High-tunnel primocane red raspberry production is beneficial in the North Central Region of the United States where production is dependent upon the length of the growing season. Advantages of growing Rubus iadaeus L. (red raspberries) in high-tunnels include improved climatic conditions, reduced winter-cane injury, decreased disease incidence, improved berry size and quality, advanced fruit maturation and harvest, and a 50% extension of the harvest season. Improved climatic conditions of high-tunnels also have affected primocane red raspberry growth and production negatively, by reducing shoot growth, fruiting lateral development, yield, and berry quality, due to increased PAR and air- and root-zone temperature in high-tunnel systems. With the expansion of high-tunnel primocane red raspberry production in an area with seasonal climatic extremes, such as the North Central Region, there is a need for simple management practices for tunnel environment manipulation. The primary objective of this study was to evaluate the relationship between temperature and irradiance (photosynthetically active radiation, PAR) and their effect on primocane growth, development, and production. Sub-objectives of this investigation were to evaluate the efficacy of shadecloth in reducing PAR and air-and root-zone temperature during high-tunnel primocane red raspberry production, and the potential of soil mulch in reducing temperature of high-tunnel primocane red raspberry at the root-zone depth.

The influences of PAR and root-zone temperature on vegetative and flowering growth responses of primocane red raspberry `Autumn Britten' were examined. Dormant, one-year-old crowns were planted in 2012, under three identical polyethylene-covered tunnels and in a field plot. A split-plot, randomized complete block design was used. Light transmission through the tunnel polyethylene cover was reduced by 17% before treatments were applied. Whole plot target PAR reduction of 50% was achieved with the addition of shadecloth with a 33% shade factor. In both production years, shadecloth was applied in early June and removed when mean PAR fell below 600 micromole·m^-2·s^-1 in late September. At planting, crowns were either mulched with Panicum virgatum L. (switchgrass) mulch groundcover or grown with no mulch.

The combination of the shadecloth and the polyethylene-cover reduced mean tunnel PAR by an average of 49% in the two years of the experiment. Seasonal climatic extremes provided no differences in treatment temperature at the height of cane growing point. Root-zone temperature had a greater effect on primocane growth, cane architecture, and yield components compared to air temperature or PAR. Root-zone temperature was consistently greater in tunnels without shadecloth or nonliving mulch. The combination of reduced PAR from the 33% shadecloth and the presence of the nonliving mulch provided the greatest reduction of temperature at primocane raspberry root-zone depth, and optimized cane height and adventitious lateral count before terminal bud formation. A positive correlation between flower lateral length and number of flowers produced per lateral was found at root-zone temperatures most similar to the field. Similar associations of increased cane density, leaf area and count, and flower lateral count were found with root-zone temperature most similar to the field. Establishment year decreased fall root-zone temperature, as a result of shadecloth or soil mulch, increased cane density and shoot growth the subsequent spring.

Root-zone temperature was the most influential climatic parameter, accounting for 71% of yield variability among treatments in year two, followed by PAR (29%), and air temperature (0%). Primocane yield in high-tunnels was optimized by maintaining root-zone temperature in the low to mid 20 °C range. Fresh berry weight was not influenced by differential PAR and root-zone temperature. Differential climatic conditions of air- and root-zone temperature and PAR did not influence the timing of bud initiation, though time-to-ripening was delayed a result of shading and root-zone temperature up to 23 °C. Sugar content of field-grown red raspberries increased as the season progressed, but soluble solids concentration decreased with the advancement of the harvest season in tunnels.

It is beneficial to manipulate the tunnel root-zone temperatures between flower-bud developmental stages of petal fall and first harvest. For high-tunnel primocane red raspberry production in the North Central Region, optimizing root-zone temperature favors the vegetative to flowering transition, and increases yield as a result of increased cane density, fruiting lateral count and length, and quantity of flowers produced per lateral. Basic manipulation techniques such as applying a nonliving mulch groundcover or obtaining a target PAR reduction of 50% can optimize primocane red raspberry growth of tunnel environments in the North Central Region of the United States.