Competition between Trifolium subterraneum L. and established seedlings of Hypericum perforatum L. var. angustifolium DC

Abstract

St John's Wort (Hypericurn perforatum) and Trifolium subterraneum were grown in mixtures and monocultures in 16 planting combinations. The experiment was biased to favour H. perforatum by establishing it 49 days prior to sowing T. subterraneum seed and by growing plants in a nutrient-rich medium.In monocultures maximum dry matter yields of H. perforatum after 188 days were half those of T. subterraneum at similar densities. Leaf areas of the two species were similar at all harvests. In mixtures, a single T. subterraneum plant completely suppressed H. perforaturn growth and caused extensive mortality, even at its highest density 16 plants per pot within 140 days, the period of the experiment. Increasing the numbers of H. perforaturn plants in mixtures had little effect on the growth of a single T. subterraneum plant. Final yields of T. subterraneum in such mixtures were similar to those of comparable densities in monocultures.The competitive superiority of T. subterraneurn in mixtures was attributed to its canopy height which overtopped most H. perforaturn leaves. The consequent reduction in photosynthesis and subsequent death of lower leaves of H. perforaturn contributed to its lower competitiveness and mortality in mixtures. The application of these findings to agronomic practices designed to prevent re-establishment by H. perforaturn is discussed.