The allelopathic potential of water extracts from Wedelia trilobata (L.) Hitchc. at the ratios of 1:10, 1:20 and 1:40 were studied on onion root mitotic cell division, seed germination and seedling growth in test plants. The water extracts at various concentrations decreased the root length, mitotic cell division of the onion and mitotic index (MI). Wedelia trilobata (L.) Hitchc. extracts affected seed germination and seedling growth of the test plants, especially at 1:10 concentration inhibited root length completely. The Wedelia trilobata (L.) Hitchc. extracts at 1:10 ratio also inhibited rice (Oryza sativa L.) RD 43 and mung been (Vigna radiata (L.)) seed germination, meanwhile the extracts at all ratios decreased shoot and root lengths in rice and mung been. In the foliar application tests of the Wedelia trilobata (L.) Hitchc. extracts to marigold (Tagetes patula L.) seedlings every 5 days in pots for a period of 8 weeks., it was found that the after foliar application decreased root lengths of the marigold plant. However, the extracts also increased shoot lengths, chlorophyll a, chlorophyll b and total chlorophyll contents of marigold plant, particularly at the high extraction ratios.