The occurrence of weed species with hemp significantly affects the development and yield of hemp (Cannabis sativa L.). Three experimental studies were conducted to examine whether weed species like Amaranthus tuberculatus L., Eleusine indica L., Ipomoea spp. L. suppress the growth of hemp. The results show that the presence of these weed species was responsible for the reduced growth of hemp in terms of height, stem diameter number of leaves except when some weed densities were reduced with a grass-specific herbicide, but the weed species did not have any association with the N (nitrogen) and C (carbon) assimilation of hemp. Additionally, utilizing a de Wit replacement series design, a greenhouse study investigated the competitiveness of the two hemp cultivars ‘Jin Ma’ and ‘NWG’ and three weed species i.e., A. tuberculatus, Glycine max L., and Setaria faberi L. to see how they affect each other through both intra- and interspecific competition. The outcome demonstrated that the weed species were responsible for the loss of yield as well as suppressing the growth of hemp. However, there were some mixtures where the plants were overyielding as well as the effects of two different hemp cultivars varied in response to the three weed species. Moreover, the effect of the three weed species was different based on hemp cultivars and mixture. These two studies suggest that controlling the weedy species in hemp fields is important to reduce the loss of hemp yield. Additionally, another separate greenhouse study looked at the accumulation of Trigonelline (TRG) in the leaves of two hemp cultivars ‘Jin Ma’ and ‘NWG’. The finding of this experiment demonstrates that the amount of TRG varies between when the plants grow under control and drought stress treatment. However, the two cultivars did not vary in terms of TRG accumulation. The three experimental studies showed that weeds have a significant effect on hemp growth parameters, and the accumulation of trigonelline varies based on drought conditions.