Population characteristics of expansive perennial herbs

Some perennial herbs which have expanded in recent times in the temperate zone of Europe were evaluated using criteria for an 'ideal weed' adapted from Baker (1965). Data were gathered on relative growth rate; production dispersibility and viability of diaspores: intensity of vegetative spread; timing of the first generative reproduction ; and maximum height of species. The range of conditions suitable for germination , and self-compatibility were also considered. On the basis of these data, semiquantitative and qualitative criteria were used for a comparison of the species based on an estimation of their expansive ability. A high expansive ability was found in all of the following species listed in decreasing order: Urtica dioica; Cirsium arvense; Typha angustifolia/latifolia, Chamerion angustifolium, Artemisia vulgaris, Baldingera arundinacea; Calamagrostis epigeios; Tanacetum vulgare; Agropyron repens and Tussilago farfara. The best strategy, facilitating a species expansion is the high production of easily dispersed diaspores (in space and/or time), in combination with efficient vegetative propagation, unless the establishment of species is limited only to very specific habitats.