Poisonous to Touch Weed Series: Cow Parsnip (Heracleum maximum)

Cow parsnip (Heracleum maximum) plant in flower. Photo credit: Alex Katovich, Bugwood.org

Cow Parsnip (Heracleum maximum) aka Common Parsnip or American Parsnip, is a native plant that widely grows in north America. It is a large plant rising up to heights of 1-2.5 m (3-8 ft.) when in flower. Like giant hogweed, cow parsnip contains organic chemicals within it’s sap called furanocoumarins. This chemical is activated by ultraviolet radiation and the result is severe burning, blistering and rash. Although the burns are less severe than those of Giant Hogweed they still cause nasty burns that take weeks or months to heal and can leave scars.

Where Does Cow Parsnip Grow?

Cow parsnip has scattered distribution throughout Canada, except the most northern regions. In the U.S. it has scattered distribution in all states except in the extreme southeastern regions. It can also be found in Eastern Asia.

Like giant hogweed it is particularly fond of rich, moist soils in seldom disturbed areas. You can often find cow parsnip and giant hogweed growing together. Habitats include low laying woodlands, borders of woodlands, meadows in wooded areas, riversides and streambanks, and partially shaded roadsides.

Identification of Cow Parsnip

Overall growth habit: A vigorous, single-stemmed, bi-annual (or short lived perennial), that grows from a taproot or a cluster of fibrous roots. It can reach heights of 1-2.5 m. (3-8′) tall.

Leaves: The compound leaves are large measuring up to 0.5m (20 in), each with 3 leaflets. The leaflets are palmately lobed (several lobes whose midribs all radiate from one point) and somewhat resemble the shape of a maple leaf. They are coarsely-toothed with fuzzy undersides. The leaves somewhat resemble those of giant hogweed but are smaller with more rounded lobes and not so deeply cut. The basal leaves are arranged in a rosette the first year of life, then become widely spreading alternating up the stem, with the bulk of the leaves concentrated along the lower half of the central stem. The leaves grow smaller the higher up the stem they go. The petioles of lower to middle leaves are 3-10″ long, light green, hairy, and relatively stout (especially at the base) (Illinois Wildflowers n.d.).

Cow parsnip (Heracleum maximum) leaf. Photo credit: Rob Routledge, Sault College, Bugwood.org

Stem: is hollow and grooved, green in colour (few to no purple spots), with soft and fuzzy hairs. They measure about 5cm (2 in) thick. The stem is usually unbranched.

Flowers: are white compound umbels more or less flat-headed. The central stem terminates in a compound umbel up to 8″ across. Additional compound umbels are produced from the axils of upper leaves on long peduncles (flowering stalks); these axillary umbels are up to 6″ across (Illinois Wildflowers n.d.). Cow parsnip blooms in late May to late June, which is typically earlier than giant hogweed (Department of Environmental Conservation n.d.).

Roots: a taproot or a cluster of thick roots.

Seed: Individual immature fruits are about 8 mm. (1/3″) in length, 6 mm. (1/4″) across, and somewhat flattened (Illinois Wildflowers n.d.). They are light green (with white margins) maturing to brown. Each fruit contains 2 seeds.

Life Cycle: Dry seeds are wind dispersed but do not germinate the first year. It is believed they require a cold moist period, followed by a warm moist period, followed by another cold moist period, germinating the second year (The Friends of the Wild Flower Garden, Inc. n.d.)

Once germination takes place the plant forms a large rosette of leaves and begins putting on a strong tap or root cluster of fibrous roots. At the end of the season the cow parsnip plant dies to the ground and returns from the root the following (early) spring. In the second year a single, thick flower stock is sent up and by mid May large white umbel flowers up to 8″ across bloom atop them. They bloom for about 1 month then begin setting seed. When the seed is rip it is dispersed on the wind or carried on water or spread by human activity.

