Giant Hogweed Plant Profile

Toxicity and Special Considerations

Thicket of mature giant hogweed

ElcovaLana / Getty Images

Giant hogweed is a public health hazard due to the toxicity of the plant. What makes giant hogweed so vicious is its sap. When it gets onto the skin and is exposed to sunlight, it creates painful burning blisters and long-lasting scars. And if the sap gets into the eyes, it can lead to temporary or permanent blindness. Thankfully, giant hogweed in its mature stage is relatively easy to identify due to its height and large leaves.

Toxicity of Giant Hogweed

The dangerous sap, which is clear in color and watery, is present in any part of the plant: the hollow hairy stem, the petioles, leaves, flowers, fruit or seeds. The sap contains furanocoumarins, toxic chemical compounds that heighten the skin’s sensitivity to UV light, with the result of inflammatory skin reactions.

If you come across giant hogweed, stay at a safe distance and avoid any physical contact without protective gear. In many areas, local government agencies such as the County Noxious Weed Control Board, urge you to report it so its occurrence can be mapped.

Giant hogweed patch
kbwills / Getty Images

Symptoms of Hogweed Burns in Humans

The sap of the giant hogweed can cause a range of symptoms varying in severity. The symptoms usually develop within fifteen minutes after contact, and peak between 15 minutes and two hours after contact:

  • In less severe cases, mild redness and skin irritation.
  • Painful blisters and burns that form within 48 hours. They later become dark and pigmented.
  • The resulting scars remain for months, sometimes years.
  • The affected skin becomes permanently sensitive to skin.
  • If sap gets in contact with eyes, it can lead to irritation and severe eye injury, including temporary or permanent blindness.

Giant Hogweed Also Toxic for Pets

Giant hogweed is also hazardous for pets, with similar symptoms as in humans. With dogs’ noses being particularly sensitive, just sniffing a giant hogweed can hurt your dog. If you spot a giant hogweed, keep your pets away from it too.

Warning

Avoid any physical contact. Not everyone is equally susceptible to giant hogweed, but you don’t want to find out the hard way.

Gardening Considerations

Under the Plant Protection Act, giant hogweed is classified as a federal noxious weed that is prohibited from entering the United States, and being transported across state borders.

A single giant hogweed produces up to 20,000 seeds. When not controlled, it spreads quickly, choking out native and desirable plants.

After the plant flowers and sets seed, the plant and the root die but that’s not the end of it. If the root has developed side shoots, new plants may emerge from it.

Botanical Name Heracleum mantegazzianum
Common Name  Giant hogweed, giant cow parsnip, cartwheel flower
Plant Type Biennial or perennial
Mature size 15 to 20 feet tall
Bloom Time Summer
Flower Color White

Identification

True to its name, giant hogweed is a giant in every respect. The mature plant is easy to identify. It grows up to 15 to 20 feet tall, its compound leaves can reach a width of up to five feet, and the flower clusters can have a diameter of up to two and a half feet. The stems and leaf stalks are sturdy, dark-reddish spotted, and hollow, between two and four inches in diameter. The wide, flat-topped flower clusters consist of many small white flowers. Flowers appear in June or July, and seeds set it August.

Giant hogweed stem with characteristic hairs and blotches
Giant hogweed stem with characteristic hairs and red spots. Fritz Geller-Grimm / Wikimedia Commons / Creative Commons Attribution-Share Alike 3.0 Unported license
Giant hogweed (Heracleum mantegazzian)um in early April
Giant hogweed in early April. Spantax / Wikimedia Commons / GNU Free Documentation License, Version 1.2 or later
Field of dried dead giant hogweed with new plants emerging
Dzurag / Getty Images 

Ideally you should identify a giant hogweed long before its towering and mature. The growing cycle can give you cues for its identification. As a biennial or perennial, the vegetative parts, basal-leaf rosettes, die back in the winter. The plant starts regrowing in the early spring. It can take a giant hogweed three to four years until flowering.

Where It's Found

Giant hogweed grows in different habitats. It is frequently found along roadsides, unattended lots, and along stream banks and rivers.

How to Remove Hogweed

When removing a giant hogweed, always wear protective clothing and cloves. Make sure that there is no gap between your sleeves and gloves that could exposes bare skin when you move your arms.

Small plants can be manually pulled. Larger plants need to be dug up. If the plant is already large, make sure to remove it before it produces any seed. Digging up a mature giant hogweed is work, because it has a taproot up to two feet long, and you need to make sure to remove the entire root system.

Giant hogweed in bloom
Giant hogweed in bloom. lucentinus / Getty Images

If the plant is mature, you must also protect your face, so it does not get exposed to any of the plant parts when you take it down.

For plants that are too large to discard in the household trash, at a minimum dispose of the flower heads in order to prevent the seeds from spreading. Move the remainder of the plant to the compost, or to a location out of the reach of humans and pets where it can decompose.

Mowing giant hogweed is not an option, because above-ground plant parts that are cut before the plant flowers and sets seeds may regrow.

Giant Hogweed Lookalikes

There are several plants resembling giant hogweed:

A unique characteristic of giant hogweed is the stem. None of the above lookalikes has a stem with purple splotches and white hairs. Angelica has a smooth purple stem, and poison hemlock has a smooth stem with purple blotches, but neither has hairs.