The present study investigated LL in plant canopies by using field measurements to constrain the Warland and Thurtell (2000) (WT2000) analytical Lagrangian equation. Measurements of two sources, the net and soil CO2 fluxes, along with in-canopy profiles of CO2 concentrations taken in a cornfield and a mixed forest near Toronto, Canada, provided the information required to solve for LL in each location. This nonlinear system of equations is unstable and capable of producing an infinite number of solutions. A test of the optimization with wind tunnel data of turbulence statistics, a known heat source, and temperature profiles from Coppin et al. (1986) shows that the parameter estimation routine is capable of closely estimating the measured Eulerian length scale. The optimization with the cornfield data found a general Gaussian-shaped LL profile for the majority of the summertime measurement intervals. Magnitudes of the optimized LL varied across each half hour interval and did not scale with any measured statistic. This suggests that either (a), there are factors influencing the value of LL that are unrelated to Eulerian measurements; or (b), other processes may be working to relate the sources and concentrations that are not captured by the WT2000 equations. Results from the optimization for the forest canopy were less clear than the corn canopy.