Allelopathic Phenomenon of Eucalyptus (Eucalyptus camaldulensis) in Taiwan

River red gum (Eucalyptus camaldulensis), native to Australia, has been introduced into Taiwan in the past decade for the purpose of paper manufacturing. Field survey of weed flora inside and outside of this eucalyptus forest area and subsequent seed bank investigation of soils collected from the forest showed that the germination and growth of several indigenous weed species were suppressed by eucalyptus trees, possibly due to competition, shading and/or allelopathic effect. Seedling test of seven weed species at 5 to 7-leaf stage cultivated in medium premixed with different amounts of eucalyptus leaf powder showed that the growth of three weeds of Amaranthaceae family were significantly retarded by this powder at 17% (v/v) rate, and the injury index of slim amaranth was dramatically increased even at a rate of 9% (v/v). In addition, sedge weeds were also affected, though at a less extent, and Gramineae spp. weeds were the least susceptible. Seed germination of edible amaranthus and some Brassica spp., such as field mustard, Fung-Shan cabbage and radish, was significantly inhibited within 2 to 3 days by the water extract of eucalyptus leaves. In addition, the radicle growth of five vegetables was all significantly inhibited more than ca. 70% by this water extract. The allelopathic potential of this eucalyptus species may be worth considering in the development and/or application of bioherbicides in organic farming system.