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Giant Hogweed vs. Cow Parsnip vs. Water Hemlock vs. Wild Carrot

The Toronto media is ablaze with reports that Giant Hogweed (Heracleum mantegazzianum) has been found within the borders of the GTA. Normally, when an invasive species is found, it does not generate this kind of buzz. Why the hubbub? The sap of Giant Hogweed, on the skin, can lead to a severe skin inflammation called phytophotodermatitis. Many people, however, don’t know how to identify Giant Hogweed or know how tell it apart from Cow Parsnip (Heracleum maximum), a native species (and only somewhat phototoxic).

With the caveat that you should have an knowledgeable plant person confirm any identification, let me quickly outline how you figure out if that plant is Giant Hogweed or something else.

The confounding factor here is all four plants above have white flowers in an umbel (a botanical term to describe the arrangement and shape of the flowers—I’ve included an illustration of what an umbel structure looks like below).

Flower structure: Umbel

Giant Hogweed Dichotomous Key

So beginning with the assumption that you have a plant with an umbel of white flowers in the GTA, you can try your hand at identification with this key. Please be aware that this key will only work with a mature plant, and Giant Hogweed can take two years to reach the flowering stage. The key is dichotomous, meaning that you will be given two choices. Answer each question with one of the provided choices, go to number and you will come up with an identification.

1: Look at the flower.
If it is 15 cm in diameter (measured from flower edge to flower edge) or larger, go to 3
If it is 14 cm in diameter (measured from flower edge to flower edge) or smaller, go to 2

2: Good news! A small flower means that this is probably not Giant Hogweed. When mature, Giant Hogweed will have flowers up to 1 meter in diameter. It could be Queen Anne’s Lace or Water Hemlock. Note that Water Hemlock is extremely toxic if consumed.

3: Look at the stem of the plant. Is the stem reddish or purple, with spots and stiff bristles?
If yes, go to 4
If no, go to 5

4: This plant is probably Giant Hogweed. To confirm, have a look at the flower. Giant Hogweed has flowers with over 50 rays (a part of the flower structure which I’ve illustrated above). Cow parsnip will have between 15-30 rays.

5: Good news! This plant is probably Cow parsnip. It will have fine hairs, but no stiff bristles. The stem may be purplish, but is mostly green with no blotching or spots. Keep in mind that Cow Parsnip is phototoxic, too.

Keys to a Giant Hogweed ID

Just in case you hate the idea of a dichotomous key, or have questions about a non-flowering plant, here are the key differentiators between Giant Hogweed and other similar-looking plants:

  • large plant, often over 2.5 m high (when mature)
  • flowers in umbel shape, larger than 15cm in diameter
  • flowers have more than 50 rays
  • hollow stems are between 3-8 cm in diameter
  • stems have stiff white bristles
  • stems have spots or blotching, red or purple in colour
  • large, lobed leaves up to 1m in size
  • leaves compounded in three

20 replies on “Giant Hogweed vs. Cow Parsnip vs. Water Hemlock vs. Wild Carrot”

Question: I have been trying to figure out if there is a difference between Parsnips, Wild Parsnip, Cow Parsnip and Water Parsnip, and what they are. Your description above has been the most helpful I have found. Can you tell me, for instance, is the Wild Parsnip mentioned in Wiki to be poisonous just escaped cultivated parsnips or something totally different? Are ‘wild parsnip’ and ‘cow parsnip’ one and the same or different? I have heard of Wild Carrot, and Water Parsnip, Water Hemlock, and Cow Parsnip, but see Wild Parsnip mentioned in Parsnip Wiki as poisonous and invasive, but not mentioning if it is the same parsnip of the cultivated root vegatable.

Please keep in mind that I’m not an expert, and when you’re dealing with poisonous plants (e.g. Water Hemlock) you should be very cautious when differentiating plants from each other. That said, here’s how I understand the differences between the plants you mention:

-Parsnip, the root veggie we eat is also known as Pastinaca sativa. It’s a plant in the carrot family (Apiaceae).
-In North America, Wild parsnip is Pastinaca sativa growing wild (or as you say, escaped from cultivation).
-Cow Parsnip is also known as Heracleum maximum. It’s related to parsnip at the family level.
-Water Hemlock (Cicuta virosa) is another plant in the Apiaceae (carrot) family.

