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The Thermal Constant (TC) is the units of accumulated heat that a plant needs to complete its cycle, since temperature is one of the most decisive climate elements in plant development. This research was done with the purpose of defining the TC of Hylocereus undatus, unknown value for this species. The reproductive phenology of three selections of dragon fruit (‘Ana’, ‘Juana’ and ‘Carlos’) was studied in the 2013 and 2014 cycles, established on the INIFAP Campo Cotaxtla field in Medellín, Veracruz, México. The register dates of the phenological phases were determined: flower budbreak, anthesis, fruit set, and fruit maturity, as well as the duration in days of the phenological stages (budbreak-anthesis, anthesis-fruit maturity, and complete period). The TC was calculated in degrees-day-development (DDD), using the residual method. The reproductive period of the dragon fruit began after the maximum temperature values were found (<34 °C). The TC in each phenological stage did not vary statistically among fluxes, cycles, or among selections. For the stage of budbreak-anthesis, the TC was 360 DDD, for anthesis-fruit maturity the TC was 537 DDD, therefore, to complete the budbreak-fruit maturity period, the dragon fruit took 897 DDD.
