Burdock (Arctium lappa L.) is a biennial herb belonging to the Asteraceae family. Continuous cropping of burdock in the same field decreases the crop yield year by year (replant problem). Allelopathy has been reported to be one of the causes of replant problems in some other plant species. However, there is very limited information on allelopathy of burdock. The present study aimed to assess allelopathic potential of burdock leaves, roots, and cultivated soil. All extracts of burdock leaves, roots, and cultivated soil inhibited the growth of barnyard grass, foxtail fescue, ryegrass, cress, lettuce, and burdock. The level of inhibition increased with increases in extract concentrations. Those extracts inhibited the root growth of monocotyledonous plants (barnyard grass, foxtail fescue, and ryegrass) more than their shoot growth. The leaf extract of burdock showed higher growth inhibition than the root extract. The level of sensitivity of burdock to the extracts was intermediate among the six test plant species. After purification of the root extract by several chromatographic runs, a growth inhibitory substance, methyl caffeate, was isolated. Methyl caffeate significantly inhibited the growth of barnyard grass, cress, and burdock at concentrations > 1 mM. The concentrations of methyl caffeate for 50% growth inhibition were 1.7–3.0, 1.1–1.4 and 2.6–4.2 mM for barnyard grass, cress, and burdock, respectively. The purification of growth inhibitory substances in the extracts of burdock leaves and cultivated soil is currently in progress. The present results suggest that the extracts of burdock leaves, roots, and cultivated soil had allelopathic potential. In order to more fully investigate this potential allelopathic involvement in the burdock replant problem, further identification of growth inhibitory substances in burdock leaves and cultivated soil will be necessary.