Determining the Last Effective Bloom Date Utilizing Heat Unit Accumulation in Georgia

A significant number of acres of Georgia cotton are planted behind wheat each year.  Often, planting dates extend into mid- to late-June.  In some cases the crop becomes delayed, especially in dryland environments, and continues to flower well into the season and will not mature.  In many cases, having an understanding of when in the growing season a bloom will typically be able to accumulate enough heat units to mature would help growers make more informed decisions about late-planted cotton .  The existing literature indicates that a boll requires anywhere from 700 to 850 DD60’s to mature.  However, this work was conducted in normal growing months, when heat unit accumulation is much faster than in September and October in Georgia.  The last effective bloom date in Georgia has been traditionally stated to land between September 1^st and September 15^th (depending upon latitude and year).  Using historical weather data, the heat units required to mature a boll during this part of the season must be less than 700, or the last effective bloom date would land in August every year.  Therefore, bolls were tagged between the last week of August and the third week of September in numerous locations across the state.  Each tagged boll will be evaluated for maturity at harvest.  Heat unit accumulation will be estimated for bolls that matured from nearby weather stations and will be used to determine the heat unit requirements for late-season boll maturation.