Four kinds of plants with tap, tuber and fibrous root system were chosen and their root volume was manipulated by tillering control in this study. Plant tillering could be described by Logistic function, means that controlling the initial planting density in the wetland is unnecessary. Therefore, the minimum planting density for each plant could be considered, and this will have cost benefits for planting wetlands. There is a positive correlation between plant tillering and root development, especially on reed and Juncus effusus L, coefficient of association between above-ground plant weight and root weight, as well as plant tillering and root volume of which reached 0.93–1.Those coefficient of association of Cannaceac and Cyperus alternifoliusL were in the range 0.43–0.69. In the process that root breakthrough the substrate, root could open up the clogged soil, which increased porosity of substrate by 3–44% than the non-planted one, although root also occupied the space in the substrate. The adjoint transverse pressure of root growth would homogenize the substrate, which randomly increased Peclet number of substrate by 6–90% than the non-planted one. No regularity correlation was found between wastewater purification and above-ground plant biomass or root volume. Root plays more role on porosity improvement than substrate homogenization and water purification in VF for domestic wastewater treatment.

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