Many invasive species show allelopathic effects that contribute to competitive impacts on native vegetation for which few control measures exist. We investigated the potential for pyrolized organic material used as a soil amendment (“biochar”) to sorb allelochemicals and mitigate allelopathic effects on seed germination and early seedling development in three common invasive plants in Canada: garlic mustard (Alliaria petiolata), tree of heaven (Ailanthus altissima), and yellow sweetgrass (Melilotus officinalis). We hypothesized that biochars would mitigate effects on germination and early seedling development (radicle extension and cotyledon development) through sorption of allelochemicals. Laboratory assays of seed germination and early seedling development of two agricultural crops (Lactuca sativa and Raphinus raphanistrum) and two native grass species (Andropogon gerardi and Poa palustris) were conducted using water extracts from leaves. Seeds were treated with plant extracts exposed to four different biochars (red oak (Quercus rubra), jack pine (Pinus banksiana), shipping pallet and construction waste, and high-carbon wood ash) using a range of extract and biochar dosages. Treatment of allelopathic plant extracts by biochars significantly (p < 0.05) mitigated effects on seed germination for Alliaria and Melilotus. Effects on seed germination and early seedling development depended on extract concentration, biochar dosage, and the target species assessed. Controls treated with biochar leachate alone showed significantly (p < 0.05) increased germination for two of the biochars tested (jack pine biochar and high-carbon wood ash). Results indicated that biochars can mitigate allelopathic effects of invasive species through sorption of allelochemicals; however, this will be effective in only some cases.