Giant Hogweed

Most (un)wanted plant of the month: 
Giant Hogweed
Heracleum mantegazzianum 

A member of the parsley and carrot family, giant hogweed can be distinguished from most other plants in that family by its massive size. It is a perennial herbaceous plant with enormous, dark green leaves and hollow stems with purple blotches that can reach a height of 2.5 to 5 metres when in flower. The flowers are borne in large umbrella-shaped clusters which can be up to 0.8m across. The seeds are winged and oval and approximately 1cm long. Each giant hogweed plant can produce up to 50,000 seeds. Giant hogweed produces toxic compounds which, when human skin comes into contact with it, can cause \”phyto-photodermatitis\”. This makes the skin extremely sensitive to sunlight resulting in painful blistering and scarring of the skin, akin to third degree burns.

Giant hogweed grows best in rich, moist soils in both open and partially-shaded areas. It will take from 3 to 8 years before it produces seed, depending on how good the growing conditions are.

What makes it invasive?
Giant hogweed produces copious seeds, grows from a dense taproot which keeps producing leaves and is also very dangerous to human health, making it difficult to remove. In the UK, giant hogweed is considered toxic waste.

How to control it:
Control must be conducted very carefully so as not to touch the leaves and get the sap on your skin. Waterproof clothing including gloves, boots and safety goggles should be used. The best way to rid an area of a hog weed infestation is to excavate the plants with the soil, and to bury the waste under at least 2 metres of soil. As this may not be an option for most individuals, another option is to repeatedly remove all vegetation, cut the root crown at an angle (below the soil surface) with a sharp spade, and mulch the area with deep soil or bark, planting with shrubs. Remember to use extreme caution when handling giant hogweed. If the plants have already gone to seed, it is possible to simply cut the seed heads and bag them for disposal, then cut the remaining vegetation and leave on-site to decompose. It will be necessary to return to the site repeatedly over the next couple of years to maintain a hogweed-free site.