A comparison of two North-American asters invading in central Europe

Perennial North-American asters have become common in central Europe since the 17th century when they started to be cultivated as ornamental plants. Today, they are widely spreading especially in alluvial sites in lowlands. Two of the most common aster species, Aster lanceolatus and Aster novi-belgii, were investigated for their generative reproduction, including seed production and germination, and vegetative propagation (growth of rhizomes). Experimental sites with and without competition from the surrounding vegetation were used to test for differences between the species. Germination experiments showed that the asters produced a relatively high number of viable and easily dispersed seeds, which often germinated immediately after ripening and usually did not require any special treatment. Some aster clones produced more than 100 new rosettes from one initial rosette within a year. Apparently, asters are able to form large polycorms in a relatively short time. However, their rapid vegetative spread can be reduced by competition from established vegetation.