Interspecific competition is often cited as one of the main reasons for the successful expansion of plant species, and it can result in irreversible homogenization of plant communities. Allelopathy is a special type of interference competition. There have been relatively few studies on allelopathy, especially in woody species, even though allelopathy is increasingly regarded as a mechanism underlying the expansion of various plant species. Its importance in the structure of plant communities has been demonstrated by the fact that some allelopathic plants form monocultures in previously diverse communities. Allelopathic plants produce toxic compounds that give them an advantage over competitors. These compounds, known as allelochemicals, include a wide range of phenolic acids such as benzoic and cinnamic acids, alkaloids, and terpenoids. They are released into the environment in appreciable quantities via root exudates and leaf leachates, and via degradation of roots and other plant tissues. These substances can modify many processes in plant growth and development, including germination, early seedling growth, biomass accumulation, respiration, and photosynthesis, leading to the failure of competing species to grow and regenerate. Therefore, allelopathy is thought to be an important mechanism in the expansion of plant communities. Phyllostachys edulis (Poaceae) is a popular and useful plant indigenous to China. Its shoots and culms are harvested as a food source, but it is also an important biomass resource. Plantations of Ph. edulis have expanded rapidly in southern China. Recently, this species has come to be regarded as a weed; with its robust growth and strong rhizomes, it is capable of dominating some forest stands by displacing other native forest species. Thus, it is a threat to biodiversity when it spreads from plantations into neighboring evergreen broad-leaved forests. However, there is little information about the allelopathic effects of this species on the regeneration of the main tree species in neighboring forests. To explore the potential allelopathy in the invasion process of Ph. edulis, we determined the effects of aqueous extracts from Ph. edulis on the growth of Castanopsis sclerophylla in a pot experiment. We prepared aqueous extracts from Ph. edulis leaves and stems, litter, and soil, at three concentrations; 0.1 g/mL, 0.05 g/mL, and 0.02 g/mL. We measured the height, ground diameter, and photosynthesis of C. sclerophylla seedlings after treatments with the extracts. The allelopathic effects depended on the concentration of the aqueous extract and its source. The aqueous extracts of Ph. edulis affected the height, ground diameter, and chlorophyll relative content of C. sclerophylla seedlings. The highest extract concentration (0.1 g/mL) showed significant inhibitory effects on these parameters, while the lowest concentration (0.02 g/mL) showed stimulating effects. The 0.1 g/mL extract reduced net photosynthetic rate and stomatal conductance of C. sclerophylla seedlings. The light compensation point of C. sclerophylla seedlings was increased by treatments with high and low concentrations of the extracts. The effects of the extract differed depending on the source. The soil extracts showed stronger allelopathic effects than did the stem and leaf extracts and the litter extracts. Soil extracts showed inhibitory effects at all concentrations (0.1 g/mL, 0.05 g/mL, and 0.02 g/mL), while stem and leaf extracts and litter extracts showed stimulating effects at the lowest concentration (0.02 g/mL). The changes in chlorophyll content and photosynthetic performance were among the most important allelopathic effects of Ph. edulis aqueous extracts on seedling growth. In these experiments, Ph. edulis showed allelopathic effects against C. sclerophylla seedlings. These findings indicate that as Ph. edulis invades into new environments, it may interfere with the regeneration of the main tree species and harm the surrounding forest. To better understand the importance of allelopathy, future research should focus on determining the chemical composition of aqueous extracts from Ph. edulis and its mechanisms of action.