Distribution data for Heracleum mantegazzianum (Giant Hogweed) were collected at three different levels:
- Europe-wide based on the compilation of country reports including such data as year of first record, current distribution map and data on any other closely related species;
- a country-wide study of three European countries: Czech Republic, Latvia and the United Kingdom, using relatively detailed data, e.g. 10km by 10km square records in the United Kingdom, and
- a regional study focussing on the detailed current and historic distribution in the county of Hertfordshire, United Kingdom.
This process combined with genetic studies of Heracleum material from across Europe identified three main taxa of large leaved hogweeds in Europe: H. mantegazzianum, H. persicum and H. sosnowskyi. A map was compiled showing the distribution of these three species across Europe with H. mantegazzianum being widespread and present ion most countries, H. persicum largely confined to Norway, and H. sosnowskyi from the Baltic states and other ex-soviet countries. The map is used to show those parts of Europe that are free of these species, distinguishing between those in which the environment is unsuitable for large leaved hogweeds, e.g. Mediterranean areas, and those that might be invaded due to suitable environmental conditions, e.g. the Pyrenees.
The primary cause of spread of H. mantegazzianum and H. persicum at the international scale was the exchange of seeds between horticulturalists, mainly in botanic gardens and others such large gardens. Heracleum sosnowskyi was spread primarily by agriculturalists due to its use in the Soviet Union as a fodder crop. Within a country, the sending of seeds from one garden to another remains a key transfer agent though other media are relevant, e.g. passage down and up rivers and other linear habitats (e.g. the Czech Republic and the United Kingdom). In Latvia, however, the spread was mainly due to the distribution of the plant as a crop though, again, other factors were significant. At the regional scale, the vectors for spread were similar to those at the country level.