GIANT HOGWEED

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We recently visited a garden and spotted giant hogweed (Heracleum mantegazzianum).  It is an impressive plant, looking just like its smaller brother common hogweed but on steroids. You will be able to spot the difference as true to its name giant hogweed is MASSIVE, and truly looks like it belongs in a giant's garden.

Giant hogweed (Heracleum mantegazzianum) is a close relative of cow parsley.  It originates from Southern Russia and Georgia, and was brought over to Britain by Victorian plant collectors. Although an impressive sight when fully grown, giant hogweed is invasive and potentially harmful and therefore most gardeners will wish to eradicate it from their garden.

Chemicals in the sap can cause photodermatitis or photosensitivity, where the skin becomes very sensitive to sunlight and may suffer blistering, pigmentation and long-lasting scars. Blisters are photosensitive, triggered by sunlight, and may recur for up to 6 years after initial contact. 

The Wildlife and Countryside Act 1981 lists giant hogweed as an invasive weed (Schedule 9, Section 14), meaning it is an offence to cause giant hogweed to grow in the wild in England and Wales. Similar legislation applies in Scotland and Northern Ireland. 

Although there is no statutory obligation for landowners to eliminate giant hogweed, local authorities will often take action to remove infestations in public areas. It can be the subject of Anti-Social Behaviour Orders where occupiers of giant hogweed infested ground can be required to remove the weed or face penalties.

Identification

Giant hogweed is a tall, cow parsley-like plant with flat topped clusters of white flowers held in umbels, like though of carrots or cow parsley. It has thick, bristly stems that are often purple-blotched. It is the enormous size of the plant that distinguishes giant hogweed. Flower heads can be as large as 60 cm (2ft) across and it can reach a height of 3.5 m (11.5ft) or more. 

Giant hogweed can be either a biennial or perennial weed, forming a rosette of deeply lobed leaves in the first year and then sending up a flower spike in the second year in order to set seed. Giant hogweed seeds are tough and can germinate in the soil up to fifteen years after broadcast.

Control

;"> Care must be taken when handling giant hogweed as contact with the plant can result in serious injury. When working near giant hogweed always wear gloves, cover your arms and legs and wear googles or a face mask. 

The sap within the plant can remain active for 2 hours after cutting, so contaminated clothing and tools are potentially hazardous too. Wash any skin that comes in contact with the plant with soap and water immediately. 

Where there are many plants it is best to treat chemically, in order to minimise contact with the plant.  Applying a tough weedkiller containing glyphosate or triclopyr.

Ideally, spray the young foliage in May which is 1 metre high or less and re-treated in August or September as necessary. Mature plants are likely to need more than one treatment to kill them. 

Giant hogweed is a controlled waste, similar to Japanese knotweed, so if it is taken off site it can only be disposed of in licensed landfill sites with the required documentation. Avoid this by allowing plant material to die and then burning in a garden incinerator on site.

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Common British weeds
Giant Hogweed
Japanese Knotweed