Giant Hogweed

Giant Hogweed and Cow Parsnip: Which is Which and Why You Should Care

This summer we fielded calls from several frantic landowners who were certain they had giant hogweed growing on their property. Heracleum mantegazzianum is a weed worthy of panic, as its sap causes severe phytophotodermatitis in humans. When skin is exposed to the sap and subsequently exposed to the sun, it will redden, swell, and form painful blisters that resemble burns and often leave scars. If the sap gets in your eyes it may cause permanent blindness. Fortunately, giant hogweed does not live in the Rocky Mountain States. But its close relative, Cow Parsnip (Heracleum maximum) does.

Giant Hogweed History

Giant hogweed is native to Asia and was introduced to New York State as an ornamental in the early 1900s. Due to its “Dr. Suessian” charm, it became a popular addition to hobby gardens in coastal areas throughout North America. When it became evident that giant hogweed could spread into natural environments and displace native plants, and that its caustic sap made it hard to remove, it was classified as a federally listed noxious weed that was illegal to buy or sell.

Giant Hogweed in America

When giant hogweed was first discovered growing in Virginia this summer (2018), the news spread like cheatgrass, igniting a frenzy on social media about the spread of an invasive weed that “burns and blinds.” The news reached Teton County around the flowering time of a closely related native plant called cow parsnip. Many residents mistakenly identified the cow parsnip as giant hogweed, and our phones started ringing.

Spot the Similarities

Cow parsnip, Heracleum lanatum, shares the family AND genus of giant hogweed, and therefore shares several diagnostic traits with it:

  • White flowers arranged in umbels (umbrella-like clusters)
  • Leaf bases that form sheaths around the main stalk
  • Large leaves that are divided into leaflets
Spot the Differences

But there are also some BIG differences between these two plants. Cow parsnip is a rather large plant, reaching heights of over 7 feet tall and sporting leaves that are 2.5 feet in diameter. But giant hogweed dwarfs the cow parsnip, topping out at 14 feet tall with leaves spanning 5 feet across!

Cow parsnip stalks are about 1 inch in diameter and solid green, while hogweed stalks can be 2.5 inches in diameter and are mottled with reddish spots, just like another member of the carrot family, poison hemlock.

Speaking of poison, the family that these plants share (apiaceae, a.k.a. the “carrot family”) is infamous for including some of the deadliest plants on Earth. Both poison hemlock and water hemlock are in this family and ingesting a small amount of any part of these plants will kill a human. Those that don’t kill can burn and blind, like giant hogweed. Even our native cow parsnip is not totally benign. It has been reported to cause phytophotodermatitis reactions as well, though not nearly as severe as those caused by giant hogweed. In the plants’ defense, the reaction is caused by furocoumarin chemicals in its sap that evolved to defend the plant against herbivorous insects. We were not the intended target. Also, there are several friendlier members of this family that we couldn’t do without, including parsley, dill, cilantro, and, of course, carrots. But it is safe to say, plants in the carrot family should be treated with caution whenever encountered outside of the grocery store, unless you are sure of their identity.

The take home message is, learn to recognize plants in the carrot family (remember, umbels and divided leaves), be cautious around any plant you are not familiar with (you don’t want to be the unintended target of an arms race between plants and insects), and have compassion for our coastal friends who have to live with giant hogweed and poison oak. Our stinging nettle pales in comparison.

View the Difference

Cow Parsnip
Giant Hogweed

Both plants have large leaves dissected into 3 leaflets, but leaflets of cow parsnip (left) have rounded lobes and giant hogweed leaflets (right) have pointed lobes. Also, giant hogweed leaves are twice the size of cow parsnip leaves.


Both cow parsip and giant hogweed have white flowers arranged in umbrella-like clusters called umbels, but the differ significantly in size.


Cow parsnip resembles giant hogweed but is much smaller, lacks the red spots on the stems, and is far less dangerous.

Giant hogweed

Giant hogweed stalks are mottled red like its close relative poison hemlock. Cow parsnip stems are solid green.

Giant Hogweed Burns

The sap in giant hogweed contains furocoumarin chemicals that cause phytophotodermatitis in humans.

Giant Hogweed

Giant hogweed is a federally listed noxious weed due to its ability to displace native plants and its caustic sap.