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Seriphium plumosum encroachment in South Africa has converted extensive areas of grassland into less productive shrubland-grassland. In addition to its competitive ability, it is hypothesised that the encroachment of S. plumosum is linked to the allelopathic potential of the plant. Interference between S. plumosum and four plant species (Eragrostis curvula, Festuca arundinacea, Solanum lycopersicum and Lactuca sativa), and between mature shrubs and seedlings of S. plumosum, was investigated in the field and greenhouse. The germination of all species was suppressed (P < 0.01) by both high-concentration and low-concentration S. plumosum extracts derived from air-dried or fresh material, with fresh material having a greater effect than air-dried material on the germination of dicot species. Seeds of the grass species were less sensitive to the extract than those of the dicot species. Seriphium plumosum seeds had a mean germination of only 68% after six weeks, a dormancy period of 12 months for optimal germination and 48% germination after three years. The allelopathic potential of S. plumosum was neutralised after 10 weeks when living shrubs were removed from the soil and >15 weeks when dead plant material remained on the soil. These findings indicate that S. plumosum can inhibit the recruitment of other plant species under natural conditions.