Peat is widely used as a growth substrate in horticultural soilless cultivation, but it is an expensive and non-renewable resource. The objective of the present study was to assess the feasibility of using peat substitutes in the production of New Guinea impatiens (Impatiens hawkeri) by selecting garden waste compost, mushroom residue compost, soil, and peat as the media. No significant difference had been observed in total root length, root tips, and root fractal dimension at prophase when using 100 % peat substrate with garden waste compost and peat mixed in a 1:1 treatment, whereas at a later period, the 1:1 mixture resulted in faster growth rate. Root fractal dimension and root abundance improved to different degrees when using garden waste compost and mushroom residue compost as substitutes; moreover, using garden waste compost contributed to maintain root growth, whereas mushroom residue compost was effective for generating fibrous roots. Hence, the results suggested that (1) garden waste compost could be ideal substitutes for peat in New Guinea impatiens production and have great potential during the later period of root growth and (2) a box-counting fractal model could be used for the study of plant root morphology and architecture as well as being an effective method for analyzing root geometry and functional properties quantitatively.