The spatial distribution of tillers within a pure grass stand and fluctuation over time result from an interaction between environmental factors and tiller population behaviour. Simulating the latter would provide a tool for ecological analysis. The population behaviour is composed of death, displacement and birth of tillers, the birth coming from clonal processes in perennial grasses. On each tiller, every time a new leaf becomes adult, its axillary bud can grow, according to an evaluation of the environment and of the neighbourhood. If the ratio of red to far-red light is low enough and if the level of available nitrogen is high enough, the bud is put in growth. Otherwise, it becomes dormant. After a gestation period, a new daughter tiller is born close to its mother. Other new tillers can be born farther from an existing tiller, after growth of dormant buds through the old sheaths. Most tillers die at vegetative stage, mainly when they are young and when they are dominated by the canopy. On this basis, an individual-based, discrete events and spatial simulator was built. The space is managed both by indices of cells and by continuous coordinates. Environmental conditions are given by external files. The tillering is mechanistic, according to the positions of the neighbouring tillers and to environmental data. The death is stochastic. Tillers are placed and move according to given rules. The simulator accurately reproduced the fine-scale spatial heterogeneity of the local tiller densities in a pure tall-fescue sward observed eight times throughout 2 years. In spite of necessary improvements, this modelling shows the determining part of plant processes in the observed heterogeneity and in its dynamics.