Biorefinery residues from non-food biomasses are promising sources of sustainable agrochemicals. The molecular properties of water-soluble extracts from ligno-cellulosic biomass pretreated first by steam-explosion and then by enzymatic hydrolyses at different buffer doses, were assayed for bioactivity on maize. 13C and 31P nuclear magnetic resonance (NMR) spectra showed that extracts varied in phenolic and carboxyl content, while high performance size exclusion chromatography and diffusion ordered spectroscopy NMR revealed that Ox-BYP 1 obtained from wastes treated with a greater buffer dose contained small-sized molecules associated in apparently large metastable aggregates. Ox-BYP 2 separated from wastes treated with smaller buffer concentrations showed a more stable conformation. Both hydrolysates revealed a positive dose-dependent bioactivity toward maize growth. Ox-BYP 1 promoted plant fresh and dry weights and root length at 10 and 100 ppm but decreased seedling growth at 1 ppm. Instead, Ox-BYP 2 increased the whole plant growth at all assayed concentrations. Their different biostimulation effects were attributed to the toxicity of easily bioaccessible lignin-derived phenolics at small concentrations of Ox-BYP 1, which was removed by molecular selfassembly at greater concentrations. Conversely, the more strongly associated Ox-BYP 2 exerted a positive bioactivity even at small doses. The bioactivity of extracts from biorefinery wastes appeared to depend on molecular composition and, in turn, on waste pretreatments.