Look Alike Plants

  • Giant Hogweed, Heracleum mantegazzianum NOXIOUS WEED
  • Wild Parsnip (Pastinaca sativa) NOXIOUS WEED
  • Poison Hemlock, (Conium maculatum) SECONDARY NOXIOUS WEED
  • Angelica Angelica atropurpurea NATIVE WEED
  • Queen-Anne’s Lace, Daucus carota ) NOXIOUS WEED…..but not poisonous nor dangerous

Are Cow Parsnip Plants Edible?

Some foragers do eat the leaves and shoots of this plant. Extreme care however must be taken in the handling of them and in properly identifying them; poison water hemlock, an extremely deadly poisonous plant is often mistaken for cow parsnip.

Symptoms of Cow Parsnip Reaction

All parts of the cow parsnip plant contain a phototoxic sap. When an individual gets the sap on their skin it causes a reaction called phytophotodermatitis. This is not an allergic reaction as with poison ivy rather it is a chemical reaction that occurs when the organic chemicals called furanocoumarins, contained in the sap, become activated by ultraviolet radiation. The activation of this chemical damages your DNA and changes the way your skin protects itself from ultraviolet (UV) light (Goldman R. Updated on Dec. 2018). This results in severe burns when exposed to the sun. Just how severe these burns are depends on how long the sap remained on the skin and how long the skin was in the sun after exposure. The burns are less severe than those of Giant Hogweed but they still cause nasty burns that take weeks or months to heal and can leave scars.

Symptoms may include:

  • intense local burning sensation
  • red rash
  • blistering
  • irregular patches on the skin
  • Discoloration and sensitivity to sunlight in the affected areas can remain for up to 2 years.

What To Do If You Come Into Contact With Cow Parsnip?

  1. Wash the affected area thoroughly with soap and COLD water as soon as possible.
  2. Keep exposed area away from sunlight for 48 hours.
  3. If a reaction is severe, seek medical attention.
  4. Topical steroids applied early can reduce the severity of the reaction and ease discomfort.
  5. If sap gets in your eyes, rinse them with cold water, wear sunglasses and seek medical help.
  6. If a reaction has occurred, the area of skin may be sensitive to sunlight for a few years. Keeping the area covered or wearing sunblock should help.
  7. Immediately launder any clothing that has come into contact with cow parsnip.
  8. Wash down the surfaces of anything that has come in contact with the plant using gloves and protective clothing.
  9. If the plant was in a public area report the sighting to your local region.

How To Get Rid of Cow Parsnip Plants?

If you have cow parsnip plants on your private property and decide to remove them yourself wear the proper protective clothing close-toed shoes or boots, long pants, long sleeved shirt, water proof gloves and ideally face and eye protection. Do not touch your face or any bare skin with your gloved hand. Also try not to touch the surface of anything other than the tools you are working with. When you have finished removing the cow parsnip wash your tools and any surface you may have touched. Wash your shoes and shoelaces off and change and wash your clothing

Physical Control:

Ideally dig up the young plants in spring before they get to large and while the digging is easier. Place all plant debris in sealed heavy black garbage bags and label them poisonous plant material. If you are unable to remove the entire root, chop or cut the root below the ground level. If they have seed heads on them, place a bag over the seed heads and tie it at the bottom, before you begin. Check on the site later in the season for any new sprouts coming from the root stock and chop them again. Continue to monitor in subsequent seasons for any new seedlings and remove any you find.

Chemical Control

Glyphosate, a systemic herbicide, is most effective when applied in the spring to the smaller plants, followed up by a summer application for missed plants or those that may have re-sprouted. Mulching the area 10-14 days after application may help to manage seedling germination. Continue to monitor in subsequent seasons for any new seedlings.

Summary

Cow parsnip is a native plant to North America and as such there are checks and balances in place in nature (such as insects and disease), to keep their populations under control. However they are quite common and care should be taken when hiking or working in areas where they grow. All parts of the plant are phototoxic and can cause severe burns to skin that comes in contact with them and then is exposed to sunlight. Even though the leaves and shoots are edible, unless you are a very experienced forager with excellent identification skills I would strongly recommend against this.

