Vinasse, a liquid fermentation residual of bio-ethanol production that also contains solid particles in suspension, is commonly used as a soil amendment. Previous studies reported vinasse reduced seed germination and seedling establishment, suggesting herbicidal activity. Laboratory experiments were conducted to determine whether vinasse herbicidal activity is present in the liquid or solid phase, and whether it affects plants during seed early germination (i.e., imbibition), late germination (i.e., embryo growth and radicle protrusion), or seedling growth. Most of the herbicidal activity was associated with the liquid phase, and for most species, seed viability was predominantly affected after the imbibition phase. Susceptibility to vinasse was species dependent. Lettuce (Lactuca sativa L.) germination was <8% when seeds were imbibed and germinated in vinasse solutions or imbibed in water and germinated in vinasse. Conversely, imbibing lettuce seeds in vinasse solutions and germinating them in water did not change their germination in comparison with seeds imbibed and germinated in water (>80% germination). Wheat (Triticum aestivum L.) and sicklepod [Senna obtusifolia (L.) H. S. Irwin & Barneby] germination decreased 10% and 35% when seeds were imbibed and germinated in vinasse, respectively, while Palmer amaranth (Amaranthus palmeri S. Watson) and southern crabgrass [Digitaria ciliaris (Retz.) Koeler] germination decreased >90%. All evaluated species reduced radicle growth as vinasse concentration increased. Filtered liquid vinasse with reduced concentration of salt and ionic compounds inhibited radicle growth similarly to unfiltered vinasse, indicating that the herbicidal activity was not due to osmotic effects and was likely present in the organic liquid phase. Amaranthus palmeri, S. obtusifolia, and D. ciliaris increased the proportion of dormant seed more than 2-fold when they were imbibed or imbibed and germinated in vinasse solutions. Vinasse might be useful for weed management to reduce germinable weed seedbanks by increasing seedling mortality and seed dormancy either by properly timing of its application as a soil amendment or by purifying herbicidal compounds and using them directly for weed control.