<p>Humic acid–based biostimulants are increasingly used in plant production due to their ability to influence plant performance. However, their effectiveness is highly variable and strongly dependent on environmental conditions. Root zone temperature represents a key regulator of seedling quality, influencing growth patterns and nutrient acquisition. To clarify the environment-dependent mode of action of humic acids, this study investigated their effects on the morphology, physiology and mineral composition of cucumber (<i>Cucumis sativus</i> L. ‘Sargo’) seedlings grown under different root zone temperatures and irrigation timings. Root zone temperature emerged as the primary driver of seedling performance. Warmer conditions enhanced shoot growth, increasing shoot dry mass by 10% and leaf area and plant height by approximately 30%, while suppressing root development. In contrast, cooler root zone conditions promoted root growth, increasing root dry mass by 10% and improving resource-use efficiency by more than 10%. Application of the humic acid biostimulant did not significantly affect seedling morphology or physiological traits but exerted a clear temperature-dependent effect on nutrient accumulation, enhancing mineral nutrient concentrations predominantly under cooler root zone conditions. These findings demonstrate that humic acid–based biostimulants primarily regulate nutrient acquisition rather than visible growth responses and that their efficacy is strongly governed by root zone temperature, highlighting the importance of environmental context in biostimulant application strategies.</p>