Last Effective Bloom Date in Cotton

Given the time of year, here is a re-post of an article from a few years ago:

The last effective bloom date is the last date that a white flower will accumulate enough heat units to mature into a harvestable boll.  Generally speaking it takes 850 heat units for a white flower to mature into a harvestable boll.  Using 30-year historical weather data from the National Climatic Data Center, the last effective bloom date for areas in the Highway 6 vicinity and north is August 18th.  For those in the south Delta, the last effective bloom date for areas around Yazoo City is August 21st.  However, keep in mind that weather patterns over the last few years have been erratic and no one knows for certain what the remainder of the year will bring.  August and September of 2012 turned off cooler than normal which resulted in a crop that was up to that point, 2-3 weeks ahead of schedule moving back into a “normal” timeframe.  However, the later part of August and September 2013 were warmer than normal and helped us produce a record crop in 2013.  If temperatures stay warm and we get an extended harvest season, last effective bloom dates could be extended several days.

The following data were also obtained from historical weather station data from the National Climatic Data Center.  These data represent from nearly 50 years to almost 100 years of historical data at each individual location.  The earliest recorded (for years data was available) freeze at Hernando, Stoneville, and Yazoo City was October 9, October 6, and October 20, respectively whereas the average date of the first freeze at these locations is November 5, November 8, and November 13.  The latest freeze at Hernando, Stoneville, and Yazoo City was December 2, December 14, and December 30, respectively.  The reason there is a larger difference in average freeze dates compared to the last effective bloom date is due to the lack of accumulation of heat units at the end of the season.

Generally it takes about 21 to 31 days for a square to develop into a flower.  Historically, it has taken about 30 days from square to bloom; however, over the last few years blooms have been observed in as few as 21 days after square formation.  Using this information, you can backtrack on your calendar from the average last effective bloom dates above to determine which fruit have the greatest likelihood of going into the picker basket.  Keep in mind if we are blessed with a long, favorable fall, younger fruit in the upper portions of the plant could end up in the picker.