In order to better understand the processes of allelopathic interactions and to elucidate the impact of various herbal extracts on seedling growth, investigations were initiated using isothermal calorimetry as a monitor. Seeds of wheat, mustard, rape and clover were germinated on aqueous herbal extracts from arnica, hypericum, milfoil, ribwort, sage and sunflower for 24 h (until the root was visible). Then, five seedlings were put into a calorimeter ampoule with herbal extracts. The specific thermal power (=heat production rate) of the seedlings during their growth was measured by isothermal calorimetry at 20 °C. Heat rate data were collected for 48 h. As a control seedlings were grown on water. The patterns of the thermal power–time curve during seedling growth on the herbal extracts and on water were completely different. In comparison with the water control, seedling growth on the herbal extracts was accompanied by a strong exothermic peak (first phase), whereas in the second phase distinct endothermic peaks were observed. The time after which the maxima of exo- and endothermic peaks occurred strongly depended on the seedling species and the origin of herbal extract. Similarly, the total thermal effect connected with seedling growth was correlated with the seedling species and herbal extract type.