The performance of a model for simulating increase in leaf area index (L) was evaluated for potato (Solanum tuberosum L.) and wheat (Triticum aestivum L.) cultivars across environments (years and sites). Rate of L expansion just after emergence was assumed to depend on temperature. After a predefined L, Ls, expansion was assumed to increase in proportion to leaf dry weight increase that depended on intercepted radiation, henceforward: radiation-limited expansion. The Ls value at which the model performed best was considered to be the most realistic L at which expansion shifts from temperature to radiation-limitation. An Ls value of zero leads to solely radiation-limited expansion, whereas a value larger than maximum L leads to solely temperature-limited expansion. The criteria used to evaluate the model were constancy of calibrated model parameters across environments, and predictive ability. For potato and wheat, parameters were most robust across environments, when Ls was neither zero nor at maximum L. Model parameters did not vary with genotype. The model’s predictions were best at an Ls of 1.0 for potato and 1.5 for wheat. Using these Ls values, the coefficient of determination between observed and predicted values was 91% for potato and 88% for wheat. Sensitivity analysis revealed that smaller Ls values led to larger changes in rate of leaf area expansion and crop dry weight than larger values did. Crop dry weight was hardly affected by an increase in Ls. Implications of the results for modeling are discussed.