Seed Germination and Seedling Growth of Four Bedding Plants in Substrate Containing Coal Bottom Ash Mixed with Coir Dust

Coal bottom ash (BA) is a by-product of coal-fired power generation and can be utilized as a growing substrate for ornamental plants. The physical and hydraulic properties of BA-mixed substrates (coir dust:BA, 10:0, 9:1, 8:2, 7:3, and 6:4, v/v) and commercial germinating media (BM2; Berger Peatmoss) were investigated, and the effects of the substrate mixtures on seed germination, seedling growth, and heavy metal concentrations (Cd and Pb) were evaluated for four common bedding plants (periwinkle (Catharanthus roseus), globe amaranth (Gomphrena globose), impatiens (Impatiens walleriana), and petunia (Petunia multiflora)). As the BA:coir dust increased, the air space rate in the substrate increased from 25.5 to 28.0%, providing the substrate with sufficient porosity. However, the container capacity and amount of easily available water decreased from 63.1 to 53.7% as BA proportions increased. In the final germination percentage and days to germination of the four bedding plants, no significant differences were detected among the substrate mixtures. Although the impatiens and petunias displayed poor growth (46–55% and 42–56% of dry weight, respectively) in the BA-mixed substrates compared to the BM2, no apparent differences in the seedling growth of periwinkles and globe amaranths were found between 7:3 (coir dust:BA) substrate and BM2. These results indicated that the BA-mixed substrates had the potential to replace the commercial germinating media. The plants grown in the BA-mixed substrates contained Cd, but it was unlikely to be derived from the BA.