The list of registered vascular plants includes 1059 taxa (Supplementary ESM S1). The average number of species per mound was 69.9 ± 22.6 SD (min 27, max 152). We identified 971 plants to species level, accepted 3 taxa as aggregates or species groups, and identified 85 taxa to genus level. The floristic diversity was confined to 82 vascular plant families. Flowering plants (Angiosperms) made up the majority of the observed species diversity and only seven species belonged to other groups: one horsetail—Equisetum hyemale, two ferns—Polystichum aculeatum, Pteridium aquilinum, and four Gymnosperms—Juniperus communis, J. oxycedrus, Pinus nigra, P. sylvestris. There were 23 families represented by more than 10 taxa, the most species rich of these were Asteraceae—126 taxa (11.9% of the established taxa), Fabaceae—111 taxa (10.5%), Poaceae—106 taxa (10%), Lamiaceae—61 taxa (5.8%), Brassicaceae—58 taxa (5.5%), Caryophyllaceae—57 taxa (5.4%), Rosaceae—48 taxa (4.5%), Apiaceae—46 taxa (4.3%), Boraginaceae—31 taxa (2.9%) and Plantaginaceae—31 taxa (2.9%). Twenty-three other families (28.1%) were represented by a single species. The flora of the studied mounds was composed primarily of perennial herbaceous plants (61%), followed by short-lived plants, and a low number of shrubs and trees. Generalists and grassland specialists dominated the species composition of the mounds. A major part (98%) of the established vascular plants consisted of native species (Table 2). Only 21 plants belonged to other categories (alien (status unknown)—13, naturalized alien—5, in large-scale cultivation—2 and doubtfully native—1). Invasive alien plants included Acer negundo, Ailanthus altissima, Amaranthus albus, Conyza canadensis, Cuscuta campestris, Datura stramonium, Erigeron annuus, Phytolacca americana, Robinia pseudoacacia, Sorghum halepense, Xanthium orientale subsp. italicum and X. strumarium. These taxa represent 20% of all plants included in the list of invasive or potentially invasive alien plants in Bulgaria and three of them (Acer negundo, Ailanthus altissima and Robinia pseudoacacia) are among the “top 10” invasive alien plants in Bulgaria. The largest number of invasive plant species we registered on a single mound was four. A substantial number of the studied mounds (40 or 35.4%) had at least one invasive alien plant.

Critically endangered species present on the mounds included Anchusa stylosa and Limonium asterotrichum. Endangered plants present on the mounds included Astragalus haarbachii, A. wilmottianus, Chamaecytisus frivaldszkyanus, C. kovacevii, Dianthus pallidiflorus, Erysimum cheiranthoides, Festuca thracica, Goniolimon besseranum and Jurinea ledebourii. Fourteen of the registered taxa were Balkan endemics: Achillea clypeolata, A. pseudopectinata, Armeria rumelica, Astragalus wilmottianus, Asyneuma anthericoides, Cytisus eriocarpus, Dianthus moesiacus, Festuca thracica, Heptaptera triquetra, Koeleria simonkaii, Dichoropetalum vittijugum, Polygala supina subsp. rhodopea (syn. P. rhodopaea (Velen.) Janch.), Scabiosa triniifolia and Thymus longidentatus. Eleven plants were included in the CITES Convention.