The cultivation of American ginseng (Panax quinquefolium L.; Araliaceae) is frequently hindered by problems during replanting. Our previous studies showed that p-coumaric acid significantly inhibited the radicle growth of American ginseng embryos grown on filter paper. The objectives of the study were to determine the allelopathic effects of p-coumaric acid on embryos of American ginseng grown in soil, to evaluate the effects of p-coumaric acid on adult plant growth, and to identify the physiological mechanism involved. The role p-coumaric acid played in the growth of American ginseng embryos was investigated using natural soil to mimic real conditions in the field. We also studied how p-coumaric acid affected the growth of 2-year-old American ginseng roots grown in nutrient solution. During the seedling emergence and fruit set stages, we monitored changes in plant growth and physiological index. In the experiment with embryos, American ginseng seeds were planted in soil to which either 0.0024, 0.012, 0.06, 0.3, 1.5, or 7.5 mg p-coumaric acid per gram of soil had been added; after 7 days of treatment, the radicle and shoot lengths of the embryo were measured. In the hydroponic experiment, the nutrient solution was amended with 0.012, 0.06, or 0.3 p-coumaric acid per mL of solution. Plant growth data, including expansion of leaves, plant height, and canopy growth, were collected at 5-day intervals starting from the day p-coumaric acid was added to the solution. The aboveground biomass, the new grown fibrous root biomass and the activity of phenylalanine ammonia lyase (PAL) in the fibrous roots were measured at the leaf expansion (10 d), blooming (20 d), and fruit set (30 d) stages. Moreover, photosynthetic parameters, including net photosynthetic rate (Pn), apparent photosynthetic electron transport rate (ETR), and maximum photochemical efficiency of photosystem II (Fv/Fm), were determined after leaf expansion. The results showed that in soil containing 0.0024-7.5 mg/g p-coumaric acid, the radicle and shoot lengths of American ginseng embryos decreased by 28.52%-100% and 1.09%-100%, respectively, and this inhibitory effect was dose dependent. p-Coumaric acid significantly inhibited the above-ground growth of American ginseng plant at all tested concentrations (P < 0.05, Dunnett t-test) and also delayed leaf expansion. Furthermore, the aboveground biomass was significantly reduced by 17.17%-54.55% at the fruiting stage (P < 0.05). Moreover, Pn and ETR of leaves were also significantly reduced (P < 0.05) by p-coumaric acid; however, the values of Fv/Fm were not affected. For fibrous roots, their PAL activity in the 0.06 mg/mL p-coumaric acid treatment was 69.05% higher than in the control at the fruiting stage, and then both their biomass and PAL activity were lower than in the control after this growth stage. When p-coumaric acid was at concentration of 0.3 mg/mL, the PAL activity and biomass of American ginseng fibrous roots were lower than in the control in all the three growth stage. These observations indicated that p-coumaric acid in the natural-soil rhizosphere of American ginseng embryos exerted profound allelopathic inhibitory effects, and it also inhibited the growth of adult American ginseng plants above ground while decreased the photosynthetic capacity of leaves. The fact that PAL activity in fibrous roots was higher than the control at seedling emergence stage and lower than the control at blooming stage in 0.06 mg/mL p-coumaric acid treatment, associated with fibrous root biomass was decreased at blooming stage, also demonstrated that p-coumaric acid may be a potential stress factor during the growth of American ginseng root systems. In conclusion, our data showed that p-coumaric acid exerts autotoxic effects on both embryos and seedlings of American ginseng and that these allelopathic effects might be partially achieved via the inhibition of photosynthesis in leaves of adult plants.