Allelopathy can be regarded as a component of biological control in which plants are used to reduce the vigour and development of other plants. Allelopathy refers to the direct or indirect chemical effects of one plant on the germination, growth, or development of neighbouring plants. The allelopathic effects of leaf extracts of Thymus vulgaris, Lavadula sp, Rosmarinus officinalis and Eucalyptus citriodora at five levels (0, 10, 20, 30 and 100 %) on seed germination and some growth characteristics of Solanum nigrum and Amaranthus retroflexus were investigated. Seed germination factors of the weeds shown different reactions. Highest seed germination was in control treatment. Increasing in the level of treatments cause reduction in seed germination percent and MDG increased. The internode length and plant height, node number and leaves total chlorophyll content decreased with increasing rate of extracts. There were significant different between the medicinal plants that used. Lavand extract exhibited a higher inhibitory effects than others and eucalyptus had lowest influence on seed germination and rowth parameters. There were significant differents between all treatments and control.

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