What is the Species?
- Heracleum mantegazzianum (giant hogweed)
- ODA “A” rank
Giant hogweed is a striking, highly visible species capable of creating significant ecological damage. Native to south-central Russia, hogweed has been known as an invasive throughout Britain and Ireland for decades, and was the target of a sizable eradication effort in Seattle in the late 1990s.
Why are we concerned?
Hogweed is known to form dense patches of extremely tall plants, particularly along streams. Each plant can produce thousands of seeds, creating large seed reserves in the soil. Its sap is also known for making skin highly photo-sensitive. Hogweed infestations can lead to:
- Reduced plant diversity as other species are displaced
- Increased risk to human health.
How does it spread?
Hogweed is spread only by seed, not fragments. Each plant can create tens of thousands of large, slightly inflated seeds. Hogweed seeds are moved around by:
- Running water
- Possibly rodents or birds
- Trading among gardeners
What does it look like?
Giant hogweed is very tall: typically 10-12 feet tall, or more. Leaves can be up to 4 feet wide, with heavily-toothed edges and deep lobes. Plants tend to have a very large central seedhead (18-24 inches across), with several smaller seedheads “in orbit” around it. The hollow stem is covered with dark purple blotches, especially near the base, each with its own tiny hair.
Are there any lookalikes?
Yes. Cow parsnip (Heracleum lanatum) is a native cousin with a sap that can similar effects, though usually much less potent than hogweed. Cow parsnip tends to be shorter, with seedheads all about the same size, and relatively flatter leaves, but these differences can be very subtle.
Poison hemlock (Conium maculatum) is a weedy non-native that is frequently confused for giant hogweed. Poison hemlock has small seedheads and carrot-like leaves, resembling a very large Queen Anne’s lace. While not being managed by ED/RR, it is a species around which caution should be used.
How do we deal with it?
- Manual: Digging up hogweed is effective, but the risk of injury from exposure to sap is substantial. Flowerheads are also clipped off mature plants, bagged and put in the garbage. Landowners who attempt to remove hogweed themselves should dress appropriately, though relying on professionals to do the removal is recommended. Landowners who remove hogweed personally are also still REQUIRED to notify the Oregon Department of Agriculture of its presence, for tracking and surveying purposes.
- Herbicide: On patches of more than a few plants, herbicides are used in combination with the manual methods described above.
How can folks help?
If you suspect you’ve found hogweed, call either the City of Portland or contact the Oregon Department of Agriculture. Both Portland and ODA have programs in place to manage this species for you and at no cost. ODA also REQUIRES landowners to report the presence or suspicion of target species like giant hogweed.
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