Giant hogweed identification and control

About this weed

Giant hogweed is a regulated Class A noxious weed. This means eradication is required state-wide. Giant hogweed is also on the Washington quarantine list.

Giant hogweed is known as Heracleum mantegazzianum, it is in the carrot family. Another common name used for this plant is giant cow parsley, not to be confused with cow parsnip which is a completely different plant that happens to look similar.

Why it's a problem

Giant hogweed is a toxic perennial (plant that has a life cycle of 2 or more years) that can grow more than 15 feet tall. It can easily be confused with the native plant cow parsnip, Heracleum maximum.

In King County, it grows mostly in urban areas, such as yards, green open spaces and empty lots. It is a public safety hazard. Hogweed stems produce a lot of clear watery sap. Getting the sap on your skin, followed by exposure to sunlight, can cause severe burns with blistering and painful dermatitis. Hogweed scars can last for as long as 6 years and skin sensitivity to sunlight can continue beyond that. 

Plant description

Giant hogweed is originally from the Caucasus Mountain region in southwest Asia. It was introduced to the United States as a garden plant in the early twentieth century.

Giant hogweed is a huge plant, capable of growing up to 15 to 20 feet tall. It takes the plant 3 to 5 years to grow that big. 

It can grow in full shade to full sun but does best in partial shade in moist to wet soils. 

The compound leaves of giant hogweed may grow as large as 5 feet wide. Leaves appear to be jagged. Each leaflet is deeply cut or lobed, and the leaf edges are sharply toothed. Leaves are mostly hairless and may appear somewhat shiny, except for hairs on the ribs on the underside of the leaf.

Its hollow stem can be 1 to 6 inches thick, covered in stiff white hairs, and ridged with dark reddish-purple raised blotches.

Flowerheads appear between May and July, when the plant is in the last year of life. Flowers are giant, up to 2 and a half feet wide umbrella-shaped flower clusters densely packed with tiny white flowers.

Roots on younger plants are white inside and shaped like a skinny carrot. Flowering plants have a thick central root ball that can extend 6 or more inches into the ground. 

Seeds are in dry fruits that are 3/8 inch long, oval, with brown resin canals. The resin canals look like long brown teardrops against the lighter colored seed surface.

Plants appear to die back in late summer, but their roots keep living underground. Giant hogweed will grow again in late winter or early springtime from existing roots. At the same time new young plants will sprout from previous year’s seeds.

Be aware of look-alike plants

Giant hogweed is often confused with poison hemlock, which only grows up to 10 feet tall and has smooth, hairless stems and fern like leaves. It can also be mistaken for our native plant, western coltsfoot.

Another native plant, cow parsnip (Heracleum maximum) is in the same plant family as giant hogweed. While the 2 plants look similar, there are a few key differences you can look for.

Giant hogweed is almost exclusively found in western Washington, while cow parsnip can be found growing throughout the state. Cow parsnip is shorter, typically growing up to 6 feet tall. Cow parsnip has soft hairs on the surface of the leaf undersides. Cow parsnip leaves are not as deeply lobed and sharply cut/toothed (incised) on the edges and typically only get up to 1 foot wide. Cow parsnip’s umbel flower clusters are smaller, up to 1 foot across. Cow parsnip seeds are more heart shaped compared to giant hogweed seeds, which are oval or elliptical.

Caution should still be used around cow parsnip as it too can cause skin irritation and blistering if its watery sap gets on bare skin, but it is not as severe as giant hogweed. 

Learn more about giant hogweed's look-alike plants:

Cowparsnip

Giant hogweed and cow parsnip: which is which

Giant hogweed look-alikes

Queen Anne’s lace/Wild carrot

When in doubt, take photos and share them with us or report them on iNaturalist.

What to do if you find it

Please notify us if you see giant hogweed growing in King County. Property owners are required to control this plant, but please do your research first to ensure you’re not endangering yourself or others. Make sure to wear protective gear.

Our program staff can provide the property owner or appropriate public agency with site-specific advice on how best to remove it. We map all known locations of regulated noxious weeds to help locate new infestations in time to control them.

Control methods

We recommend using a combination of methods to control noxious weeds. In areas with few weeds, it is important to act quickly before they become harder to control. Make a long-term plan as it often takes several years to get rid of most weeds. Start in the least infested areas first and then move into more heavily infested areas. 

For more in-depth control information, read Best Management Practices (BMP) for Giant Hogweed (490KB)

Use caution. Wear proper clothing: gloves, long sleeves, long pants, closed toed shoes, and eye protection when attempting any control measures. Avoid skin contact. Watery sap in leaves and stems can cause burns, blisters, and scarring.

Manual control

Small patches of mature giant hogweed plants can be dug up. For bigger plants, it is best to cut the main stalk and leaves off, taking care not to get any sap on your skin, and pull them aside so you can get to the main root. Try to dig at least 6 inches down to get as much of the root as possible. Younger plants and seedlings can also be dug up or pulled if the soil is soft enough. If they break off, or you cannot get the whole root, chemical control might be a better choice for that area.

Mechanical control

Mowing can suppress young hogweed that is growing in a lawn, but only if repeated 4 to 8 times during a growing season. Mowing is less effective on mature plants. Take care not to get any sap on your skin if mowing. Do not weed whack. It will not kill the plants and will spread sap all over. 

Biological control

Cattle and pigs can eat hogweed without apparent harm. Deer and horses have also been observed eating hogweed (being trampled by hooves also damages the plant). There are no biocontrol insects available for hogweed at this time.

Chemical control

Stay safe when using herbicide: 

• Always read the label before use. 
• Wear a long-sleeved shirt, long pants, shoes, and eye protection. 
• Follow state and local regulations.

Chemical control is most effective if done before hogweed begins to flower, in early to late spring.  Apply herbicide to the entire leaf and stem surface of actively growing plants. Wait at least 14 days before cutting any hogweed that has been treated to give the herbicide time to work. 

Glyphosate (non-selective) is effective for hogweed that is not growing with grasses. Triclopyr (broadleaf selective) is effective in grassy areas. 

Disposal instructions

All plant parts should be put in garbage bags and disposed of as trash or taken to the transfer station. Do not put in compost or yard waste bins.

Noxious Weed Disposal - Washington State Noxious Weed Control Board

Are your weeds piling up? - Noxious Weeds Blog