Milestone & Turner Rd Cow parsnip, J Lentowski

American cow-parsnip (Heracleum maximum) growing in disturbed soil around a paper birch near the intersection of Milestone and Turner Roads.

If you’re out and about on Nantucket this month and see super-sized umbels of white flowers that resemble Queen Anne’s lace but are the size of dinner plates, topping plants with beefy stems and large coarse leaves, it’s likely you have come across American cow-parsnip (Heracleum maximum). Typically found at the edges of wetlands and salt marshes, this native wildflower can also pop up unexpectedly in clearings where the soil has been disturbed and the soil is fairly rich and damp.

American cow parsnip Leaf, Williams Ln, Ellen Phelan

A compound leaf of American cow-parsnip.

American cow-parsnip flies under the radar its first season, producing only a bunch of leaves, but in the second spring it makes a grand entrance by sending up flower stalks usually 3-6′ tall, and producing umbels that are around 8″ in diameter. While a plant of this size is sure to make a splash no matter what, American cow-parsnip produces an added frisson of fear in a lot of people, because it is a close relative to giant hogweed (Heracleum mantegazzianum), which is a serious invasive weed introduced from Eurasia. Giant hogweed was introduced as an ornamental by Victorian gardeners, who also brought us such delights as Japanese knotweed (Fallopia japonica) and black swallow-wort, also known as dog-strangling vine (Cynanchum louiseae). The Victorians loved novelty in the form of oversized plants, particularly ones with a sinister twist, apparently. Giant hogweed (also called giant cow-parsnip by some) causes a severe blistering skin reaction that leaves permanent scarring, so it has made the blacklist of “Most Dangerous Weeds” in a lot of places.

Amer Cow Parsnip portrait, KAO

American cow-parsnip showing off its compound umbels and compound leaves.

So, if you stumble across an enormous white-umbeled plant that looks like a Queen Anne’s lace from the Little Shop of Horrors, how can you tell whether it’s giant hogweed or American Cow-parsnip? And why should you care?

First off, it’s wise not to handle ANY plants in the Apiaceae (Carrot & Parsley Family) casually, so take a look rather than wading right in. Did you know that even home garden plants like parsley, carrots, and parsnip all produce harmful furanocoumarins in their vegetation? These are chemicals that when rubbed on your skin followed by sun exposure, can cause blistering burns, a condition called phytophotodermatitis. Handling carrots or parsley is not always a problem, but if the plants are wet, or if you get the oils on sensitive skin, or in large quantities, you can wind up with fluid filled blisters like poison ivy, only less itchy. The oils quickly bond to your skin, and spending even a short time in the sun afterwards can cause blisters to form. The giant hogweed reaction is much, much more serious and debilitating.

So, take a look at your Apiaceae carefully, and hands-off. Assuming your plant has the characteristic large coarse-lobed leaves and big white umbels, the next step is to assess the number of branches in the inflorescence. An American cow-parsnip will have up to 45 primary branches in its umbels, while the invasive and much more hazardous giant hogweed will have 50 or more branches. You can figure out the number of primary branches by looking at the top of the flower head, and counting the round bunches or clumps of flowers. Each bunch of flowers in an umbel equals one primary branch.

Cow parsnip branch bundles and umbel

If your plant has umbels with 45 or less branches, you have a native American cow-parsnip, and the only thing you need to do is avoid handling or weed whacking the plants to prevent possible skin reactions, the same sort of precautions you would take with poison ivy or poison sumac. American cow-parsnip is a great pollinator plant feeding many species of native bees, flies, and wasps, and greatly enhances the biodiversity of its surroundings. Biodiversity goes way beyond pollinators, and this plant offers habitat for many other invertebrate species that feed on the leaves, and spiders that lie in wait for prey.

If on the other hand you’ve discovered a plant with 50 or more primary branches on an inflorescence, you may have found giant hogweed. This species has never been recorded as occurring on Nantucket, and we would love to keep it from ever becoming established. Please get in touch with one of the co-chairs of the NBI Invasive Plant Species Committee: either Kelly Omand at Nantucket Conservation Foundation (komand@ nantucketconservation .org) or Sarah Bois of the Linda Loring Nature Foundation (stbois@ llnf. org) to let them know about your find. Pictures and an exact location will help us determine if your plant is a concern.