Heracleum sphondylium L., Hogweed

Account Summary

Growth form and preferred habitats

Despite many modern Floras categorising H. sphondylium as a biennial (Tutin 1980; New Flora of the BI 1991), it is more accurately described as a (relatively) short-lived, semi-rosette perennial. Frequently H. sphondylium is polycarpic, only becoming monocarpic if, or when, it exhausts itself after excessive seed production (Kilburn 1983; Sheppard 1991). Hogweed is non-aromatic and extremely variable. In Europe, it has been divided into nine subspecies and numerous varieties or forma, although considerable intergradation exists between them (R.K. Brummitt, in: Tutin et al. 1968; Sell & Murrell 2009; Jonsell & Karlsson 2010). H. sphondylium stems vary greatly in height, occasionally reaching up to 200(-300) cm. They are hollow, ridged and are often covered with bristly hairs (ie hispid). Leaves, which number 2-3 at the base and 3-5 on the stem, are simply pinnate, the leaflets lobed, with toothed or crenate margins. Leaves are usually densely hairy beneath (Tutin 1980).

H. sphondylium is frequent and conspicuous in a very wide range of habitats but it is most frequent in more or less disturbed ground on roadsides, waste ground, scrub and woodland (except under the densest canopies) and in poorly grazed or infrequently mown grassland and recently disturbed bare soil. Although absent from swampy or extremely acid soils (pH 4.3 or less), it does occur frequently in rough grassland on riverbanks and in water-meadows, provided the roots of the plant do not become waterlogged for periods long enough to induce rot. H. sphondylium tolerates a high range of humidity, wind exposure and salinity, but it does not tolerate more than transient waterlogging or prolonged drought, both of which destroy seedlings and young plants. Near the coast it can tolerate moderate salinity from sea spray, but not salt-water inundation. Light requirements are also low and it can survive woodland shade down to c 5% of daylight (Sheppard 1991).

Although in fertile growing conditions it rapidly establishes a stout, branching, perennating taproot system that can penetrate to depths well below 100 cm, and above ground the plant is so vigorous it can reach heights of 2 or even 3 m, Hogweed does not reproduce vegetatively. Only the upper 'collar' portion of the taproot is capable of regeneration after ploughing or other uprooting disturbance (Sheppard 1991). The foliage and stem die right back in autumn. The lack of vegetative reproduction means that although H. sphondylium is common in disturbed, man-made or semi-natural habitats, it is not clump-forming, typically occurring as individual established flowering plants and only abundant when present as seedlings or young plants (Grime et al. 1988, 2007).

H. sphondylium relies entirely on seed for its reproduction and dispersal and the species is considered a competitive-ruderal without a persistent seed bank (Grime et al. 1988, 2007). The competitive status of the species has yet to be experimentally defined and, as it displays high morphological variability and plasticity throughout its range, this would be a major undertaking. Observation suggests that the competitive ability of Hogweed is derived from its large taproot energy store, which gives it the ability to expand its leaves early in the growing season before associated grasses and other neighbouring competitors can do so.

Germination of the large seeds (flat mericarps) peaks in March, early in the growing season when vegetation in most grassland is still short. Hogweed thus seems capable of establishing new plants even within ± closed perennial communities (Grime et al. 1988, 2007; Sheppard 1991).

Occasional cutting or grazing favours Hogweed by reducing the advantage of clonal competitors such as Ranunculus acris (Meadow Buttercup) and large, tufted grasses, including Dactylis glomerata (Cock's-foot). However, without some form of major uprooting disturbance, competitive displacement of H. sphondylium by other perennials appears extremely unlikely (Sheppard 1987).

Individual Hogweed plants can persist for many years with estimates of average life span ranging from 12 to 16 years depending on the habitat.

Fermanagh occurrence

Hogweed is very common in Fermanagh and has been recorded in 368 tetrads, 69.7% of those in the VC. Hogweed is really only entirely absent from areas of upland peat and fully aquatic habitats, where it is confined to wetland margins, but it is probably also under-recorded to some extent in well-managed farmland in eastern lowland parts of the county.

