Intervals of Upland(Gossypium hirstum L.) and Pime (G. barbadense L.) flower and boll development were monitored in days and heat units HU at different planting dates in 1983 and 1984. HU accumulation from emergence to first effective bloom producing an open bell, and the number of days from emergence to first open boll were the only consistent intervals of development between planting dates both years. The intervals from emergence to last effective bloom and open boll, and the number of days from emergence to first open boll were bloom period significantly decreased with delayed planting, but were effectively monitored by the summation of HU beginning April 1 regardless of planting date. Early, planting produced more open bolls per plant and greater lint yields were associated with increased HU accumulation for growth and development.