dichotomous key

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).

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