Fall planting of spinach (Spinacia oleracea L.) into high tunnels for harvest from late fall through early spring is widely practiced in the northeastern United States, but replicated studies focusing on this production system are lacking. The objectives of our study were to understand the effect of fall planting date (PD) and cultivar on yield and soluble solids content (SSC) of spinach. Three cultivars (Regiment, Space, and Tyee) were transplanted in unheated high tunnels in Durham, NH, in 2014–15 (Year 1) and 2015–16 (Year 2) at six different fall dates: 20 Sept., 30 Sept., 9 Oct., 19 Oct., 30 Oct., and 9 Nov. Five additional cultivars (Carmel, Corvair, Gazelle, Emperor, and Renegade) were included at the third date (9 Oct.) to compare yield and SSC among cultivars during winter months. A randomized complete block design with four replications was used for all experiments. Year and fall transplant date had a significant effect on total yield. Total yield of Year 2 was nearly double that of Year 1 for all PDs and cultivars. In both years, total yield decreased with later planting, such that yield from the 20 Sept. date was greater than a minimum of three of five subsequent PDs, depending on year. Total yield in spring (January through April) did not differ among the first four PDs in Year 1 or among any dates in Year 2, suggesting that a wide range of PDs will work well for those primarily interested in spring harvests. Combined analyses of the data from both years showed no significant differences in total yield among the eight cultivars planted only on 9 Oct. However, of the three cultivars grown at all PDs, Regiment produced significantly higher yields than Tyee. Harvest date, cultivar, and harvest date × cultivar affected leaf and petiole sap SSC in both years. SSC was most strongly negatively correlated with air and soil temperatures at a 10-day interval in Year 1 (R2 = 0.61 and 0.64, respectively) and a 7-day interval in Year 2 (R2 = 0.78 and 0.69, respectively). ‘Gazelle’ and ‘Emperor’ contained among the highest SSC in both years. Our work demonstrates total yield is highly dependent on fall PD and the growing conditions of a given year.