Growing Degree Days (GDD) or effective heat units or growing degree units are heuristic tool in phenology. It is a simple means of relating plant growth, development and maturity to air temperature. This concept is widely accepted to estimate or predict the lengths of the different phases of development in crop plants. Development refers to the timing or progress of the crop from one stage of maturity to the next. During this progress of the crop through its growth may occur (Gardener et al. 1985). Unless stressed by other environmental factors like moisture the development rate from emergence to maturity for many plants depends upon the daily air temperature. Plants require a specific amount of heat to develop from one point in their life-cycle to another. Temperature is the driving force for all biological activity. Consequently, the growth, development and reproduction of many organisms are predictable based on temperature. A growing degree day (GDD) is a measure of heat above a threshold for that day. Growing degree accumulation reflects the number of GDDs or heat units above a base temperature for consecutive 24-hour days. GDD can be used to assess the suitability of a region for production of a particular crop, estimate the growth stages of the crop, predict maturity, cutting dates of forage crops and timing of fertilizer application and to estimate the heat stress on crops. GDD can also be used to more effectively indicate suitable seeding dates as well as to assit in staggering planting dates in order to insure more orderly harvests (Edey 1977). In this study GDD for cotton crop was assessed periodically. The temperature data was collected from Annamalai University meteorological observatory during the period of January-June 2018. Growing Degree Days at different growth stages of cotton were calculated during crop period. Highest GDD was accumulated at the flowering stage (1034.7 days0C ) followed by maturity stage (705 days0C)
