The world consumption of medicinal plants as pharmaceuticals, cosmetics and as a food supplement for the improvement of human welfare is increasing day by day. In agriculture there is a world-wide effort to reduce the amount of chemicals used in production by introducing modern biological and ecological methods. One of the possible solutions is allelopathy, the utilization of the chemical interaction between plants. In this research we studied the effect of several concentrations of a hydro-alcoholic extract of St. John’s wort (Hypericum perforatum) and sage (Salvia officinalis) (undiluted, 1/5, 1/10 and 1/20 diluted) and different concentrations of Eucalyptus oil (Eucalyptus globulus)(0.1, 0.2, 0.5 and 1% v/v) on seed germination percent and rate for two weeds, Amaranthus retroflexus and Portulaca oleraceae. The statistical design was a randomized complete block design with 4 replicates. In the second experiment, we studied the effects of different concentrations of Black zira (Bunium pesicum) on lettuce and radish seed germination. Results showed that an undiluted extract of St. John’s wort and sage had a significant inhibitory effect on seed germination percent and germination rate for Amaranthus retroflexus, but not for Portulaca oleraceae. Eucalyptus essential oils had a strong inhibitory effect on the germination of both weed species at 0.7% (v/v). Bunium pesicum essential oils also had an inhibitory effect on lettuce and radish seeds. Lettuce seeds were more sensitive than radish to these essential oils. Lettuce seed germination was completely stopped at 0.2% (v/v), but radish required a much higher concentration (1% v/v). The strong inhibitory effects of Eucalyptus and Black Zira essential oils may give promising results in the organic culture of plants.

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