Heracleum mantegazzianum

Heracleum mantegazzianum, commonly known as giant hogweed, is a monocarpic perennial herbaceous plant in the carrot family Apiaceae. H. mantegazzianum is also known as cartwheel-flower, giant cow parsley, giant cow parsnip, or hogsbane. In New Zealand, it is also sometimes called wild parsnip or wild rhubarb.
Giant hogweed is native to the western Caucasus region of Eurasia. It was introduced to Britain as an ornamental plant in the 19th century, and has also spread to other areas in Western Europe, the United States, and Canada. Its close relatives, Sosnowsky's hogweed and Persian hogweed, have similarly spread to other parts of Europe.
The sap of giant hogweed is phototoxic and causes phytophotodermatitis in humans, resulting in blisters and scars. These serious reactions are due to the furanocoumarin derivatives in the leaves, roots, stems, flowers, and seeds of the plant. Consequently, the jurisdictions of many regions consider it a noxious weed.

Etymology

The genus name Heracleum is from, the Latin rendering of : 'Hercules', in reference to the giant nature of hogweeds.
The species name mantegazzianum refers to Paolo Mantegazza, Italian traveller and anthropologist.

Description

Giant hogweed typically grows to heights of. Under ideal conditions, a plant can reach a height of. The leaves are incised and deeply lobed. A mature plant has very large leaves, wide, sometimes even up to wide and up to long and a stout, bright green stem with extensive dark reddish-purple splotches and prominent coarse white hairs, especially at the base of the leaf stalk. Hollow, ridged stems are in diameter, occasionally up to diameter, and can grow to more than high. Dark red spots on the stem each surround a single hair. The umbrella-shaped inflorescence, called a compound umbel, may be up to in diameter across its flat top. The flowers are white or greenish white and may be radially symmetrical or strongly bilaterally symmetrical. The fruit is a schizocarp, producing seeds in dry, flattened, oval pairs. The seeds are approximately long, with a broadly rounded base and broad marginal ridges, coloured tan with brown lines running of the seed's length.

Life cycle

The life cycle of giant hogweed consists of four phases:

Pre-flowering plants: In the first year, leaves sprout from the seed. In subsequent years, leaves grow from the overwintering roots, producing a rosette of leaves at ground level with no above-ground stem. This pre-flowering phase can last for several years.
Flowering plants : After several years of growth, the plant flowers, sending up an erect stem from the rootstock.
Seeds : A flowering plant can produce 20,000 seeds.
Dead stems : After producing seeds, the plant dies, leaving dried stems and seed heads standing; the seeds slowly fall and are dispersed by wind and animals.

Emerging seedlings tend to be morphologically diverse, with jagged pinnate forms and more rounded leaf shapes being sighted almost always within the same seed bank. During the first few years of growth, the leaves and stem of a pre-flowering plant die over the winter. In the spring, the plant grows back from its root. In other words, the giant hogweed is a herbaceous perennial.
A giant hogweed plant usually produces a flowering stalk in 3–5 years, but plants may take up to eight years to flower if conditions are unfavourable. In the Czech Republic, a single plant reached twelve years old before flowering. In any case, when the plant finally flowers, it does so between June and July.
Seeds are typically produced in August. A single flowering plant will produce 20,000 seeds on average with seed production varying between 10,000 and 50,000 seeds per plant.
Giant hogweed is a monocarpic perennial, that is, after a mature plant flowers and produces seed, the entire plant dies. During the following winter, tall dead stems mark the locations where the flowering plants once stood.
The seeds are dispersed short distances by wind, but can travel longer distances by water, animals, and people. Most seeds are found in the top of the soil within a few metres of the parent plant. Seeds may stay alive in the seed bank for more than five years.
A seed deposited in the seed bank is initially dormant. Dormancy is broken by the cold and wet conditions of autumn and winter, and so freshly deposited seeds lie dormant until at least the following spring, at which time approximately 90% of the previously dormant seeds will germinate. The rest remain dormant in the seed bank.
Seeds normally result from cross-pollination between two or more plants but self-pollination is also possible. More than half the seeds produced by self-pollination will germinate and give rise to healthy seedlings. Hence a single isolated seed may give rise to a new colony of plants.

