High Tunnel Management for Early Ripening of Sweet Cherries: Bringing Hawaii to Michigan?

The adaptation of high tunnels for sweet cherry (Prunus avium L.) production in continental temperate, rainy climates can provide a number of potential benefits, foremost being protection from rain-induced fruit cracking. In recent years, we have demonstrated that high tunnels also can improve tree growth and facilitate high fruit quality, as well as provide protection from some insect pests and rain-disseminated diseases, key advantages that have advanced the potential for organic production systems. Sweet cherry requires a period of chilling temperatures during winter for normal annual growth and fruiting; consequently, the three-season high tunnels typically used are not covered during winter, to facilitate normal seasonal cold acclimation and dormancy (and avoid snow loads). However, we have been investigating the feasibility of early covering for potential season extension, to promote earlier bloom and ripening for premium market opportunities. In 2009, with sequential covering of high tunnel orchards beginning in late winter, we demonstrated significant and sequential advancement of bloom and ripening, by 11 and 8 days, respectively, across several varieties. In 2010, ripening was advanced into the market window supplied by California production areas, well ahead of the usual arrival of cherries from Washington and Oregon. However, earlier flowering is accompanied by an increased risk of crop damage by low temperatures. In 2011, we examined the use of supplemental heat from propane convection heaters to protect developmentally advancing flower buds from low spring temperatures. Temperature gradients were mapped from the orchard floor to the tunnel apex and along the length of the 62 m (200 ft) tree rows. More than one week of subfreezing temperatures, some as low as -8˚C (17˚F), provided multiple opportunities for frost protection experiments. Results varied based on heater design and output, with tunnel air temperatures within the canopy being raised by 1.6 to 3.3˚C (2 to 6˚F). The use of heater BTU specifications vs. tunnel space to estimate potential air temperature increases above outdoor ambient will be presented, as will estimates of growing degree requirements for predicting the progressive stages of cherry bud development during early covering. The historic probability dynamics for the occurrence of bud stage-specific critical temperatures during March-April also were developed. The management of sweet cherry high tunnels in a cold weather climate like that of Michigan requires knowledge of these factors and the development of multiple strategies relative to production risks vs. benefits during late winter and early spring.