The GUESS (Greenhouse Use of Energy Seedling Simulator) model is a lumped-parameter coupled dynamic simulation combining a carbon-based process model of seedling growth with a heat/mass transfer model of the greenhouse envelope. GUESS was created to provide nursery managers and engineers a tool to assess the impact of climate control decisions upon energy use and seedling growth rate. The source code for GUESS was written using Simulink and MATLAB. A test case was set up to validate the model. And experimented with different lighting targets to see which one offered the most growth per unit energy cost. The model was parameterized for Douglas fir production in Corvallis, OR Seedlings were started at 0.57g d.w and were harvested at 1.7g d.w Temperature regulated to 68.5±6.3°F Growth rate could be highly sensitive to carbon content conversion as well.Models can be very useful as simulation tools, but their utility depends highly upon the data used to parameterize them.