THE EFFECT OF TEMPERATURE ON YEAR-ROUND DEVELOPMENT OF ROSE SHOOTS INITIATED USING CUTTING OR BENDING

Abstract

The development of greenhouse-grown cut roses (Rosa hybrida ‘Kardinal’ and ‘Fire ‘N Ice’) in relationship to temperature was tracked during nine cycles of growth from December 2003 to May 2005. Hourly leaf canopy air temperatures were monitored continuously and used to compute the accumulated thermal units to each of five developmental stages. During cycles 1-3 all shoots on a plant were cut back, while during cycles 4-7 shoots were cut back or bent to initiate a new flush of shoot growth. For cycle initiation via cutting versus bending there was a significant difference in the thermal units required for development in only one of the four cycles. Results suggest that a thermal unit model developed for ‘Kardinal’ or ‘Fire ‘N Ice’ can be applied whether cutting or bending is used for shoot initiation. The thermal units from cycle initiation to harvest were greatest during cycles 1 and 2 (750-770), significantly less for cycles 5 and 6 (680-700) and least during cycles 3, 6, and 7 (615-625) for ‘Fire ‘N Ice’. For ‘Kardinal’ the thermal units required from cycle initiation to harvest were 680700 for cycles 1, 2, and 7 when mean daily light integral was less than 17 mol m -2 d -1 . While for cycles 3 to 6, the accumulated thermal units to harvest was significantly less, averaging 615 °C d -1 when mean daily light integral ranged from 20 to 33 mol m -2 d -1 . In cycles 8 and 9 plants were grown under 3 light levels to test different hypotheses on factors causing seasonal variation in thermal units. The effects of increasing light were largely explained by temperature at shoot level rising proportionally greater than temperature recorded within the leaf canopy. A software program has been developed to aid in rose crop scheduling which uses projected greenhouse temperatures to schedule when cycle initiation and other developmental events should occur.