Plant of the week – June 22nd, 2020 – Giant Hogweed

Heracleum mantegazzianum, the Giant Hogweed

by John Grace, The University of Edinburgh

Giant Hogweed and botanists. Photo: Chris Jeffree

The Giant Hogweed, Heracleum mantegazzianum, is one of the most striking of the many ornamental plants introduced into Britain in the 19th Century. It is truly a giant. In good conditions it grows to 4 metres (some authorities claim 5 metres) and produces 20,000 seeds. It grows best in the fertile soils around rivers, but also along roads and on waste ground. It has spread rapidly in UK, the rest of Europe, and in North America. In the 1970s it was particularly common around Edinburgh (Clegg and Grace 1974), and on the Tweed and other rivers in south Scotland (Stewart 1979). 

The plant is a member of the family Apiaceae (formerly the Umbelliferae), the family which includes carrots, parsnips, celery and coriander. Members of this family all have umbrella-like inflorescences, and usually they take more than one year to reach maturity. Most of them, including H. mantegazzianum die after fruiting (‘monocarpic perennials’).

Where did it come from? There are records of seeds being received at Kew Gardens of several ‘giant’ species of Heracleum from 1817, whilst at the RBGE the records start in 1888. Loudon’s Hortus Britannicus mentioned Giant Hogweed as early as 1820. However, the name H. mantegazzianum was not used until 1895 by S. Sommier (a Swiss Doctor) and E. Levier (a French amateur botanist), who travelled in Europe and Asia. They thought their species was somewhat different from the Giant Hogweed already in cultivation, and they named it after their friend and companion Paulo Mantegazza. The type description, in Latin, is reproduced in the Edinburgh PhD thesis of Fiona Stewart (1979). There has always been taxonomic confusion surrounding the giant Heracleums, despite the dedicated work of the Soviet/Georgian botanist Ida Mandenova who described 90 types of Heracleum from the ‘headquarters’ of the genus in the Caucasus Mountains in 1950. Confusion persists, and it is reckoned that there may be several species which are sometimes called H. mantegazzianum: Mandenova described H circassicum and H. grossheimii and the name H. sosnowskyi was given to a very similar but smaller biotype. The modern tools of molecular genetics are required to resolve the confusion.

Worm’s eye view of inflorescence: the central primary umbel has fruits, secondary umbels produce flowers a bit later. Tertiary umbels often occur too. Photo: Chris Jeffree

The danger the plant poses to human health was alerted by a local physician, D.C. Drever in 1970: he had seen many cases of phyto-photo-dermatitis at his surgery in Dalkeith. The painful skin condition is caused when sap on the skin is exposed to the ultraviolet rays of sunlight. The chemical culprits, furanocoumarins, are present in lesser amounts in many species within the family Apiaceae, including parsnips and celery. Drever made sure local councils knew about the danger, and there began an annual ‘war’ against this alien plant which was ‘taking over’ at many riparian sites in southern Scotland. Indeed, the species has spread enormously in Britain and other parts of Europe since the 1970s (see maps in Tiley et al. 1996, Pysek et al. 2007).

The news media usually begin to publish Giant Hogweed stories when the plant starts flowering at the end of June. But 2020 has been different; the reporting season came early on 20th May with an article about how ‘Nasty’ Giant Hogweed thrives as COVID-19 lockdown cuts weed control:

https://www.bbc.co.uk/news/uk-scotland-52726060

Hybrids with the native Common Hogweed H. sphondylium were first reported by plantsman David McClintock (1973). H. sphondylium is a polycarpic perennial, and large versions of it are sometimes mistaken for Giant Hogweed (it is really nothing like it).  Stewart and Grace (1984) conducted controlled crosses between the two species. The offspring were intermediate, like those in the field, but the experimental cross only worked when mantegazzianum pollen was transferred to sphondylium stigmata; in the opposite direction it seemed that the sphondylium pollen tubes were not long enough to penetrate the maternal (style) tissues of mantegazzianum.  However, the hybrids had low fertility and they are scarce in the field.

Dense stand of Giant Hogweed at Goggarburn, near the HQ of the RBS, next to the tramline. Flowering from mid-June onwards. Photo: Chris Jeffree

The plant has attracted much attention, particularly because local Councils and landowners feel the need to control its spread. We found it could be controlled by early application of the herbicide glyphosate (‘Roundup’); others have shown that repeated cutting and using grazing animals may be successful (Tiley et al 1996). Later, an entire European project was devoted to management and control of the species (Pyseck et al. 2007), and several reviews and excellent summaries are available: https://www.cabi.org/isc/datasheet/26911

https://onlinelibrary.wiley.com/doi/epdf/10.1111/j.1365-2338.2009.02313.x

Will it ever go away? Perhaps it will, as its seeds need a winter chilling treatment. The lack of winter chilling may be why it is absent from southern parts of Europe. British winters are becoming mild, and may soon be too warm for it.

Please don’t try to grow this species, however much you admire its magnificence. Seeds of the species are no longer available from seed merchants, and growing it is forbidden by the Wildlife and Countryside Act, Schedule 9 part 2, and punishable by up to 2 years in jail.

References

Clegg L and Grace J (1974). The distribution of Heracleum mantegazzianum (Somm. & Levier) near Edinburgh. Trans. Proc. Bot. Soc. Edinb. 42: 223-229.

Drever JC, Hunter JAA (1970). Hazards of giant hogweed. British Medical Journal, 3, 109.

McClintock D (1973) Heracleum sphondylium × Heracleum mantegazzianum. – Watsonia 9, 429-430.

Pysek P, Cock MJW, Nentwig W, Ravn HP (2007)Ecology and Management of Giant Hogweed (Heracleum mantegazzianum) CABI Publishing. Egham, UK

Stewart F (1979) Hybridization between Heracleum mantegazzianum Somm. & Lev. and H. sphondylium L. (Umbelliferae) in the British Isles. PhD thesis, University of Edinburgh (available online).

Stewart F, Grace J (1984) An experimental study of hybridization between Heracleum mantegazzianum Somm. & Levier and H. sphondylium L subsp. sphondylium (Umbelliferae). Watsonia 15, 73-83.

Tiley GED, Dodd FS, Wade PM (1996) Heracleum mantegazzianum Sommier & Levier. Journal of Ecology 84, 297-319.

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