Giant hogweed (Heracleum mantegazzianum) is a member of the Parsley family (which includes Wild Carrot, Fool’s Watercress, Cow Parsley, Hogweed, Wild Angelica, Alexanders, Sweet Cicely and Hemlock) and is native to Asia. It has hollow green/purple stems with fine spines, that make them appear furry, and dark green coarsely toothed leaves. Small white flowers are arranged in flat-top umbrella-like clusters. Fattened, oval fruits with a broadly rounded base are produced by the plant. Giant hogweed can grow up to 3-5 metres tall, with individual umbels measuring up to half a metre across and leaves up to 2 metres across. The plants’ dense canopy can out-compete native species and cause stream bank erosion in riparian areas when the large shading plant dies back late in the year leaving riverbanks exposed to the elements.
Giant Hogweed reproduces through seeds produced in late summer and/or perennial buds that form on the root stalks of the crown. It can produce up to 50,000 seeds per plant (approximately 1,500 per flower head), which can be catapulted distances of up to 4 metres. Seeds can remain inactive in the ground for several years. The plant can be both a biennial or a perennial, normally flowering around May in the second, third or fourth year after germination.
Giant hogweed growing across or along the route of a public right of way is likely to attract the involvement of the Environment Agency due to the threat it presents to public health. Hairs on the outside of the stems and poisonous sap on the inside of the stems and leaves can cause severe irritation. The sap reacts with the skin and makes the skin sensitive to ultra-violet light, though no pain or irritation is felt at the time of contact. Any subsequent exposure to sunlight can cause the skin to burn and will result in large, watery blisters that do not become evident until 15 to 20 hours following contact, by which time the damage has been done. Blisters may develop into purplish or blackened scars and could persist as recurrent photo-dermatitis long after exposure. Contact with the eyes can lead to temporary (or, in some cases, permanent) blindness. Children can be most at risk, as Giant hogweed is highly attractive to them. The size of the plant forms part of the attraction, as does the long, straight, hollow stems that can often be used as ‘telescopes’, ‘blow pipes’ or ‘swords’. The consequences of these games do not bear thinking about – especially when the toxic sap has come into contact with the child’s face, eyes or mouth.
Giant hogweed is also a host for both carrot fly and the disease Sclerotinia sclerotiorum, both of which attack many horticultural and arable crops.
Individual Giant hogweed plants can be killed off with one or two applications of herbicide. However, due to the longevity and quantity of the seeds, full eradication can take a number of years to achieve. To be effective, all control measures should be undertaken each year before the plant produces seeds (early spring to early summer). Eradication of Giant hogweed through herbicide treatment alone can take a minimum of three years – and could take up to ten, due to the proliferation of seeds that form a dormant ‘seed bank’ over successive years. Any chemical application to plants found growing near watercourses will require the granting of written consent from the Environment Agency prior to treatment.
Great care should be taken whilst conducting any Giant hogweed works to ensure the plant does not come into contact with the skin or eyes. A face-shield, gloves and suitable protective clothing should be worn during any Giant hogweed operations, and it is advisable to have at least two operatives conducting works in case one of them slips and falls into the plant’s foliage. If at all possible, avoid working in direct sunlight when dealing with Giant hogweed.
The plant may be hand dug, but care must be taken to remove as much root material as possible, as a plant can grow from a section of root carrying auxiliary buds. Alternatively, the hogweed can be mechanically excavated, as long as due allowance is made for the extensive seed bank. Following excavation new seedling growth should be controlled by use of herbicide. Cutting of the plant is not an efficient long-term solution and is not recommended on any plant exceeding 1.5 metres in height due to the risk of direct contact.
Intensive grazing of Giant hogweed by cattle, sheep or goats has the potential to be a successful method of eradication if routinely carried out over a course of several years throughout each growing season. However there is a risk that the mouths and tongues of the grazing animals may be susceptible to painful blisters.