There is a world-wide effort in agriculture to eliminate the amount of chemicals used in production technology by introducing up-to-date biological and ecological methods. One of the possible solutions is allelopathy, the utilization of the chemical interaction between plants. The literature indicates that active substances of medicinal and aromatic plants have allelopathic properties. In this project we compared allelopathic activity between Soxhelet (1/20 w/v) and maceration extracts (water and hydro-alcoholic 1/10 w/v) of thyme (Thymus vulgaris) and peppermint (Mentha piperita) on seed percentage germination and germination rate of two noxious weeds, Amaranthus retroflexus and Portulaca oleracea. The experimental design was as a randomized complete block with 3 replicates. In a second preliminary experiment, we studied the effects of different extraction methods of thyme and peppermint on lettuce (Lactuca sativa) seed germination. Results showed that Soxhelet and maceration extracts (hydro-alcoholic) of peppermint completely inhibited seed germination of A. retroflexus and L. sativa (but not on P. oleracea). Maceration extracts (hydroalcoholic) of thyme completely inhibited seed germination of L. sativa and also had inhibitory effects on A. retroflexus. Soxhelet extracts of thyme completely inhibited seed germination of P. oleracea. The highest percentage germination and germination rate of the tested weeds occurred in control treatments. In conclusion, the maceration hydro-alcoholic extract (1/10) had more inhibitory activity than the maceration water extract as measured by percentage germination and germination rate of the weeds. There were no significant differences between the inhibitory effects of thyme and peppermint on seed germination using the hydro-alcoholic maceration extract. The Soxhelet extract of thyme demonstrated more inhibitory effects on P. oleracea than peppermint, but in A. retroflexus, peppermint extract exhibited more allelopathic potential than thyme.

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