Acacia melanoxylon is a naturalized Australian species that has invaded woodlands and degraded natural habitats in the north western Iberian Peninsula (Galicia) in Spain. Several phenolic (p-hydroxybenzoic, vanillic, p-coumaric, syringic, protocatequic, ferulic acids) and flavonoides (catechin, luteolin, rutin, apigenin, quercetin) were isolated from methanol extracts of flowers and phyllodes of A. melanoxylon by HPLC. Seeds of three native plants, Dactylis glomerata, Lolium perenne, Rumex acetosa and a crop Lactuca sativa were grown in perlite culture and aqueous extracts of Acacia (flowers and phyllodes) were applied exogenously at 100%, 75%, 50% and 25% concentrations. Acacia flower extract (100%) inhibited the shoot length in D. glomerata, L. perenne by 31%, and 20% of the control, respectively. Both extract reduced leaf osmotic potential in D. glomerata, L. perenne and L. sativa. Efficiency of open PSII reaction centers (Fv/Fm), quantum yield (ΦPSII), chlorophyll fluorescence quenching (qP) and non-photochemical quenching (NPQ) were decreased in L. perenne, D. glomerata, R. acetosa and L. sativa after treatment with flowers/phyllodes (100%) extract. Leaf protein contents in D. glomerata, L. perenne and L. sativa were decreased due to flower and phyllodes (100%) extract.