Effective weed management strategies are essential for producing high-quality and successful ornamentals in nurseries and greenhouses. Weeds can affect both the productivity and quality of ornamentals, especially in containers where nutrient and moisture availability are limited, due to restricted space. The objective of this study was to determine how different types of weed species at various densities and in different container sizes affect the growth of ornamental plants. Two rounds of greenhouse experiments were conducted at the Horticulture Teaching and Research Center, Michigan State University, in summer and fall 2023. Seeds of large crabgrass {Digitaria sanguinalis (L.) Scop} and smooth pigweed (Amaranthus hybridus L.) were grown separately until they attained the 4-6 leaf stage. Liners of hydrangea {Hydrangea macrophylla (Thunb.) Ser.} and syringa (Syringa vulgaris L.) were planted in containers of three different sizes 0.67 gallons (2.54 liters), 1.5 gallons (5.67 liters), and 3 gallons (11.35 liters) containing standard bark-based substrate amended with controlled-release fertilizer. All plants received 0.5 inches (1.3 cm) of water thrice daily and were allowed to grow till they were well established. Then the weed seedlings of large crabgrass and smooth pigweed were carefully transplanted to each container-grown ornamental plant at different densities of 0,1,3,6 per pot, with the density of 0 being the control set. After weed transplantation, all pots were maintained inside the greenhouse for 8 weeks. The experimental design utilized a randomized complete block design with six replications per treatment. Initial and final growth indices of ornamentals were recorded. At 8 Weeks After Potting (WAP) fresh weights of both the weed species and the ornamentals were recorded separately. All data were analyzed in SAS by ANOVA and the Tukey’s HSD test were performed to separate out the means. Results showed that final growth indices of hydrangea were significantly higher than syringa and maximum in 3-gallon containers. Overall, large crabgrass at density of 6 was more competitive than smooth pigweed and hydrangea performed better than syringa in 3-gallon container size in competing the weed species.
