Three isoflavanones, 5,7,20 ,40 -tetrahydroxy-6-(3-methylbut-2-enyl)isoflavanone (1), 400,500-dihydro-5,20 ,40 -trihydroxy-500-isopropenylfurano-(200,300;7,6)-isoflavanone (2) and 400,500-dihydro-20 -methoxy-5,40 -dihydroxy-500-isopropenylfurano-(200,300;7,6)-isoflavanone (3) and a previously known isoflavone 5,7,40 -trihydroxyisoflavone [genistein (4)] were isolated and characterised spectroscopically from the root exudate of the legume Desmodium uncinatum (Jacq.) DC. We propose the names uncinanone A, B, and C for compounds 1, 2 and 3, respectively. Isolated fractions containing uncinanone B (2) induced germination of seeds from the parasitic weed Striga hermonthica (Del.) Benth. and fractions containing uncinanone C (3) moderately inhibited radical growth, the first example of a newly identified potential allelopathic mechanism to prevent S. hermonthica parasitism.