
Allelopathy stimulate or inhibit the growth of plant by releasing allelochemicals. Thuja orientalis L is medicinal plant which belongs to family cuperacea. Aqueous extract of leaves, bark and seeds of T.orientalis. L were used at 1, 5 and 10g/L concentration with different time period (24, 48, 72 hrs.) to check its allelopathic effect on seed germination, fresh and dry weight and seedling growth of Pennesitum americanum. Result revealed that aqueous extracts of all parts of plant at all concentration and time period had significantly inhibited seed germination of P.ammericanum L. when compared with control. The inhibitory effect increases with increasing concentration of extracts and time period. The bark extract of all concentration of T.orientalis in 24hr and 48hrs show stimulation in seedling growth. Inhibitory effect when compared with different parts were leaves > bark >seeds. The test of phytochemical screening revealed that aqueous and acetone extract contain terpenoid, tannins, flavonoid and volatile oil. These phytochemicals are secondary metabolites which act as allelochemical. Hence it is concluded that aqueous extract had inhibitory effect due to these water soluble allelochemicals. These chemicals can be used as herbicides.