This paper reports new floristic data of 159 taxa (158 species and one hybrid) from the Heves–Borsod Plain (5 micro-regions between the Mátra and Bükk Mountains and the Tisza River valley). The data were collected between 1999 and 2018 and supplemented with collected specimen (altogether 93 herbarium sheets). This study is the first part of a thematic series that analyzes the distribution patterns of forest, forest steppe and dry grassland ‘steppe’ species, beyond reporting floristic data. The data were derived from 51 flora-mapping quadrats (CEU), based on 5.395 field collected data records. Some indicator species of the three species groups have been assessed in detail (with the addition of distribution maps of 25 species). I have also tested two East-West direction ‘lines’ (the 100-meter isoline and the Csörsz Ditch) for the possible existence of North-South chorological gradients in the area. True forest species (mostly Querco-Fagetea elements) show a dispersed pattern, their representatives can be linked to forest blocks appearing in the landscape. Streams from the adjacented hilly areas (even if they are strongly modified) are very important in the dispersal of the forest species. Their significance and role are decreasing towards the South. For some steppe and forest steppe species (e.g. Brachypodium pinnatum, Campanula bononiensis, Clematis recta, Elymus hispidus, Lychnis viscaria, Ranunculus illyricus, Sanguisorba minor, Stipa spp., Teucrium chamaedrys, Trifolium alpestre, Vinca herbacea) the examined lines indicate a regional area boundary, while for other species (e.g. Phlomis tuberosa, Thalictrum minus) gradient-like distribution differences were not observed. In the distribution of many species an additional southern gradient running along the northern boundary of the former Heves Floodplain was detected. Southwards to this line, the representatives of the selected species are already very sporadic, occurring only in synantropic habitats (e.g. Vincetoxicum hirundinaria).