Sphaeranthus suaveolens (Forssk.) DC. and Argemone mexicana L. plant species are agricultural weeds that threaten crops and native plant diversity in East Africa, particularly in Tanzania. Yet, there have been few studies to assess whether their allelopathic effects inhibit plant germination and growth. Experiments were carried out in petri dishes and pots to investigate the inhibition effects (here referred to as negative allelopathy) of S. suaveolens leaf (SsL) and A. mexicana leaf (AmL) crude extracts on Zea mays L germination and seedling growth. Zea mays seeds and seedlings were treated with different crude concentrations of SsL and AmL to investigate their inhibition potential. The results showed that SsL and AmL crude extract concentrations delayed and/or reduced Z. mays germination, particularly at higher concentrations (70% and 100%). At these higher concentrations, few seeds germinated compared to the number of seeds germinated at lower concentrations (0%, 25%, and 50%). Further, it was found that Z. mays growth vigour was negatively affected as evinced by lower stem and root lengths, diameters, and total fresh biomass at higher concentrations of SsL and AmL. Although this study demonstrates the allelopathic effect of S. suaveolens and A. mexicana on Z. mays germination and growth, it also recommends further laboratory and field research experiments to investigate their allelopathic effects on other crops and native plants. However, this study advises that the management and control of these weeds be taken into account as their inhibitory effects could subsequently lead to a loss of plant diversity and crop productivity.