Heracleum mantegazzianum Sommier & Levier, Giant Hogweed
Account Summary
Introduction, neophyte, an invasive, occasional garden escape.
1938; Praeger, R.Ll.; streamside at Maguiresbridge.
Throughout the year.
Fermanagh occurrence

This hugely tall biennial or perennial (ie in reality hapaxanthic – requiring 2-7 years before achieving flowering), monoecious, strong- or sharp-smelling herbaceous umbellifer species up to 5 m tall was recorded on the banks of the Tempo, Maguiresbridge and Ballinamallard rivers prior to 1951. While these sites remain the centres of its Fermanagh distribution, it is now considerably more widely scattered in lowland areas and has been recorded in a total of 26 tetrads (4.9%), principally in the eastern lowlands of the VC.
The primary colonisation sites of this invasive alien are typically in the deep, loamy or boggy soil of relatively undisturbed waterside habitats along the banks of rivers and streams, where it may then form large, dense stands (Tiley et al. 1996; Jonsell & Karlsson 2009). Fortunately, so far in Fermanagh, it has not succeeded in establishing anything more than a token presence around or close to Upper and Lower Lough Erne. It has begun to appear, however, in other disturbed, ruderal, or unmanaged, damp lowland sites further away from water bodies, which means it has already started the process of secondary colonisation in Co Fermanagh.
Growth form, introduction and preferred habitats
Giant Hogweed is a native of the NW Caucasus mountains between Russia and Turkey, where it is a plant of wet places at forest edges and in glades. In its native region, it often grows by stream-sides in montane areas which have quite a damp, humid climate and an annual rainfall of between 100-200 cm. H. mantegazzianum was introduced into B & I in the 1820s for horticultural purposes, being regarded as very dramatic and especially suitable for waterside plantation in larger gardens. Unusually, for an introduced species, it escaped into the wild almost immediately (by 1828 in Cambridgeshire) and it has been spreading in both B & I ever since (Tiley et al. 1996). In Ireland, the National Botanic Garden at Glasnevin was offering seed of H. mantegazzianum for exchange in 1889, which appears to be the first mention of the plant in print in the country (Wyse Jackson 1989).
Throughout Britain and Ireland, it colonised new lowland territory extremely rapidly in the last 30 years of the 20th century. Surveys carried out in the late 1980s and 1990s suggested that if left unchecked it would embark upon a secondary phase of invasion, colonising sites in the general countryside remote from water courses, mirroring the situation it has already accomplished in Sweden and the Czech Republic (Lundström 1984; Pyšek 1991; Tiley et al. 1996).
Secondary colonisation involves the invasive occurrence in low-lying rough grassland on roadsides, hedgerows, railway embankments, refuse tips, wasteground, cemeteries and disused farmland, in some of which habitats H. mantegazzianum has already appeared in Fermanagh.
Flowering reproduction
The reproductive potential of the plant is prolific and almost as enormous as the physical scale of the plant itself! It is monocarpic in B & I (ie it flowers once and dies). The plant typically takes up to four year's growth to reach flowering capacity. The flowers are self-compatible, requiring no pollinator(s), and a vigorous individual plant may then produce up to 50,000 seeds in a season, although estimates of this vary from 5,000 upwards, peaking at the higher figure (Tiley et al. 1996). The seed is dispersed by wind and water and can remain viable in the soil for up to 15 years.
Variation and hybrids
Little intra-specific variation has been detected in H. mantegazzianum and no subspecific taxa have been described although there appears to be some confusion as to the specific limits and a number of other apparently very similar taxa have been named (Page et al. 2006; Sell & Murrell 2009).
Hybridisation is unusual between species in the family Apiaceae, but occasional hybrids between H. mantegazzianum and H. sphondylium have been reported in Europe where both species grow in proximity. Fertility in hybrids is low (seed set less than or equal to 1%) and introgression has not been detected (Weimarck et al. 1979). The hybrid is multi-stemmed like H. sphondylium, but clearly larger. The hybrid is convincingly intermediate in all characters, although rather closer to H. sphondylium in plant height, stem diameter and ray number, and to H. mantegazzianum in fruit length (Weimarck et al. 1979; Stace et al. 2015). The paucity of fruits and presence of some malformed ones is an additional hybrid character.
British and Irish occurrence
H. mantegazzianum distribution is currently biased towards both the SE and the NW of both islands, but the species is spreading at an amazing rate – the change index calculated between 1962 and 2002 for the New Atlas being one of the highest of any species at +2.09. This suggests that many of the present gaps in the hectad map could soon be filled unless weed control can be achieved (Preston et al. 2002).
The species' preference for the cooler, moist oceanic climate of N and W Ireland and N Scotland may possibly be associated with the requirement for sufficient soil moisture at the seedling stage, but an ecological preference for the more continental climatic conditions of SE England is more difficult to account for, except perhaps in terms of the prevalent warmer environment fostering its rapid growth rate (Tiley et al. 1996).
On the other hand, there may be an economic, historical and social reason for the current greater frequency of Giant Hogweed in SE England, since this is undoubtedly the region where the plant was first introduced and became a popular plant with the wealthy owners of large gardens. The impression created by its grand scale will have appreciated quite some time before its overly aggressive and dominant nature became apparent. The danger posed by the allergic blistering sap is a further drawback which only came to light much later, but is now very well known and the general public has been frequently alerted to avoid the plant.
European and world occurrence
The species was similarly introduced, is actively spreading and becoming widely naturalized in 13 other European countries; it has also followed the same invasive pattern during the last 50 or 60 years in both Canada and the United States (Tiley et al. 1996; Page et al. 2006). In Sweden, human interest and activity, for example collecting the impressive fruiting heads, also contributed to its spread (Lundström 1984).
Toxicity
The sap contained in the hollow stem, leaves, branches and surface hairs contains substances called furocumarins, which on contact with the skin, but only in the presence of strong sunlight, can cause a severe allergic blistering reaction. The damage to skin is permanent, since its ability to filter ultra-violet light is compromised, and future exposure to the sun will give rise to recurring severe sunburn, a condition that will last for several, or perhaps many years. The sunburn will be accompanied by rashes, blistering and swelling of the skin (Powell 1988). The painful blisters engendered by the sap develop into pigmented scars that can last for up to six years, but which more typically disappear after a few months (Page et al. 2006).
On account of this danger the plant should never be touched without protective clothing. Having said this, the health threat from the plant can easily be exaggerated: for instance, while H. mantegazzianum is common and locally abundant on the banks of the River Clyde, a survey of Glasgow hospitals and general practices close to large stands of the plant strongly suggested that the sap is not the serious medical problem it is often purported to be (Dickson 1998).
Weed control
Eradication is difficult and expensive, since early season spraying with glyphosate may not be totally effective for five to ten years (if ever). Manual cutting must be done below ground level otherwise vigorous growth is only encouraged. Ploughing is effective, but again it needs to be repeated since the larger roots will regenerate the plant.
Rather amazingly, cattle, sheep, pigs and goats can browse the plants perfectly safely. Foraging by pigs that damages the roots or heavy and prolonged grazing by sheep both eventually eradicated Giant Hogweed from pastures in Denmark (Tiley et al. 1996).
On account of its invasive nature and potential danger to humans, especially children, it is now illegal to plant Giant Hogweed or to transport its seeds. Dead plant material and the soil around the plant must also be disposed of in a recognised landfill site run by an appropriate local government authority.
Threats
Eradication requires sustained, deliberate destruction.