Abstract
Seeds of 'Sunbright' sunflower (Helianthus annuus L.) were planted weekly throughout the year as part of an overall scheduling experiment. Plants grown from seeds and planted on 12 December 2001 through 30 January 2002 were exposed to naturally occurring frost/freezing temperatures during 27-28 February and on 1 March 2002. The purpose of this paper is to report on the observations made on these plants after they made additional growth and flowered subsequent to the frost/freeze events. These temperatures where not low enough to kill the plants, but were cold enough to kill differentiated flower buds. Vegetative buds were not damaged. Thus, sunflower plants that had set terminal and lateral flower buds and had flower buds in different stages of maturity were damaged by the freeze, resulting in an altered plant morphology. Mature plants with flower buds within days of opening were most severely damaged while immature plants with only vegetative buds where not damaged by the freeze event. However, those plants with dead differentiated terminal and lateral flower buds continue to grow vegetative lower lateral buds that were not damage by the frost/freeze events as they were vegetative at that time and subsequently flowered with short lateral stems very low on the main stem. While all plants with differentiated terminal buds had the buds killed, plants from planting dates closer to the frost/freeze events had more vegetative buds higher up the main stem. Plants with a small differentiated terminal bud were only thermally pinched resulting in a well branched plant with long lateral stems and numerous small marketable flowers from the surviving vegetative buds. Plants that were all vegetative at the time of the frost/freeze events produced single stem flowers without damage.
