Seasonal growth curves of sugar beet (Beta vulgaris L.) were simulated by a CSMP computer model called SUBGRO. The hypothesis on which the simulations were based was a hierarchy of priorities for photosynthate partitioning. In order of importance, these priorities were as follows: respiration, top growth, fibrous root growth, and storage root3 growth including sucrose acculation. Simulations agreed well with field observations when these priorities were used. Simulated growth did not match field observations if the sequence was changed. The rate of use by each sink was further regulated by its growth potential and by its environment. Functional leaf and fibrous‐root surfaces played key roles in regulating the growth rates through a quantitatively expressed set of partitioning functions. To test the partitioning hypothesis, simulated recovery patterns from partial defoliation or root pruning were compared to experimental observations. Simulated recovery was similar to that of real plants.