| Summary: | Using growing degree days (GDD) for expression of duration of plants’ developmental stages is preferred over calendar time. Therefore, determination of GDD and zoning of regions is very important for on time and correct agronomic practices and also for better land use. To determine the required GDD for developmental stages of spring safflower cultivars, the data obtained from planting-date experiments in two years at Kabotar Abad Agricultural Research Station of Isfahan, was used. In each year, three cultivars of Arak, Goldasht and Zendehrud were evaluated in 8 planting dates. Zoning of the province was done by calculated GDD and temperature data of 51 synoptic and climatologic stations of Isfahan province and neighboring provinces. Interpolation of GDD was performed in geographic information system (GIS). Based on the first suitable planting date in every region and the required GDD, pertinent maps were drawn in GIS. Results showed that the required mean GDD for planting to emergence, planting to heading, planting to flowering and planting to maturity were 152, 924, 1275 and 1972, respectively. Zoning of the province showed that if safflower is planted at suitable date, GDD for all developmental stages will be provided in all parts of the province. For this purpose, by considering the plant's base temperature and required temperature for emergence of seeds in soil in every region, suitable planting date must be chosen.
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