Fourteen medicinal plants from northern Thailand, which dominate and form large colonies, were dried, powdered, and evaluated for growth inhibitory activity using a modified sandwich method. All the plants showed inhibitory activities on the growth of lettuce seedlings: Cymbopogon nardus had the strongest activity, completely inhibiting the growth of the lettuce hypocotyls and radicles; Piper retrofractum completely inhibited the hypocotyl growth and 93.5% of the radicle growth; and the remaining plants inhibited hypocotyl and radicle growth by 30.9 57.4% and 44.1 75.3%, respectively. Inhibitory active compounds may have been released from the plants into the sandwich method medium and inhibited the growth of the lettuce seedlings. The present results suggest that C. nardus and P. retrofractum are possible candidates for developing alternative natural herbicides in sustainable agriculture because of their strong inhibitory activity.

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Aqueous methanol extracts of Acmella oleracea inhibited the growth of roots and shoots of cress (Lepidum sativum), lettuce (Lactuca sativa), Italian ryegrass (Lolium multiflorum), and barnyard grass (Echinochloa crus-galli) with the extract concentration-dependent manner. The extracts were then purified and 2 growth inhibitory substances were isolated and identified by spectral data as (E,E)-2,4-undecadien-8,10-diynoic acid isobutylamide (compound 1) and nona-(2Z)-en-6,8-diynoic acid 2-phenylethylamide (compound 2). Compound 1 inhibited the growth of cress roots and shoots at concentrations greater than 0.3 mM and that of barnyard grass root at concentrations greater than 0.03 mM. Compound 2 inhibited the growth of cress shoots and barnyard grass roots at concentrations greater than 1 mM. The inhibition on the seedling growth indicates that both compounds had growth inhibitory activity and may contribute to the inhibitory effect of A. oleracea extracts. This is the first report of having growth inhibitory activity of compounds 1 and 2 on plants.