A Monte Carlo type model describing dynamics of three pairs of annual plants living in a homogeneous habitat is presented and discussed. Each plant follows its own history with growing, fecundity and survival chances determined individually as functions of the plant's condition and environment. The three plants - Valerianella locusta, Mysotis ramosissima and Cerastium semidecandrum differ by the weight of their seeds, which in the model determines the competition preference. Heavier seeds have a better chance for germination from a site containing seeds of different plants. Better colonisers produce more seeds and disperse them over a larger distance. I show that without absolute asymmetry in the impact effects between better competitors and better colonisers and in a spatially and temporarily homogeneous habitat, coexistence of species is possible, however only in a limited time. This is different from statements coming from models using mean-field type methods. I demonstrate also that in a system of two species clustering of plants of the same type are more frequent. From the calculated survival chances of seedlings and adult plants it follows that elimination of plants occur mostly at the early stages of the plants life cycle, which agrees with the field data. I show that this competition/colonisation trade-off model is sufficient to maintain coexistence and I determine the conditions for dominance of one type of plants.I show that the time of extinction of the weaker species goes down with increasing observation time as a power function with the exponent independent of the type of plants.