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Allelopathy is one of the mechanisms that help to explain the invasion success of some plant species. Invasion of Phragmites australis through allelopathy is not robust enough to provide reliable information that could integrate the existing knowledge to sound on-ground reality. This study analysed the chemical characteristics of soil and water and monitored both over four seasons taking into consideration the phenological cycle of P. australis in the field. A series of bioassays has been conducted to test phytotoxicity using field concentrations of allelochemicals on different plant species in laboratory. Significant changes in soil and water chemistry were observed in invaded area compared to uninvaded area. Soil-water (rhizosphere and surface) and whole plant-leachate significantly inhibited germination and α-amylase activity of Lactuca sativa as well as the adventitious root formation of Phaseolus aureus. Seasonal impact on allelopathic interference of P. australis in terms of germination and growth of L. sativa, Melaleuca ericifolia, and Poa labillardierei showed distinct but inconsistent variation. Soil biota played an important role in reducing the phytotoxicity in natural soil. In addition, P. australis infested soil showed a lower arbuscular mycorrhizal fungus inoculum potential in terms of Zea mays roots colonization. A synthesis from field studies and associated laboratory experiments may provide a more logical understanding towards invasion mechanisms of P. australis through allelopathy, leading to more realistic management decisions.

Australia is facing challenges in controlling Chrysanthemoides monilifera subsp. monilifera (boneseed). However, burning has achieved some success in this regard. We aimed to investigate the comparative phytotoxicity of boneseed dried powder and ashes (burnt at 450°C and 250°C). Phenolic compounds in powder and ashes were measured using Folin-Ciocalteu assay and HPLC. The phytotoxicity of boneseed powder and ash extracts was assessed through germination bioassay on Lactuca sativa and the phytotoxicity of litter and ashes was evaluated using field soil, both in growth chamber. Burning of boneseed reduced total phenolics in ashes of boneseed organs by 99% and 100% both at high and low temperatures. The four phenolic compounds that were detected in boneseed were either absent or at negligible levels in the ashes, with inversely related to temperature. Both boneseed ash extracts and litter ash-mediated soil significantly reduced phytotoxicity displaying increased germination, biometric and biochemical parameters of test species compared with unburnt powder extracts and litter powder-mediated soil respectively, with greater reduction of phytotoxicity found for ashes produced at the lower temperature. Interestingly, the ash extracts and litter ash-mediated soil were found to stimulate some of those parameters of the test species compared to control. There was no excessive reactive oxygen species (ROS) produced in test species exposed to ash extracts compared with unburnt powder extracts. These findings suggest that burning of boneseed is an appropriate method of weed control and that this approach will reduce phytotoxicity of this species on native plants.