Natural History
Geographical Origin and History
Giant hogweed is native to the Caucasus mountains of southwest Asia. It was brought to Europe by 19th century naturalist explorers (CABI, no date) and subsequently escaped, spreading throughout much of Europe and the UK.
Giant hogweed first arrived in North America in the early 1900s. It was introduced to southeastern Ontario by the 1940s (Morton 1978). In 1944 it was recorded from Vancouver, British Columbia, and in 1953 from Seattle, Washington. It is now established in parts of the northeastern US and adjacent Canada (USDA-NRCS 2004), including southeastern Ontario, where it has become locally common (Morton 1978). It is also established in Washington State and in British Columbia (USDA-NRCS 2004, Dawe and White 1979).
Giant hogweed first appeared in the upper Great Lakes states in Lower Michigan in 1991, where a small population was discovered along a small stream next to a mowed area (Case and Beaman 1992). Within the last several years it has appeared at a number of new sites in Lower Michigan, western Upper Michigan, and adjacent northeast Wisconsin, as plantings or recent escapes (S. Trull, pers. comm.). It is likely to continue showing up in new locations, assisted by gardeners looking for something unusual to plant.
Characteristics
Giant hogweed is a biennial or (more often) a monocarpic perennial. Immature plants typically form a rosette of 3-4 (or sometimes a few more) arching leaves reaching 1-2 m (3.3-6.6 ft) tall. Stressed, depauperate plants may only have 1-2 leaves for their first few years until they accumulate enough reserves to produce a rosette. As plants mature they develop a stout, branching taproot, with the crown 8-12 cm (3.1-4.7 in) below the ground (Tiley et al. 1996). The root crown may reach 15 cm (5.9 in) across before the plants bolt and flower.
The rosette leaves of immature plants and the lower stem leaves of mature, flowering plants are deeply lobed and toothed (pinnatified), with a strong midvein and a fairly short petiole (Tiley et al. 1996).
The leaves of flowering plants become progressively smaller, more shallowly lobed and sessile towards the top of the stem (Tiley et al. 1996). The leaves are fairly densely covered with short, stiff hairs, and are alternately arranged on the stem. The stems are stout and hollow, with purple blotches and scattered short, stiff, thick-based (pustulate) hairs. These stiff hairs tend to stick to the skin and break off, causing significant irritation (Morton 1978).
At peak flowering (anthesis), plants typically have a single straight, central stem with several short to fairly long branches near the top (Tiley et al. 1996). Each branch bears a flat-topped flower cluster (or umbel) composed of 50-150 primary rays (branches of the flowerhead), each supporting a cluster of shorter secondary rays, with each of these secondary rays tipped with a white flower (the whole thing forming a structure called a compound umbel) (Gleason and Cronquist 1991).
The paired, light brown seeds are flattish, elliptical, and about 8-11 mm (0.3-0.4 in) long (Gleason and Cronquist 1991). The seeds are easily separated. Each seed has 3-5 conspicuous, dark brown oil glands extending from the top of the seed about 2/3 of the way down (Tiley et al. 1996).
The characteristic that makes giant hogweed so distinctive from all our other herbaceous plants is its size. Flowering plants may reach 5 m (16 ft) tall, with lower leaves to 3 m (10 ft) long and 1.5 m (5 ft) wide (Gleason and Cronquist 1991). The main stem may reach 10 cm (4 in) in diameter at the base (Tiley et al. 1996). Flowerheads (umbels) can reach as much as 0.5 m (1.6 ft) across.
Similar Species
Several of our native plants resemble giant hogweed to one degree or another. In our region the plant most likely to be mistaken for giant hogweed is cow parsnip. Cow parsnip or bibigwewanashk (Heracleum lanatum) is closely related to giant hogweed, but is a smaller plant. Its umbels generally have 15-30 primary rays. Flowering cow parsnip plants typically reaching about 1-2 m (3.3-6.6) feet tall, though they can reach 3 m (9.8) ft tall (Gleason and Cronquist 1991). The stems of cow parsnip lack the blotches and thick-based bristles characteristic of giant hogweed. Unlike giant hogweed and wild parsnip, cow parsnip is a true perennial, with mature plants bolting and producing seeds each summer.
Purplestem angelica (Angelica atropurpurea) might also be mistaken for giant hogweed. Our largest native herbaceous plant, this impressive monocarpic perennial may reach 3 m (9 ft) tall (Voss 1985). It is more common south of the ceded territory but is found in some of the larger wetlands and floodplains in the Lake Superior region, including along the Fish Creek slough in Ashland County, Wisconsin. Angelica's leaves are more dissected than those of cow parsnip and giant hogweed, and the stem is smooth and dull reddish-purple (not purple-spotted and blotched). Angelica also differs from giant hogweed and cow parsnip in that its white flowers are in rounded, globular umbels instead of nearly flat umbels.
