Assessment of the spread potential of some cultural refugees from the Graz area.
Occurrences of numerous cultivated ornamental plant species as garden escapes has been observed within the municipal area of Graz. Some of them are able to spread generatively, by seeds (Clematis tangutica, Lavandula angustifolia, Symphytum asperum and Symphytum caucasicum) and sometimes assisted by baccivorous birds (Berberis julianae, Prunus laurocerasus and Viburnum rhytidophyllum). Several others are successful in vegetative spread when carelessly disposed with garden waste (Chaenomeles × superba, Chamaecyparis lawsoniana, Euonymus fortunei, Kerria japonica and Lamprocapnos spectabilis). Some feral species were observed for several years, such as Allium ramosum s. lat., which rapidly spreads via bulblets, and species like Berberis julianae, Clematis tangutica, Euonymus fortunei, Kerria japonica Prunus laurocerasus and Viburnum rhytidophyllum. Others survive only for a few growing seasons and have disappeared again (Chamaecyparis lawsoniana, Lamprocapnos spectabilis, Lavandula angustifolia, Symphytum asperum, Symphytum caucasicum). The potential spread of garden escapes depends mainly on the number of diaspores, on presence and topographical extent of suitable habitats (mainly forests, forest edges and ruderal areas) and on commonness of horticultural use. Most of the garden escapes described here are characterized by a low potential spread. Only shade-tolerant, evergreen shrubs like Berberis julianae, Prunus laurocerasus and Viburnum rhytidophyllum may spread their fruits by birds and thus propagate in all the forest outskirts of Graz. In contrast, a progressive growth of Allium ramosum s. lat on rocky slopes and ruderal areas is expected, involving adverse impacts on the native flora of rocky stands.