Microwave technology has wide applications, including extraction of active compounds in biomass and compost for agricultural use. A study was carried out to determine the effects of microwave power level from 0 (control) to 1000 W on the properties and active microbial groups in vermicast, and how it may impact the photosynthesis, plant growth, and yield of kale (Brassica oleracea var. sabellica) ‘Red Russian’. Heat accumulation in the vermicast increased rapidly to a peak of 86 °C at 400 W before declining to 68 °C at 1000 W. Vermicast water loss increased exponentially up to 800 W before declining. The C:N ratio of the vermicast was reduced at ≥600 W while the pH remained the same. In a 2D-principal component analysis biplot, vermicast treated at 600, 800 and 1000 W were associated with Gram-positive (G+), GGram-negative (G−), G + G− bacteria, protozoa, and fungi groups while the 0, 200, and 400 W treated vermicast were associated with eukaryotes. However, the trend for total microbial mass was 200 W = 400 W > 0 W > 600 W = 800 W = 1000 W. Kale leaf anthocyanin, chlorophylls, and carotenoids were significantly (p = 0.001) increased by the 400 W or 600 W treatment compared to the other treatments. Stomatal conductance, transpiration, and photosynthesis rates were increased by the 400 W followed by the 600 W. As a result, yield of kale grown in the 400 W microwaved vermicast was the highest. Future studies will explain the functions of specific microbial populations and elemental composition in microwaved vermicast.