• Giant hogweed (Heracleum mantegazzianum)

Giant hogweed (Heracleum mantegazzianum)

Giant hogweed originated in South-west Asia and dominates vegetation in marginal habitats. Under the Wildlife and Countryside Act 81 it is an offence to “plant or otherwise cause (GHW) to grow in the wild.” It grows up to 2m in height and each flower produces up to 1500 seeds in late summer. Most seeds fall within 4m of the plant but can be spread much further by wind, traffic turbulence, on water, by ploughing, etc. Seeds can remain viable for 7 – 15 years.

It is dangerous to humans because contact with the bristles or the sap can induce photo dermatitis, a burn-like symptom that recurs in sunlight and can last for years. Contact with the eyes can produce blindness, especially in children. It does not affect buildings but because of its health and safety implications it can reduce land values and, if it grows across pathways, can effectively close them.

Giant hogweed does not spread from rhizomes so each plant can be killed with herbicide or by being dug out before it sets seed.The most practical control method is treatment with a herbicide before the plant flowers in early August, and certainly before it sets seed. A second or third visit in the same season will deal with any plants that have germinated when the larger plants die back. An ongoing programme over several years will be necessary if the ground is left open, partly as killing one plant allows light in which enables further seeds to germinate, and seeds can remain dormant in the soil for a number of years.

An alternative solution is to excavate the giant hogweed and the soil around it, and remove this from site as controlled waste to a licensed landfill. How much soil is a matter of judgement and depends on the situation of the plant. Excavating the soil need only be to a depth of approximately 300mm, but the extent of ground to remove is unclear as the seeds from previous years could have been spread a long way.  Even if the soil is excavated, there is no guarantee that seeds have not been spread  by wind or water from other plants. Excavating and removing the plants and soil likely to contain seeds is possible at any time of the year.

Anyone working amongst giant hogweed plants needs to wear PPE that covers the whole body including gloves, hood and face visor.