Poisonous to Touch Weeds Covered in This Series Include:

  1. Poison Ivy (Toxicodendron radicans and Toxicodendron Rydbergii)
  2. Giant Hogweed (Heracleum mantegazzianum)
  3. Cow Parsnip (Heracleum maximum)
  4. Wild Parsnip (Pastinaca sativa)
  5. Poison Sumac (Toxicodendron vernix)
  6. Poison Oak (Toxicodendron diversilobum) and (Toxicodendron pubescens)

Other poisonous to Touch Plants Not Covered in This Series Include:

  1. Poke Weed (Phytolacca decandra): besides being mildly to highly toxic this plant can cause a rash in sensitive individuals.
  2. Stinging Nettle (Urtica dioica): the leaves and young stems of this herbaceous plant are fitted with stinging hairs tipped with formic acid and other irritants. If touched, these needle-like hairs inject the stinging acid into the skin, triggering a burning, tingling sensation and an itchy rash. Thankfully the symptoms usually do not persist longer than 24 hours.
  3. Monkshood (Aconitum napellus): This is a highly toxic plant. The neurotoxins, aconitine and mesaconitine can be absorbed through the skin and cause severe respiratory and cardiac problems. So do not pick or handle this plant without gloves, especially by the root.
  4. Oleander (Nerium oleander): This is a highly toxic plant. The gummy sap can cause skin irritation or possibly dermatitis in those who are sensitive. If you are growing this plant as an ornamental in your garden be sure to were gloves, long sleeves, long pants, closed shoes and protective eye wear before pruning this plant.
  5. Bloodroot (Sanguinaria canadensis): the flower of this plant produces a reddish toxic sap that looks very similar to blood and contains the toxin sanguinarine. If this sap comes in contact with bare skin it can cause burning. If you get a high concentration of sap on your skin it can even eat away the flesh. If ingested, it is deadly to animals.
  6. Wood Nettle (Laportea canadensis): like stinging nettle have stinging hairs along its stem.

References

Department of Environmental Conservation n.d., Cow Parsnip, Retrieved on Mar. 2, 2021 from: Cow Parsnip – NYS Dept. of Environmental Conservation

Esser, Lora L. 1995. Heracleum maximum. In: Fire Effects Information System, (online). U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Retrieved on Mar. 2, 2021 from: Heracleum maximum (fs.fed.us)

Goldman R. Updated on Dec. 2018, Medically reviewed by Shilpa Amin, M.D., CAQ, FAAFP , Health Line, Retrieved on Feb. 24, 2021 from: Hogweed Burns: How to Avoid Giant Hogweed and Treat Burns (healthline.com)

Illinois Wildflowers n.d., Cow Parsnip, Retrieved on Mar. 2, 2021 from: Cow Parsnip (Heracleum maximum) (illinoiswildflowers.info)

Knoke D. Giblin D. n.d., Heracleum maximum American cow-parsnip, American hogweed, Retrieved on Mar. 2, 2021 from: Burke Herbarium Image Collection (washington.edu)

MacDonald F. and Anderson H. 2012, Giant Hogweed (Heracleum mantegazzianum): Best Management Practices in Ontario. Ontario Invasive Plant Council, Peterborough, ON., Retrieved on Feb 24, 2021 from: Giant-Hogweed-OFAH.pdf (invasivespeciescentre.ca)

The Friends of the Wild Flower Garden, Inc. n.d., Retrieved on Mar. 2, 2021 from: Cow Parsnip, Heracleum maximum Bartram (friendsofthewildflowergarden.org)

The Ministry of Natural Resources and Forestry 2018 (rev. oct.2019), Wild Parsnip, Retrieved on Mar. 1, 2021 from: Wild parsnip | Ontario.ca

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