Alien herbaceous ornamental plants as a risk of biodiversity degradation in habitats of urban areas

The problem worldwide is a loss of biodiversity of ruderal areas and native or semi-native habitats located in or near urban areas. The horticultural industry has gained a prominent role in alien plant naturalization. The aim of the study is to compare ornamental herbaceous plants – the already known garden escapers and newly introduced taxa showing escape depending on taxa biological properties in the light of probable threat to biodiversity outside plantings in Latvia. Eleven target species were examined. The ascertaining of the spread of the species was performed in Riga – the capital of Latvia hosting a population over 620,000 people rich in gardens. The biological characteristics of the species – growth pattern, reproduction peculiarities – were studied by observing ex situ specimens in the Botanical Garden of the University of Latvia. The results show that newly introduced taxa, Aster umbellatus Mill., Boltonia asteroides (L.) L’Her, Kalimeris incissa (Fisch.) DC., resemble already known garden escapers by dominant index values, adaptation and invasiveness risk degree. This leads to the suggestion that these taxa may become garden escapers, especially if their use in horticulture is not restricted.