Heracleum

This article is about the plant genus. For the hero after whom it is named, see Heracles. For the invasive species, see Giant hogweed.

Heracleum is a genus of flowering plants in the carrot family, Apiaceae, whose members are known as hogweeds. It has between about 60 and 120 species, depending on how narrowly the species, and the genus itself, are drawn.¹² Hogweeds grow throughout the temperate Northern Hemisphere and in high mountains as far south as Ethiopia. The genus is richest in the Caucasus, with 26 species, and in China, with 29.¹

Most hogweeds are unremarkable plants of meadows and woodland edges, among them the native common hogweed of Europe, H. sphondylium. Three of the largest, H. mantegazzianum, H. sosnowskyi and H. persicum, were carried out of the Caucasus and south-west Asia as garden curiosities and fodder, and are now among the most troublesome invasive plants in Europe: the giant hogweeds.¹

Linnaeus published the genus in 1753.³ Its name is usually traced to Heracles, by way of the “all-heal of Heracles” of classical writers, although the plant those writers meant is identified by their translators with Opopanax, a different genus.⁴ Linnaeus himself, in 1737, listed Heracleum among the names that honour physicians, and glossed it as commemorating Heraclides, the father of Hippocrates.⁵

Classification

Heracleum belongs to the tribe Tordylieae, subtribe Tordyliinae,⁶ in the subfamily Apioideae, the largest of the family’s four.⁷ The family is large: 442 genera and 3,575 species by one count.⁸ It supplies much of the kitchen, including carrot, parsnip, celery, coriander, cumin, dill, fennel and parsley,⁹ and some of the most poisonous plants of the temperate world, among them poison hemlock and the water hemlocks.¹⁰¹¹

As traditionally drawn, Heracleum is not a natural group. Molecular studies have found it polyphyletic, and it holds together only if nine Chinese species are excluded and two small Caucasian genera, Mandenovia and Symphyoloma, are absorbed into it.⁶ Several former species have already been transferred to Tetrataenium and Semenovia.² The genus probably arose in the western Palaearctic and spread east by three routes; its many Chinese species may be the product of a recent radiation in the Hengduan Mountains.⁶

Heracleum is considered a relatively young genus,¹² and the large hogweeds evolved so rapidly that the insects feeding on them did not radiate alongside.¹³ This is one reason why no enemy specific to the giant hogweeds has been found; see enemy release hypothesis.

The giant hogweeds

The giant hogweeds belong to section Pubescentia, one of the sections into which the botanist I. P. Mandenova divided the genus in 1950. In molecular studies it appears as a natural group.¹⁴

Names

  1753 --o-- Linnaeus names the genus Heracleum
         |
  1817 --o-- a giant hogweed on the seed list at Kew
         |
  1819 --o-- H. pubescens: the first tall hogweed named
  1841 --o-- H. persicum, from seed sent from Persia
         |
         |   the plants spread through the gardens
         |   of Europe under the names of others
         |
  1895 --o-- H. mantegazzianum, from Abkhazia
  1944 --o-- H. sosnowskyi, from Georgia
         |
  2007 --o-- DNA: three species, closely related
  2013 --o-- Britain: five kinds, by the fruit
  2024 --o-- northern Russia: the "sosnowskyi" is
         |   probably mantegazzianum
  2025 --o-- "twin species"; perhaps only one
         |
         v   the plants, meanwhile, go on spreading

The giant hogweeds were named late. H. pubescens, described in 1819, was the first tall hogweed to receive a name, and many plants were afterwards referred to it. H. persicum followed in 1841, H. mantegazzianum only in 1895 and H. sosnowskyi in 1944.¹⁵¹ By then giant hogweeds had been grown in European gardens for most of a century, and every early record had been made under the name of some other species. Russian botanists of the twentieth century, drawing species more narrowly, added a second layer of synonyms to the first.¹⁵

In 2007 an analysis of the DNA of 189 plants from 72 populations sorted the confusion into three distinct but closely related species. In each, the invading plants were genetically close to plants of the native range.¹ The same analysis exposed a run of misidentifications. Some European “H. sosnowskyi” were H. mantegazzianum. The Scandinavian “Tromsø palm”, long known as H. laciniatum, is H. persicum. And plants labelled as giant hogweed in two London parks, Kensington Gardens and the grounds of Buckingham Palace, were closer to H. persicum.¹

Later work has reopened the question. DNA barcodes failed to separate plants identified as H. mantegazzianum from those identified as H. sosnowskyi, and all the large hogweeds sampled in Murmansk oblast and the Komi Republic proved probably to be H. mantegazzianum. The authors suggest that the two species hybridised at the Polar-Alpine Botanical Garden in Kirovsk, where the first Soviet trials were grown (see Sosnowsky’s hogweed).¹⁶ A 2025 study calls them “twin species” and notes genetic evidence that they may be one.² In Britain, a study of fruit characters found five distinct kinds of naturalised giant hogweed, and its authors proposed amending the schedule of the Wildlife and Countryside Act under which the plants are banned.¹⁷

Whatever they are called, all of them burn. Every naturalised giant hogweed examined in Britain, whatever its identity, contained furanocoumarins able to cause phytophotodermatitis.¹⁷whatever it is called this year

Chromosomes and genome

All the species of Heracleum examined so far are diploid, with 22 chromosomes.¹ Flow cytometry of Czech giant hogweed gave a nuclear DNA content of 3.56 pg (2C), or about 1,741 million base pairs in a single set.¹⁸ In the same survey of the Czech alien flora, naturalised plants had on the whole smaller genomes than their relatives that had not become invasive.¹⁸

Hybridisation

Hogweeds hybridise readily, which complicates their identification.¹⁵ Giant hogweed crosses with the native common hogweed where the two grow together. The hybrids are intermediate in form and virtually sterile, and backcrossing is rare. The chromosomes of the two parents differ by two translocations and two inversions.¹⁹ In experiments the cross succeeded only with the common hogweed as the seed parent.²⁰

The two species are kept apart by several barriers: selective pollinators, which mostly keep to one species (see Umbel § Pollination), poor seed set, and the sterility of the hybrids. The first barrier, distance, was removed when people brought the giant into the gardens of Britain.¹⁹

In Russia, H. sosnowskyi was deliberately crossed with other species of the genus in breeding programmes, and some of the hybrids may since have run wild.¹⁵

Chemistry

Furanocoumarins, the compounds that make hogweed sap burn in sunlight, have been reported from more than a hundred species of Heracleum.² A comparison of the invasive H. sosnowskyi with the smaller, non-invasive H. sibiricum found both to carry the genes of the whole biosynthetic pathway. The invader accumulated more of the phototoxic xanthotoxin and expressed more copies of the genes that make it; the production can be switched on by ultraviolet light, heat and attack by herbivores.² See furanocoumarin.

See also

• Giant hogweed
• Sosnowsky’s hogweed
• Umbel
• Enemy release hypothesis
• Hogweed (disambiguation)