Though generally much smaller and more delicate than giant hogweed, common water hemlock or abagwasi'gans (C. maculata) could possibly be mistaken for it. Common water-hemlock is usually a branching plant that can reach as much as 2 m (6.6 ft) tall. Its lower leaves are divided twice or three times, with leaves becoming less divided upward. The leaflets are relatively long and narrow, but often more than 5 mm (0.2 in) wide and usually toothed. It produces numerous umbels of white flowers to 12 cm (4.7 in) across. A second species, bulbous water hemlock (C. bulbifera), has long, very narrow leaves that are less than 5 mm (and often 1-2 mm, or less than 0.1 in) wide. It rarely gets over 1 m (3 ft) tall. It can also have umbels of white flowers, but usually produces only numerous small bulbs in the axils of the leaves. Both these plants are fairly common in swamps and floodplains, but may also occur in roadside ditches and other disturbed, wet areas.
WARNING: The roots and other parts of water hemlock and certain other members of the parsley family are EXTREMELY POISONOUS!! Never eat roots or other parts of any wild plant, unless you are certain of the plant's identity, and that it is safe to eat!
Reproduction and Dispersal
Over most of its European and North American range, giant hogweed is a monocarpic perennial, though some Russian plants are apparently polycarpic (Shumova 1973 in Tiley et al. 1996). Also, Tiley et al. (1996) report that when the flower stalk is damaged or cut off before the plant is done flowering, plants may survive to produce weak flower stalks the following year. Hogweed is also capable of producing side shoots from the root crowns, which can grow into new plants (Morton 1978).
Giant hogweed plants take at least two growing seasons to reach reproductive size. In one study cultivated plants were found to flower in the second, third, or fourth growing season, with a few plants waiting 5 seasons to flower (Stewart and Grace 1984, in Tiley at al. 1996). Flowering in the third spring appears to be most common. As with wild parsnip and some other monocarpic members of the parsley family, there appears to be a threshold taproot size that must be attained by fall for plants to flower the next spring (Tiley at al. 1996).
The flowers of giant hogweed are pollinated by a variety of insects and are self-compatible, so that a single isolated plant can produce viable seed. More than 80,000 flowers may occur on a single plant (Tiley at al. 1996). Each flower normally produce 2 seeds, so single plant is capable of producing over 100,000 seeds (Tiley at al. 1996). In Europe new colonies are often started by a single plant (Tiley et al. 1996).
The seeds are easily dispersed by water, leading to potentially rapid dispersal in riparian habitats (Morton 1978, Pysek and Prach 1993). The seeds have been observed to float for 1.5-2 days in turbulent water, and 3 days in quiet water (Clegg and Grace 1974). Wind is an important agent for short-distance dispersal of the seeds in the fall (Tiley et al. 1996). Neiland (1986, in Tiley et al. 1996) found that seeds dropped from a height of 2 m found that seeds traveled over 2m (6.6 ft) in a 3 m/sec (10 ft/sec) wind, and over 10 m (33 ft) in a 14 m/sec (46 ft/sec) wind. According to Miller (2001), birds may consume and spread the seeds also, though Clegg and Grace (1974) found that birds in Scotland ignored hogweed seed, even when it was set out with other bird food. Isolated seeds may be dispersed on the feet of mammals (Tiley et al. 1996).
The seeds of giant hogweed are believed to need cold stratification to germinate (Shumova, E. M. 1973 in Tiley et al 1996). Seeds stored dry at room temperature have survived for as long as 7 years (Morton 1978). The seedlings readily establish on disturbed ground (Caffrey 1994). Seedlings need ample soil moisture to survive, but established plants are moderately drought-tolerant (sources in Tiley et al. 1996).
Habitat Preference and Tolerance
In is native range in the Caucasus mountain region, giant hogweed typically occurs along forest edges, in forest glades, and along streams (Mandenova 1950, in Tiley et al. 1996). The continental climate of this region is disconcertingly similar to that of the upper Great Lakes region, with warm summers, cold winters, and ample rainfall.
In Europe and in North America, giant hogweed has colonized a variety of moist to wet, sunny to partly shaded habitats, from roadsides, ditches, old fields, and right-of-ways, to fields, marsh edges, moist to wet meadows, and woods edges and openings. Urban waste areas such as old dumps, railroad right-of-ways, and rubble piles are commonly colonized in Europe (Tiley et al. 1996).
Giant hogweed does best on soils that are constantly moist, and is tolerant of poorly-drained soils and seasonal flooding, but not permanent flooding (Tiley et al. 1996). It seems to tolerate a wide range of soil textures, from gravel and sand to loams and clays, as long as they are of sufficient depth to allow development of the taproot (Tiley et al. 1996). It colonizes sites that are relatively low to relatively high in organic matter, and appears to be somewhat tolerant of salt (which also tends to raise soil pH). Data from sites in western Scotland strongly suggests that giant hogweed may be intolerant of even moderately acid soils, however.
Giant hogweed is somewhat shade-tolerant and does well in light to moderate shade, though plants in full sun grow the largest (Morton 1978, Tiley et al. 1996). On both continents giant hogweed has proven to be particularly aggressive on streambanks and in floodplains, where rich, moist soils supports hogweed's rapid growth and water carries the seeds downstream (Morton 1978, Pysek and Prach 1993). Expansion to new sites appears to be greatly aided by disturbance to the existing plant community (Tiley et al. 1996).