More info for the terms: cover, density, peat
Descriptions of giant hogweed habitats in North America are less detailed than those of its nonnative European habitats. Characteristics of invaded European habitats may provide information about the susceptibility of habitats in North America, where giant hogweed is still expanding its range.
Giant hogweed is often described along roadways and waterways in North America, which have been important in the early spread of giant hogweed in Europe (see Spread, Impacts, and Control). Along French Creek on Vancouver Island, giant hogweed initially established on alluvial stream bank soils but more recently, giant hogweed has spread into adjacent woodlands (Wall, personal observation, as cited in [42]). This same pattern was noted in northern Vancouver (Page, personal observation, as cited in [42]). In Washington and Oregon, giant hogweed occurs in riparian areas, disturbed sites, and along roads [11]. In the eastern Midwest, giant hogweed habitats include roadsides, stream banks, railway embankments, and abandoned fields [7]. Giant hogweed occurs in the sandy gravel along roadsides and ditches in eastern Ontario and western Quebec (Darbyshire, personal observation, as cited in [42]). Open woodlands, canals, streams, roadsides, and vacant lots support giant hogweed populations in New England and New York [28,36].
In Europe, giant hogweed is common in unplowed pastures, meadows, riparian areas, and forest clearings in high-humidity areas with nitrogen-rich, basic soils [61], although many soil types are tolerated. During a survey of giant hogweed-invaded habitats in northwestern England, none of the habitat charactersitics measured (altitude, slope, distance from water, bank top width, and water quality) consistently predicted giant hogweed's occurrence. Researchers proposed that either habitat characteristics were too broad or giant hogweed's tolerances too general for good prediction and concluded that "almost all United Kingdom banksides (we)re susceptible to invasion" [10]. Using logistic regression modeling, researchers found that human population density (P<0.0001), low winter temperatures (P=0.002), and moraine clay (P=0.001) significantly predicted the distribution of giant hogweed on a national scale in Denmark. At the regional scale within Denmark, predictive variables were human population density (P<0.0001), alluvial sand cover (P=0.001), and spring precipitation (P=0.01) [37]. In West Bohemia, researchers comparing invaded and uninvaded sites found that giant hogweed did not occur in vegetation types with acidic soils, including peat bogs, birch (Betula spp.) woodlands, and spruce (Picea spp.) forests. Habitat susceptibility increased with increased light availability (P=0.0117), increased soil reaction indicators (P=0.0004), increased dispersal potential (P<0.001), and increased disturbance (P<0.001) [57].
Climate: The climates in giant hogweed's native and nonnative ranges are similar. In the Western Greater Caucasus region, giant hogweed occupies habitats from 160 to 7,200 feet (50-2,200 m) [68]. In this area, January temperatures can range from -33 to 61 °F (-36 to 16 °C) and July temperatures from 27 to 95 °F (-3 to 35 °C). Based on 30 years of data, annual precipitation averaged 34 inches (863 mm) [45]. Several reviews have summarized the prevailing climates in giant hogweed's nonnative range. Plants generally grow best in northern temperate climates [70]. Cold winters are common in giant hogweed-invaded habitats and are likely necessary for successful germination and establishment (Tiley and others 1996, as cited in [13]). Giant hogweed seedlings are frost resistant (Godefroid 1998, as cited in [42]). While moist sites are preferred, moderate summer droughts are also tolerated [70]. In Canada, giant hogweed is considered "most invasive" in cool, moist climates, and populations occur as far north as Kapuskasing, Ontario. Giant hogweed is also found on seashores and tolerates salt spray [42].
During population and growth studies conducted in Germany, giant hogweed suffered high mortality during a hot dry summer, but reproduction and recruitment from drought survivors was high [18]. In the Czech Republic, giant hogweed's distribution was significantly related to January isotherms (P=0.003). Giant hogweed was less common in areas with warm winters. Average June isotherms, annual precipitation, and elevation did not significantly affect giant hogweed's distribution [51]. Distribution was also affected by human population density; for more about this see Spread, Impacts, and Control. When germination, establishment, and growth of giant hogweed were evaluated along elevation gradients in the United Kingdom and the Czech Republic, researchers found that limitations to dispersal explained giant hogweed's distribution better than climate. At all sites tested from 30 to 2,000 feet (10-600 m), more than 80% of giant hogweed seed germinated. Second-year survival of giant hogweed was lowest at the highest-elevation, coldest-climate site but was still almost 30% [77].
Soils: In most reviews, giant hogweed is described on deep, moist soils with nearly neutral pH and moderate to high nutrient content. Soil textures in giant hogweed habitats may be gravels, sands, loams, or clays. Moist soils are preferred, and waterlogged and winter-flooded soils are tolerated but extended periods of inundation are not [13,28,42,70]. Giant hogweed seeds rot quickly on sites inundated for long periods [70]. The pH reported from giant hogweed habitats ranged from 4 to 8.5 [68].
In southern Ontario, giant hogweed is often found on deep, rich, moist soils [33]. In Germany, giant hogweed was most common on loamy soils. Giant hogweed-invaded sites had an average pH of 5.6, experienced rapid nutrient cycling, were well aerated, and received a "good supply" of water [65]. In Edinburgh, Scotland, giant hogweed populations occurred on soils with pH levels ranging from 5 to 9 and organic matter contents ranging from 3.1% to 8.7% [6].