So they’re all kinds of carrots. That family, however, has over 3000 species, so it’s more important to compare the genus names (the first of the two names in the latin binomial naming system). You can see that the Parsnip, Cow Parsnip & Water Hemlock are all different genera. Common names can confuse–there is a “Wild Parsnip” in Europe which is another plant species all together. So to figure out a difference, you need to know just what species of plant is being referred to as “Wild Parsnip” before you decide if they’re the same or different species to the cultivated parsnip we eat. Hope that helps.

I have Cow Parsnip and your description was very good. I live 130 km north of Toronto, Ontario, Canada. My flowers are 2-3 inches in diameter and plants are 4 feet high. Stems are green and very pliable, bend over easily. These are native and therefore I can leave them. Is this true, native to the Carolinian area?

I am not an expert, but have some botnical and horticulture training.Don’s plant sounds like Queen anne’s lace or wild carrot
(daucus carota) or a type of angelica that is sold as an ornamental.Cow parsnip (Heracleum lanatum) and giant hog weed (Heracleum mantagazzianum) both grow over 6 feet tall in the flowereing phase. Giant hog weed is an invasive species, cow parsnip is native and an important food crop for deer. Cattle will eat cow parsnip, hence the name. I have cow parsnip in my garden on purpose. I have had one grow as tall as nine feet in a wet area.They prefer flood plains and wet ground, but will grow anywhere. They are bi-annual, which means they flower the second summer and then the plant dies. If the seed head is cut off before seeds ripen, it will not spread. I have had some that took three to four years to flower, especially if I mow them over when they are small. They are difficult to tell apart to the untrained eye. Giant hog weed is larger. Cow parsnip flowers early to mid June, Hog weed flowers mid to late summer. The hairs on the stalk of cow parsnip are soft, and stem is mostly green with some red. Young ones are all green. I fear cow parsnip will be eliminated because of the fear surrounding giant hog weed. I have actually seen cow parsnip used in professional landscaping, it is a beautiful plant in my opinion.

I am undecided on what I have from the pictures and descriptions of Cow parsnip, and Angelica arguta I am very confused on what plant I actually have how can I tell the difference, each plant is used differently and I would really like to know? Can you help?

You failed to differentiate water hemlock Cicuta bulbifera/C. maculata from wild carrot Daucus carota. Water hemlock has a hollow stem and grows in wet places.

Thanks for your comment Lewis—my goal here was to differentiate Giant Hogweed from other similar plants, not all similar plants from each other.

Those of your commenting here would probably like the facebook groups Edible Wild Plants & Plant Identification.

What about the one purple center flowerlet on the blossom? This is Queen Anne’s Lace? Wild parsnip and hogweed do not have this?

Hi Linda, yes it is Queen Anne’s Lace that has the small number of (usually 1 to 5) red florets (or flowers). It’s important to note, however, that there are some populations of Queen Anne’s Lace that have completely white umbels, so while diagnostic, the lack of the red flowers in the umbel doesn’t mean it has to be hogweed or cow parsnip.

Hi there, I have a plant growing in my backyard… looks a lot like the type you are taking about, except the leaves are a tad different. I don’t know how to identify this plant properly. Can you help????

If you are unfortunate enough to be in contact with giant hogweed, be aware that it is the oil that causes the painful rash. Immediately cover the area with clothing or something that will not expose the area to the sun. It is the action of the sun on the oil on your skin that reacts. As soon as you are able wash the area with something like liquid dish soap or good old fashioned Sunlight bar soap.

Would giant hogweed ever be small enough to confuse with Queen Anne’s Lace in the Rocky Shield area such a the Frontenac Biosphere.

Terry, in my experience, no. If you’re comparing mature plants (in flower) at this time of year, their difference in size should be quite obvious.

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