We investigated seasonal variations in allelopathic potential of Artemisia princeps var. orientalis. Aqueous and methanol extracts and volatile substances were prepared in the laboratory from samples collected monthly (April through October). Their impacts were then assessed on the germination and seedling growth of Lactuca sativa and Achyranthes japonica. The allelopathic potential varied with the time of sample collection and the concentration tested. For example, germination of L. sativa was not inhibited by the aqueous extract but seedling growth (shoots and roots) was, with its seasonal effect being significant. For A. japonica, seed germination was not inhibited at lower concentrations (except for August samples). However, at higher concentrations and in certain months (especially July), germination was more negatively affected. The degree of seedling growth inhibition also differed by month and by extract concentration, with roots being impacted more than shoots. Volatile substances also had a time-dependent influence on the germination and seedling elongation of A. japonica. In a separate experiment, the ethyl-acetate and water fractions of a crude methanol extract were prepared monthly from A. princeps var. orientalis. Here, we examined their antimicrobial activities against three gram-positive bacteria (Bacillus cereus, Bacillus subtilis, and Staphylococcus aureus), two gramnegative bacteria (Escherichia coli and Pseudomonas fluorescens), and one lactic acid bacterium, Lactobacillus plantarum. The ethyl-acetate fraction that was sampled in September was remarkably potent against B. cereus and B. subtilis, whereas the water fraction collected in August and September showed great antimicrobial activity against the grampositive and -negative bacteria. In contrast, L plantarum was not inhibited by the water fraction, regardless of the sampling month. Likewise, the ethyl-acetate and water fractions collected in April and October had the lowest levels of antimicrobial activity.