Lettuce (Lactuca sativa) is an economically important leafy green widely cultivated in greenhouses, yet the interaction between cultivar and air temperature remains poorly characterized for many of the cultivars currently marketed for controlled environment (CEA) production. Even in climate-controlled greenhouses, internal air temperatures can exceed general recommendations, leading to bolting, excessive stem elongation, bitter flavors, and reduced yields. This study aimed to generate benchmark yield and morphological data for 20 lettuce cultivars grown hydroponically in a greenhouse during a fall (20 °C mean air temperature) and summer (28 °C mean air temperature) production cycle, with harvests at 9 (juvenile stage) and 21 (mature stage) days after transplanting (DAT). Lettuce cultivars were grown in a common nutrient film technique (NFT) system with average pH and electrical conductivity (EC) of 5.6 and 1.5 dS·m−1, greenhouse day and night air temperature setpoints of 21 °C and 18 °C, and a target average daily light integral (DLI) of 17 mol·m−2·d−1. The experiment was set up as a randomized complete block design with two blocks. Depending on the cultivar, air temperature, and harvest time, lettuce shoot fresh mass (SFM) and projected canopy area (PCA) were significantly different. However, regardless of the cultivar or harvest time, yield (kg·m−2·year−1) was always greater at 20 °C than 28 °C. Supraoptimal air temperatures increased stem and leaf elongation, potentially improving light interception and SFM per plant early in production, but required more area per plant to grow without significantly overlapping with neighboring plants, thus reducing planting density and yield potential. Supraoptimal air temperatures decreased specific leaf area (SLA), resulting in thicker leaf lamina. Chlorophyll concentration was more affected by cultivar than harvest date or air temperature. Benchmarking greenhouse lettuce yield and morphology across cultivars and seasons offers growers a valuable tool to reliably assess productivity, select appropriate cultivars, and adjust planting density. These insights also inform breeding efforts to improve the yield from CEA systems, with particular attention to plant architecture and leaf traits suitable for automated harvesting and packaging in greenhouse systems.
