With the rapid development of urban forestry, the waste of landscaping has gradually increased. The use of green waste compost (GWC) as a substrate for forestry production has high ecological and economic value. The purpose of this study was to evaluate the effects of coco coir (CC) and rice husk ash (RHA) on the physicochemical properties of GWC and the growth of cucumber (Cucumis sativus L.) and watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai) seedlings. When the amount of GWC was unchanged, CC (0%, 10%, 20%) and RHA (0%, 2.5%, 5%) with different proportions are added, respectively. This study found that the GWC optimized with 10% CC and 2.5% RHA was the best substrate. Compared with the control, the plant height of cucumber seedlings and watermelon seedlings increased by 133% and 62%, respectively; root length increased by 84% and 44%, respectively; the total dry weight increased by 68% and 44%, respectively; the total fresh weight increased by 45% and 59%, respectively; and the total chlorophyll content increased by 81% and 88%, respectively. In terms of plant growth morphological indicators, biomass accumulation and photosynthetic pigment content, cucumber and watermelon seedlings grew best under this substrate. In consequence, GWC modified by 2.5% RHA and 10% CC was an excellent substrate for cultivating Cucurbitaceous vegetable seedlings.
