Giant hogweed (Heracleum mantegazzianum) is a native of the Caucasus mountains and was introduced to Britain in 1893 as an ornamental plant. It escaped from gardens and now colonises many areas of wasteland and riverbanks throughout the UK. By the 70’s and 80’s the plant had colonised the majority of the Deveron catchment, which saw the local councils adopt an eradication scheme to control the species. By and large a great job was done and many heavy infestations were cleared allowing native wildlife to flourish once again. As the plants slowly disappeared so did the funding and the eradication scheme ceased.
The difficulty with Giant hogweed in the district is that each flower head produces several thousand seeds that are easily dispersed by water, so the plant spreads rapidly along watercourses. Project research has found that there is a huge seed bank held in suspension in the river Deveron and its tributaries. These seeds can be preserved in clay sediments for many years.
After germinating the plant is a perennial, taking up to four years to mature and flower, after which it dies after going to seed. People tend to misjudge our native cow parsley as the plant when walking close by watercourses. There is room for this error as the cow parsley is the Scottish cousin to this foreign invader. Although there can be no mistake when scale is involved, as the giant hogweed has features that are over 300% larger than its native relative. The biosecurity threat posed by the species is that due to its size it suppresses the growth of native plants and grasses, leaving the river banks bare of vegetation in winter, increasing the risk of erosion and consequently silting up of valuable spawning grounds for wild salmon and trout.
Identifying Giant hogweed
Giant hogweed is closely related to common Cow Parsley – the two are similar in shape and colour. The stem is green with dark red spots, is ribbed and had sparse, spiky hairs. Its stems may be as thick as 10cm in mature plants. The leaves can be up to a metre across and over two metres long. The plants flowers are white, up to half a metre across and comprise clusters of smaller flowers forming an umbrella-shaped structure. Giant Hogweed can grow in excess of 4 metres high and is naturalised by rivers.
How to control
It is important that the plant is kept under control, even eradicated where possible. Using mechanical methods, the plant can be cut, mown or pulled. Physical control should never be undertaken unless protective clothing is worn.
Cutting before flowering will only achieve temporary control – the plant will regrow the following season.
Cutting after flowering has no benefit once the seeds have been formed. Small infestations can be controlled by digging out the whole plant. The relevant competent authority (SEPA, SNH) must be satisfied that the management plan is appropriate.
The plant can also be controlled chemically. This method should only be considered after other practical methods have been ruled out. Properly used, the herbicide glyphosate (Roundup) is most effective. Prior agreement with SEPA must be obtained before a herbicide can be applied in or near water.
The plants may be sprayed with Glyphosate at a rate of 5 litres per hectare when growing actively but less than a metre tall, usually between April and May.
Control measures will only affect mature plant and so due to the longevity of the seeds, regular annual checks should be made for germinating plants.
Beware of Giant hogweed…!
If the threat posed by the species to local biodiversity is not bad enough, there is also a huge health risk to think about. Giant hogweed contains furocoumarins chemicals which can cause phytophotodermatitis (plant-light-skin inflammation), a severe reaction from exposure to the sun after coming into contact with the sap.
This causes burns and blisters to appear two to three days after exposure. These blisters may last months with subsequent reaction occurring for up to 25 years, every time the affected area is exposed to sunlight where it may re-blister years after healing. Contact with the eyes can cause temporary and possibly permanent blindness.
Generally children are attracted by its size and hollow stems which they pick for telescopes or pea shooters. Children playing locally throughout the district are almost certain to come into contact with the plant at some stage as they pass it by. If you come into contact with Giant hogweed please seek medical advice as soon as possible.