Phenotypic integration and developmental canalization have been hypothesized to constrain the degree of phenotypic plasticity, but there is little evidence for the relationships among the three processes in different environments, especially for plants under natural conditions. To address this issue, we conducted a field experiment by subjecting plants of Abutilon theophrasti to low, medium and high densities, under infertile and fertile soil conditions, measured a variety of traits and analyzed canalization (coefficient of variation [CV]), integration (coefficient of integration [CI] and the number of significant correlations of a trait with other traits [NC]), and plasticity (REL RDPIs and ABS RDPIs) in these traits and their relationships at two stages of plant growth. Our results showed an increase in mean CV, NC and ABS RDPIs of traits with density, and the positive correlations between trait NC and ABS RDPIs became stronger with higher densities but weaker over time in fertile soil, while correlations among trait CV, NC and ABS RDPIs became stronger over time in infertile soil. Results suggested shared or cooperation mechanisms among phenotypic integration, canalization and plasticity. Soil conditions and growth stage may affect responses of these correlations to density via modifying plant size and competition strength. The attenuated canalization and enhanced integration may be helpful for the production of plasticity, especially under intense competition.