Abstract The waste material from rearing black soldier fly, Hermetia illucens (L.), (Diptera: Stratiomyidae), larvae (BSFL), frass, could serve as a partial replacement for fertilizer or growing media, such as peat. Studies have shown that depending on the crop life cycle, long or short, the incorporation of frass can have positive impacts on vegetable growth. Investigating the earlier stages of growth, such as emergence, can provide more information on the acute impacts of insect frass on plant production. Two greenhouse studies were designed to investigate the use of frass in vegetable production at the seedling stage: BSFL frass as a partial replacement for peat in a growth germination medium and BSFL frass as a fertility replacement in a growth germination medium. Tomato, lettuce, and arugula seeds were grown in a standard control growing media, a growing media amended with frass to partially replace peat, and a growing media with frass applications to substitute the fertility source. In all treatments and varieties, the control growing media produced better or comparable results to the treatments. As a peat replacement, the two tomato varieties, lettuce, and arugula seedlings grown in frass had comparable seedling vigour and days to emergence to those grown in the control. As a fertility replacement, seedlings grown in frass had significantly lower percent emergence in all four cultivars when compared to the control. The tomato varieties also had significantly lower seedling vigour in the BSFL fertility treatments than the control. Depending on crop and growing media composition, the impacts of frass on early plant development vary. The adoption of frass into growing practices should include preliminary tests for nutrient composition and microbial contamination.