Flowering reproduction

The flowering period is from June to September. In individual plants, flowering can occur from about the third year of growth onwards, but the average pre-reproductive stage is 6-7 years long. Most individuals do not have the stored photosynthetic resources to flower in successive years, but they can survive to flower repeatedly about eight times, although it appears that seeding two or three times is more normal for the species (Sheppard 1991). The flowering stem is determinate, with one large terminal umbel up to 25 cm across, and 1-9 lateral umbels below it. Smaller tertiary umbels can branch from the peduncles of lateral umbels. Each inflorescence is a compound umbel, flat or slightly convex, composed of 10-20 somewhat unequal rays, measuring 4-25 cm in diameter. Very large plants can bear up to 30 umbels of insect- or self-pollinated flowers, capable of producing 10-20 thousand seeds in a season. The schizocarp fruits (each of two mericarps) are relatively large, measuring 6-10 mm long and equipped with lateral wings. Fortunately for the landowner and gardener, the vast majority of plants produce very many fewer seeds than this estimate, the mean total being c 850 per flowering plant.

The single-seeded fruits (ie twin mericarps, each one an achene) are passively dispersed by wind in late September to early October. Being relatively large and flat, while most fruits scatter near the parent plant, a small proportion are carried up to 50 m downwind. The seed bank is transient and, after chilling, virtually all seeds germinate the following spring (Sheppard 1991).

Toxicity and herbivory

Despite containing a wide range of aromatic oils, resins, terpenes, saponins and coumarins as toxic protection against herbivores and microbes, H. sphondylium supports a wide spectrum of feeding insects (Sheppard 1991). Insect herbivory can seriously restrict flower and seed production as can also, of course, grazing by large vertebrates including sheep, cattle and pigs.

The only fungal disease Hogweed commonly suffers from is a white powdery mildew, caused by Erysiphe heraclei in late summer and autumn, but this too can seriously reduce flowering and seed production.

British and Irish occurrence

Common and widespread throughout B & I so as to be almost ubiquitous in soils of suitable pH on ground up to around 1,000 m altitude (New Atlas). Subsp. sphondylium is the common form in B & I, while subsp. sibiricum (L.) Simonk has been found in Norfolk and in wild areas around the Cambridge Botanic Garden.

European and world occurrence

The H. sphondylium species complex is common and widespread throughout temperate Europe and Asia, becoming less frequent towards the Mediterranean and N Africa (Hultén & Fries 1986, Map 1429). Subsp. sphondylium occurs mainly in NW Europe, extending into Scandinavia, east-central Europe and the mountains of the Mediterranean region (Sheppard 1991). Subsp. sibiricum is mainly a plant of NE and east-central Europe but also occurs in C & SE France (Sell & Murrell 2009).

Uses

Young Hogweed stem 'spears' have been eaten as green vegetables and are described as very succulent if cooked briefly in boiling water like broccoli (Mabey 1972, 1996). However, it should be pointed out that, like the alien Giant Hogweed (H. mantegazzianum), the plant contains furanocoumarins which can sensitize human skin to sunlight causing severe allergic reactions that are akin to scalding water, causing permanent scars. These toxins are not inactivated by cooking. A condition known as 'strimmer rash' is caused by skin contact of allergic field workers with sap from Anthriscus sylvestris (Cow Parsley) and Heracleum sphondylium (Cooper & Johnson 1998).

Hogweed has been traditionally used in herbal medicine as a mild expectorant and in a few proprietary medicines for laryngitis and bronchitis (Launert 1981). In homeopathy, the plant has been used to treat kidney complaints (Sheppard 1991). The seeds have been used as the basis for an alcoholic beverage (French 1971), and foolish young boys and gypsies have even smoked the stems as a tobacco substitute (Vickery 1995).

Names

The genus name 'Heracleum' is Greek for 'Hercules' (or 'Heracles'), 'healer', which was a name given to it by Theophrastus (Gilbert-Carter 1964; Gledhill 1985). The Latin specific epithet 'sphondylium' translates as 'rounded', but it is not clear to what aspect of the species this refers (Gledhill 1985).

The English common name 'Hogweed' derives from the fact that the plant previously was collected as fodder for pigs and rabbits (Grigson 1974), and the alternative common name, 'Cow Parsnip', also alludes to its use for domestic animals. Grigson (1955, 1987) lists a total of 70 English common names from around B & I, many of which refer to animals that presumably were known to graze the plant or were fed it by farmers. For instance, Grigson (1955, 1987) reported that H. sphondylium is (or was) used to feed pigs up and down the country, people collecting the plant and, "carrying it home in bundles as a free harvest for the sty".

Threats

None.

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