To study the effects of competition on growth and variability, dwarf marigolds (Tagetes patula) were grown in a greenhouse for 11 weeks, (1) individually in pots, and (2) at a density of 1500 individuals m^-2 in flats. Both sequential harvests and non-destructive measurements were performed weekly. Relative growth rate decreased over time for both crowded and uncrowded populations. The growth in d. wt of crowded populations was close to linear, and the growth of uncrowded plants was intermediate between linear and exponential. The leaf area of crowded individuals reached a maximum after 6 weeks, but the leaf area of uncrowded plants continued to increase throughout the course of the experiment. Size variability increased greatly and then decreased somewhat in the crowded populations, whereas the size variability of uncrowded populations remained low over the entire period of growth. From the 3rd to the 11th week, growth increment was correlated with initial size within crowded populations, but there was no evidence of a relationship between initial size and growth increment for uncrowded individuals. Similarly, size rank at 3 weeks was highly correlated with rank at 11 weeks in crowded, but not uncrowded, populations. Our results point to the limits of relative growth rate as a descriptor of plant performance, and emphasize the need to develop models of plant competition and stand structure which are: (1) based on the behaviour of uncrowded plants, and (2) consistent with observed plant growth curves.