Background and Aims Many notorious alien invasive plants are clonal, but little is known about some roles and aspects of clonal integration. Here, the hypothesis is tested that clonal integration affects growth, photosynthetic efficiency, biomass allocation and competitive ability of the exotic invasive weed Alternanthera philoxeroides (Amaranthaceae). Methods The apical parts of Alternanthera were grown either with or without the lawn grass Schedonorus phoenix (tall fescue) and their stolon connections to the basal parts grown without competitors were either severed or left intact. Key Results Competition greatly reduced the maximum quantum yield of photosystem II (F-v/F-m) and growth (biomass, number of ramets and leaves, total stolon length and total leaf area) of the apical Alternanthera, but not the biomass of S. phoenix. Stolon connections significantly increased F-v/F-m and growth of Alternanthera. However, such effects on growth were smaller with than without competition and stolon connections did not alter the relative neighbour effect of Alternanthera. Stolon connections increased Alternanthera's biomass allocation to roots without competition, but decreased it with competition. Conclusions Clonal integration contributed little to Alternanthera's competitive ability, but was very important for Alternanthera to explore open space. The results suggest that the invasiveness of Alternanthera may be closely related to clonal integration.