Conductance to CO₂ inside leaves, known as mesophyll conductance (g_m), imposes large limitations on photosynthesis. Because g_m is difficult to quantify, it is often neglected in calculations of ¹³C photosynthetic discrimination. The ‘soluble sugar method’ estimates g_m via differences between observed photosynthetic discrimination, calculated from the δ¹³C of soluble sugars, and discrimination when g_m is infinite. We expand upon this approach and calculate a photosynthesis‐weighted average for canopy mesophyll conductance (^cg_m) using δ¹³C of stem phloem contents. We measured gas exchange at three canopy positions and collected stem phloem contents in mature trees of three conifer species (Pseudotsuga menziesii, Thuja plicata and Larix occidentalis). We generated species‐specific and seasonally variable estimates of ^cg_m. We found that ^cg_m was significantly different among species (0.41, 0.22 and 0.09 mol m⁻² s⁻¹ for Larix, Pseudotsuga and Thuja, respectively), but was similar throughout the season. Ignoring respiratory and photorespiratory fractionations (^cΔ_ef) resulted in ≈30% underestimation of ^cg_m in Larix and Pseudotsuga, but was innocuous in Thuja. Substantial errors (∼1–4‰) in photosynthetic discrimination calculations were introduced by neglecting ^cg_m and ^cΔ_ef. Our method is easy to apply and cost‐effective, captures species variation and would have captured seasonal variation had it existed. The method provides an average canopy value, which makes it suitable for parameterization of canopy‐scale models of photosynthesis, even in tall trees.