Minnesota’s “bad carrots,” but several other species are, including not only giant hogweed but also wild parsnip, burnet saxifrage, Queen Anne’s lace, and several others. A guide to identifying these plants is available in the Downloads tab.
USDA, NRCS. 2023. The PLANTS Database (http://plants.usda.gov, 06/21/2023). National Plant Data Team, Greensboro, NC USA.