Variation in the Onset of Flowering of Summer Squash as a Function of Days and Heat Units

The time from sowing to flowering and maturity of summer squash ( Cucurbita pepo L.) varies depending on environment. Research was conducted over a 2-year period to assess flowering time of five squash cultivars grown at different locations and times in Georgia representing varying thermal regimes. Heat units (HUs) were calculated using a single equation to determine if this approach could account for a significant proportion of the variability in the time to onset of flowering observed over this range of environments. Depending on the cultivar and flower sex, the number of days to flowering varied as much as 20 days. There were no cultivar differences in overall mean days to first staminate flower; however, there were differences in mean days to first pistillate flower. There were cultivar differences in HUs required for staminate and pistillate flowers. The use of HUs significantly affected the variability in time to flowering as indicated by regression analyses and mean absolute differences between predicted and observed days to flowering. Summer squash (Cucurbita pepo L.) are produced extensively in the southeastern United States for processing and fresh market. In Georgia alone, >11,000 ha are grown annually, and planting ranges from early spring through the fall (Mizelle, 1993). This environmental diversity results in variable days to maturity for the crop; thus, it is difficult for growers to plan planting dates to meet predesignated harvest windows. Research is needed on variability in development of squash cultivars produced in different environ- ments. Identifying the environmental variables that strongly influ- ence development may improve production planning. Temperature is a major environmental variable influencing plant development, and heat units (HUs) or growing degree days are used extensively to normalize time to crop developmental events in response to temperature (Ritchie and NeSmith, 1991). The HU approach is used for a variety of vegetable crops including sweet corn (Zea mays L.), peas (Pisum sativum L.), tomato (Lycopersicon esculentum Mill.), lettuce ( Lactuca sativa L.), and cucumber (Cucumis sativus L.) (Arnold, 1959; Boswell, 1929; Logan and Boyland, 1983; Madariaga and Knott, 1951; Perry et al., 1986). Generally, these experiments concentrate on the time between sowing and maturity. For fresh vegetable crops such as cucurbits, harvest maturity is an ambiguous designation and may significantly vary assessment of the time between sowing and maturity. A more finite developmental indicator such as flowering may be more useful (Perry and Wehner, 1990). For this research, it was hypothesized that temperature is the primary environmental determinant of the time from sowing to flowering in summer squash. The objectives of this research were to 1) evaluate the number of days to flowering for several squash cultivars planted at different dates and locations representing varying thermal regimes and 2) determine whether HUs significantly affect the variability in the time to flowering over a range of environments. Materials and Methods 'Dixie', 'Senator', 'Elite', 'Lemondrop', and 'XPH-1671' squash were grown in 12 experiments over a 2-year period to collect data concerning time to flowering. 'Dixie', 'Lemondrop', 'Senator', and 'XPH-1671' are Asgrow lines, and 'Elite' is a Harris Moran line. 'Dixie' and 'XPH-1671' are yellow crook-neck types, 'Lemondrop' is a yellow straight-neck type, and 'Senator' and 'Elite' are zucchini types. This study was conducted at three Georgia locations: Blairsville (34 o 51'N, 83 o 56'W, 586 m eleva- tion); Griffin (33 o 15'N, 84 o 16'W, 299 m elevation); and Tifton (31 o 29'N, 83 o 32'W, 113 m elevation). Table 1 lists the location, dates of sowing, cultivars grown, and mean temperature of each experiment the first 60 days after sowing. The soil types were a Cecil sandy loam (clayey, kaolinitic, thermic Typic Hapludult), a Transylvania clay loam (fine-loamy, mixed, mesic Cumulic Haplumbrept), and a Tifton sandy loam (fine-loamy, siliceous, thermic Plinthic Paleudult) at Griffin, Blairsville, and Tifton, respectively. The experiments used similar cultural practices. Row width was 0.9 m and in-row plant spacing was 0.6 m. Cultivars