Menyanthes trifoliata is a threatened and endangered species in Korea and USA. To assess the important ecological characteristics for its conservation and restoration, we measured the level of soluble reactive phosphorus (SRP) in the water, total N and C contents in the plants, and the water-holding capacity of the surrounding floating peat mat. These factors, which limit the success of aquatic macrophytes, were examined for their effects on the growth of this species. Leaflet areas and petiole lengths ranged from $6.1;cm^2$ and 12.4 cm, respectively, at a water depth of 5 cm, to $22.7;cm^2$ and 33.0 cm, respectively, at 40 cm. The peat mat was able to retain water at a depth of 5 to 45 cm. SRPs ranged from 10 to 46 ppb, while total C and N contents were between $0.62\%$ (petiole) and $2.76\%$ (leaf), and between $39.7\%$ (petiole) and $43.8\%$ (stem), respectively. The water-holding level was not correlated with SRP, and neither phosphorus nor nitrogen content affected plant growth. However, the capacity of the floating mat to hold water as the depth subsided was positively related to growth. This suggests that such mats are good tools for restoring populations of M. trifoliata when materials with high water-holding capacities are used.