Abstract:Field experiments were conducted, using "Fixed Interval technique for sampling seedlings", to determine the rules of asexual reproduction of Scirpus planiculrnis population. Counts were recorded for the number of re-emerged plants on the overwintering stem tubers and of the re-emerged seedlings from the trailing branch system, from June 10 to August 30 at a 10-day interval. In this way, the exponential equation was established for describing the increase of the re-emerged plants of Scirpus planiculmis per 10-day period. The technique of "observation on specific individual plants" was also used to determine the rule of sexual reproduction of the plant, i. e. to investigate the ability of sexual reproduction by counting the number of seed-sets from the re-emerged seedlings on 12 overwintering stem tubers and all their progenies (2640 re-emerged plants). Then, pooled cal ulation was made to obtain the intrinsic growth rate of the Scirpus planiculmis population as r= 7.24. Thus, it is indicated that the ultimate reason for the over-grown of the species population is the high intrinsic growth rate. On the other hand, since the critical period of both the weed and rice was already estimated, it could be used (1) to determine the critical time for weeding and (2) to control the weed effectively, and (3) to provide sound basis for high grain yield of rice.