The use of biopesticides has expanded rapidly in recent years; however, their use in weed control is less advanced. Herein, we describe the development of a weed control agent by screening 208 plant extracts (104 species) for their plant growth-inhibition activities, which resulted in 142 active samples (from 89 plant species). Verbascoside, isolated from the shoots of Verbena incompta, was identified as a growth inhibitor against rice-root (EC50, 1.75 mM), and its root growth-inhibition activity was also confirmed in radish, tomato, and Lotus japonicus. Verbascoside is composed of hydroxytyrosol (EC50,12.51 mM) and caffeic acid (EC50, 4.08 mM), two poorly water-soluble phenolic components with weak growth-inhibition activities, and two sugars, which are more soluble but inactive. The plant apparently developed a more active and highly soluble compound by condensing these four components. We conclude that a biopesticide containing verbascoside may be useful for weed-control purposes.