Invasive Plant: Giant Hogweed

At a Glance

Do not touch or attempt to control this plant. Attempting to control giant hogweed by yourself is not recommended. Please follow the instructions below for how to report sightings of giant hogweed.

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  Scientific name: Heracleum mantegazzianum
• Annual, perennial, or biennial: Biennial
• Conditions it likes best: Likes unmaintained urban areas, vacant lots, roadsides, and riparian areas. Grows well in sun or shade.
• How it spreads: Through seed. Each plant can make up to 50,000 seeds that may still grow after at least five years.
• How it causes damage: Takes space, light, and nutrients from other plants.
• Oregon Department of Agriculture noxious weed rank: A
• Report patches of this plant? Yes. Oregon law requires that you report all sightings immediately for assistance. Use the online tool at Oregon Invasives Hotline or call 866-INVADER to let the state know when and where you found this plant. You may also contact  Environmental Services (contact information on this page) who will notify the state of Oregon for you and help you remove these plants.

How to Identify Giant Hogweed

Fall-Winter. Grows as a rosette (circle of leaves with no stem). Leaves can be 2 or 3 feet wide, with deep lobes.

Spring-Summer. Some plants will bolt (grow a tall stem) in May and then flower in June. Giant hogweed can be over 10 feet tall, but height is not the only identifying feature. Leaves can be very large (4 feet across) and mostly growing from the base of the plant. Stems have very small hairs and purple spots, but several other kinds of plant do, too.

How and When to Remove Giant Hogweed

Caution! Trying to remove giant hogweed yourself is not recommended. At all times of year, giant hogweed can hurt you. The sap makes your skin very sensitive to the sun, causing severe sunburns, blisters, and rashes that can last for months and leave scars. Getting sap in your eyes can make you blind.

• Requires professional management. Giant hogweed is a high priority plant and would be managed by professionals from the City or the State. This plant is on Oregon’s and Portland’s Required Eradication List, so it is important to report any plants found so they can be removed. If you think you have found hogweed, contact Environmental Services (contact information on this page), the State, or your county's weed program contact to confirm that it is hogweed and to talk about removing it.
• Check. Check the site for new plants at least once a year and especially for the first 2-3 years after removal.
• Replant. After a year or two of managing this plant, consider planting native or less aggressive non-native plants.

Prevention is Best Practice

Clean your boots, pets, and maybe even your tires when you finish a hike or trail ride in Pacific Northwest natural areas, or if you have invasive plants on your own property. Cleaning boots and pets keeps invasive plants from spreading to new places. Be careful when trading plants with neighbors and fellow gardeners to make sure you’re not trading this or any other plant of concern.

Be Aware of Look-alikes

Giant hogweed looks a lot like cow parsnip (Heracleum maximum), a plant native to
Oregon and Washington. Cow parsnip is usually only 5-6 feet tall and has smaller leaves with shallower lobes and less jagged edges. Cow parsnip does not have hairs or bumps along the base of stems, but has more obvious stem ridges, like celery.

Giant hogweed also looks a lot like poison hemlock (Conium maculatum), a non-native, invasive plant found along roadsides and trails. Poison hemlock can be 6-8 feet tall and has very small hairs with purple spots. But poison hemlock has lacy leaves, almost like a carrot. Giant hogweed, on the other hand, has large, single-part leaves.

Get help with identification from —

• Oregon or Washington Departments of Agriculture
• Local Soil & Water Conservation Districts
• Environmental Services Early Detection and Rapid Response, if you live within Portland city limits. Contact information is on this page.
• Use the online tool at Oregon Invasives Hotline or call 866-INVADER to get help with identification.

Properly Dispose of Invasive Plants

To prevent spreading invasive plants:

• Never place plants or seeds in your compost or yard debris.
• Never throw pulled plants on the ground or into the street or sidewalk. 
• Put all pulled plants, bulbs, or seeds into a plastic bag and put the bag in the trash.
• Wash all garden tools and gloves.

Resources

• Oregon Department of Agriculture's giant hogweed profile (PDF).
• Washington State Noxious Weed Control Board webpage on giant hogweed.