Fall Planting Date and Production System Effects on Broccoli Cultivar Adaptability and Yield in North Dakota

Fall production can extend the broccoli (Brassica oleracea var. italica) season, but suitable cultivars, transplanting dates, and the value of high tunnels are uncertain. Six cultivars were transplanted on four dates (10 July–25 August) in a high tunnel and the open field in Fargo, North Dakota, in 2023 and 2024, and growth, physiology, roots, yield, and marketability were analyzed with mixed models including year. Romanesco formed no curds at any planting date and is not suited to fall production. Jacaranda produced the heaviest heads (592 g) but formed heads on 53% and marketable heads on 48% of transplanted plants; Green Magic, Monty, and Gypsy reached 77–78% and 70–75%, respectively. Marketable yield per transplanted plant was highest for the 10 July (365 g plant−1) and lowest for the 25 August planting (87 g plant−1). Days from transplanting to the first frost injury were closely correlated with head formation (r = 0.74) and yield per transplanted plant (r = 0.81). The high tunnel delayed frost injury by 21 days and increased marketable heads, mainly at late plantings, but its marketable-yield advantage depended on the season. Early transplanting of these three reliable cultivars is recommended in the Northern Great Plains.