Asclepias speciosa (Apocynaceae, Asclepiadoideae): a rare or unrecognized alien species in Europe?

Studies on populations of Asclepias syriaca L. in Lithuania revealed the occurrence of a new alien plant species, the North American native Asclepias speciosa Torr. (Apocynaceae, Asclepiadoideae), in southern parts of Lithuania - the first report of the latter species in Europe. Interestingly, a thorough analysis of herbarium specimens revealed that A. speciosa had first been collected in Lithuania in 1962, but the specimen was misidentified at the time as A. syriaca. The newly discovered population of A. speciosa occupies mesic grasslands, tall-herb fringe communities and arable field habitats. Sexual reproduction of this species was not recorded; it spreads locally by means of vegetative reproduction. We present here an exhaustive analysis of morphological characteristics and differences between A. speciosa and A. syriaca and other species of the genus, as well as a key for identification of alien Asclepias species in Europe. We predict that the effect of A. speciosa on native habitats and communities, and its economic impact, are comparable to those of the highly invasive A. syriaca. Although A. speciosa currently occurs very rarely as an alien species in Europe, its existence in other regions of Europe is highly probable.