Similar species

The various species of the genus Heracleum are similar in appearance, but vary in size. H. mantegazzianum is among the tallest, typically reaching high, whereas Heracleum species native to western Europe, such as H. sphondylium, or North America, such as H. maximum, rarely exceed high. There are considerable differences in the size of the umbel, leaves, and stem of H. mantegazzianum as well.
The following table compares H. mantegazzianum and H. maximum by feature:

	H. mantegazzianum	H. maximum	H. sphondylium
Height	Typically tall	Up to tall	Up to tall, rarely 2.5 m
Leaves	Compound, lobed leaves typically wide, up to wide; mature leaf has deep incisions and serrated edges	Compound, lobed leaves up to ; mature leaf is less incised with less jagged edges	Compound, lobed leaves up to ; mature leaf is with rounded, less jagged edges, hairy
Stem	Green stems from in diameter, occasionally up to in diameter, with dark reddish-purple splotches and coarse white hairs at the base of the leaf stalk	Green ridged stems up to in diameter with fine white hairs	Grey-green ridged stems up to in diameter, coarsely hairy
Flowers	White umbel is typically in diameter, up to in diameter, with 50–150 flower rays per umbel; flowers from mid-June to mid-July	White umbel up to in diameter with 15–30 flower rays per umbel; flowers from late May to late June	White umbel up to in diameter with 12–25 flower rays per umbel; flowers from June to September
Fruit'	Narrower oval fruit 9–11 mm long	Heart-shaped fruit	Broad oval fruit 7–10 mm long

Many more species exist; in Europe, over 20 species are found of the genus Heracleum alone. Few of these reach a similar size, and several are phototoxic to a lesser degree. Two other species, H. sosnowskyi and H. persicum, do reach similar sizes, and are equally noxious. These three species, alongside H. lehmannianum and H. asperum'', are sometimes grouped into a single species complex.

Distribution

Giant hogweed is widespread throughout western and northern Europe, especially along terrains such as coastal areas and riverbanks. By forming dense stands, it can displace native plants and reduce wildlife habitats. It has spread in the northeastern and northwestern United States and southern Canada, and is an invasive species across western Europe; in sites where it has settled, it overtakes the local native species, H. sphondylium.
In Canada, the plant occurs in most provinces, except in the prairies. It has been seen in Quebec since the early 1990s. The plant's spread in Ontario began in the southwest and was seen in 2010 in the Greater Toronto Area and Renfrew County near Ottawa.
In the US, giant hogweed occurs in Maine, Wisconsin, and south to Indiana, Michigan, Maryland, and New Jersey. In June 2018, it was reported growing in Virginia and North Carolina. The plant is federally listed as a noxious weed in the US.
The closely related hogweed Heracleum sosnowskyi is widespread in Russia and the Baltic states, and present in eastern Europe.

History

Heracleum mantegazzianum is native to the western Caucasus region of Eurasia. Because of its impressive size, giant hogweed was brought to Europe and North America as an ornamental plant and garden curiosity.
The following historical information grew out of the European Giant Alien Project, which began in 2005.

Migration across Europe

Heracleum mantegazzianum was first described in scientific literature in 1895, but by that time more than a dozen European countries had already imported the plant as an "ornamental curiosity". The introduction of H. mantegazzianum was first recorded in Great Britain in 1817 when it was put on the seed list at the Kew Botanic Gardens in London. The first natural population was recorded in 1828, growing wild at Shelford in Cambridgeshire.
The spread of H. mantegazzianum throughout Europe continued unabated until the middle of the 20th century, at which time the dangers of giant hogweed had become more widely known. Despite the warnings, however, the plant continued to be used by gardeners, beekeepers, and farmers for another 50 years. Heracleum mantegazzianum was finally delisted by the Royal Horticultural Society of Great Britain in 2002.

Introduction to North America

During the 20th century, giant hogweed was transported to the US and Canada for display in arboreta and Victorian gardens. The earliest recorded planting in North America was in 1917, in gardens near Highland Park in the city of Rochester, New York.
By 1950, giant hogweed had appeared in southern Ontario, and within a quarter-century the plant was firmly established in Ontario. It was first collected from Nova Scotia in 1980 and Quebec in 1990. Giant hogweed was still available for sale in Canadian nurseries as late as 2005.
On the west coast of North America, H. mantegazzianum appeared in Oregon, Washington, and southwestern Canada, but it is not clear how the species found its way into this region. The first reports of giant hogweed in British Columbia were published in the 1930s.