The days from bag opening to primordia formation ranged from 3 to 14 days for non-sterilized substrate, 7 to 21 days for sterilized substrate. Days from bag opening to primordia formation on substrates C45 + R45 + WB10 and C30 + R60 + WB10 were significantly longer than those on R80 + WB20, and on substrate C40 + WS40 + WB20 were significantly longer than those on WS80 + WB20. Addition of cotton seed hull significantly delayed primordia formation compared to sole rice straw or wheat straw substrate. Significant difference between sterilized substrate and non-sterilized substrate in terms of days from bag opening to primordia formation occurred on all substrate except C15 + R75 + WB10.