Landscape trees are planted in commercial parking lot medians to provide shade as well as to enhance landscape environmental aesthetics. However, expansive areas of asphalt and concrete surfaces surrounding parking lot medians might expose parking lot trees to high temperatures that reduce tree growth and aesthetic potential. We studied the effect of commercial parking lots on the size of established trees of Brachychiton populneus Schott & Endl., Fraxinus velutina Torr., Pinus canariensis Sweet ex K Spreng, Pinus halepensis Mill., Prosopis chilensis (Molina) Stuntz, and Ulmus parvifolia Jacq., in Phoenix, AZ, USA. During Summer 2001, tree size and temperatures of ground surfaces under and near tree canopies were evaluated in parking lot medians and adjacent perimeter landscape beds at 15 commercial parking lots. For all taxa, mean canopy volume, height, and diameter at breast height were reduced by 64, 32, and 37%, respectively, compared with trees of the same taxa in adjacent perimeter landscape bed. Overall, size of P. halepensis and U. parvifolia was most negatively affected by parking lot medians, while size of P. chilensis was least affected by parking lot medians. Average mid-day summer temperatures of asphalt surfaces under and near tree canopies in parking lot medians approached 60oC (140oF) and were as much as 27oC (49oF) higher than surface temperatures of vegetated and non-vegetated surfaces in adjacent perimeter landscape beds.