Contribution to the Knowledge of the Invasive Alien Species of Heracleum (Apiaceae) in Bulgaria

Heracleum mantegazzianum and H. sosnowskyi (Apiaceae) have been recently published as naturalised alien species in the Bulgarian flora. The present article summarises the data about the distribution of the species in the country and provides new data about the habitat preferences of the taxa, chromosome number, genome size and reproductive potential. Diploid chromosome number of 2n = 2x = 22 has been established in both species from Bulgarian accessions. The estimated genome size is 1C = 1.81 pg for H. mantegazzianum and 1C = 1.75 pg for H. sosnowskyi. Studies of the flower/young fruit number and soil seed bank of H. sosnowskyi suggests that the species has a great invasive potential.