This study investigated the effects of soil depths and soil organic fertilizer application on the growth characteristics of Spiraea bumalda 'Gold Mound' in a extensive green roof system. The treatments were 3 soil depths (10, 15 and 25 cm) and 5 soil types in mixture of artificial soil and organic fertilizer. We measured plant height, leaf width, leaf length, number of flowers, visual quality and survival rate from March to October in 2011. The growing medium of 10 cm soil depth showed the

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... st plant growth in A1 (amended soil 100%), and the lowest plant growth in O1A4 (organic fertilizer 20% + amended soil 80%) treatment. In case of 15 cm soil depth, Spiraea bumalda 'Gold Mound' showed a high leaf length and visual quality in O1A2(organic fertilizer 33% + amended soil 67%) treatment and high leaf width and number of flowers in O1 (organic fertilizer 100%) treatment. A1 treatment without organic fertilizer showed the lowest leaf length and poorest visual quality, and O1A4 treatment showed the lowest plant height and lowest number of flowers. At soil depth 25 cm, O1A1 (organic fertilizer 50% + amended soil 50%) treatment showed greater plant height, visual quality and number of flowers than other treatments. The leaf length and leaf width were more effective in O1 treatment. A1 treatment showed a relatively low leaf length, leaf width and visual quality. The higher the organic conditioner, the better the plant growth. And, survival rates of Spiraea bumalda 'Gold Mound' showed 92%, 88% and 76% at soil depths of 25 cm, 15 cm and 10 cm, respectively, in this a extensive green roof system. Therefore, the results showed that the growth of Spiraea bumalda 'Gold Mound' was affected by both soil quality and soil depth. Different optimal mixtures of organic fertilizer and amended soil were determined, depending upon soil depth.