We present a reconstruction of the June weather conditions in SE Slovenia from 1497 to 2003 based on the De Martonne aridity index (AI). The AI were derived from oak (Quercus spp.) tree-ring series of living trees and historic wood, which exhibited a clear response to June precipitation (positive) and temperature (negative). In the reconstructed AI time series we classified negative and positive deviations from the mean as strong (+/-1.28 SD) or extreme (+/-1.645 SD), and thus identified 50 years with a likely dry and hot June, as well as 40 years with a likely wet and cool June. Historical sources and chronicles were used to validate the AI reconstruction in the pre-instrumental period before 1896. The years 1501, 1540, 1546, 1616, 1718, 1788, 1822, 1834, 1839 and 1841, with extreme or strong negative AI deviations, are mentioned in Slovenian chronicles because of crop failures, droughts or extremely hot summers. The years 1691, 1705, 1798, 1799 and 1847, with extreme or strong positive AI deviations, are mentioned as years with a cool and rainy summer. We discuss the relevance of June weather conditions for the growth of plants in the region between the Alps, the Mediterranean and the continental Pannonian lowland, and the possible changes due to the current climate change scenario.