Giant Hogweed - the not-so-friendly giant

Giant Hogweed (Heracleum mantegazzianumis) is a short-lived perennial with tuberous stalks, lasting typically between 5-7 years.
It flowers in its final year from late-spring to mid-summer, with numerous white flowers clustered in umbrella-shaped heads that can be up to 80 cm in diameter. Each plant produces up to 100,000 flattened, 1 cm long, oval dry seeds.
Gallery image

How can I identify Giant Hogweed?

Giant Hogweed typically grows to heights of 2-5 metres and has a stout, dark reddish-purple stem and spotted leaf stalks that are hollow with sturdy bristles. Stems vary from 3-10 cm in diameter.

The stem shows a purplish-red pigmentation with raised nodules. Each purple spot on the stem surrounds a hair, and there are large, coarse white hairs at the base of the leaf stalk. The plant has deeply incised compound leaves that grow up to 1-1.7 metres in width.

Why is Giant Hogweed a problem?

The sap of the Giant Hogweed is phototoxic; this means that after contact with the skin it can cause severe inflammation when the skin is subsequently exposed to sunlight. Initially, the skin colours red and itches; then, within 48 hours, blisters form as it burns, and treatment may involve hospitalisation.

If the skin is exposed to sap, the affected area should be washed thoroughly with soap and water and the exposed skin protected from the sun for several days. Even after effective treatment, the blisters can form black or purplish scars that can last several years, and the presence of even minute amounts of sap in the eyes can lead to temporary, or even permanent, blindness.

For this reason children must be kept away from Giant Hogweed, and protective clothing, including eye protection, should be worn when handling or digging it.

How can I get rid of it?

Giant hogweed is best controlled by Chemical treatment. This can be applied in a number of ways, and it very much depends on the site as to the method chosen. Normally, due to its proximity to water courses, Knotweed NI uses a glyphosate-based chemical that is licensed specifically for use on or near water courses.

Our staff are trained and qualified to carry out an extensive range of chemical control options to ensure we deliver the correct concentration of chemical directly into the target plant, but with no effect on surrounding trees and shrubs. To date we have treated a vast number of sites – ranging from a single plant in a private garden, to entire river systems choked with invasive weeds. If you want to discuss your options, please call Mark or Ken on the numbers in the sidebar.
Gallery image