The cotton plant, primarily the Gossypium hirsutum species, is the source of the world’s most widely used natural fiber. The production of harvestable fiber depends entirely on a distinct and relatively short flowering stage. The bloom marks the shift from the plant’s vegetative growth phase to its reproductive phase, which ultimately yields the cotton fiber. Understanding the timing and progression of this flowering is fundamental to cultivation.
Preparing for the Bloom
The journey to the cotton bloom begins after the seed is planted, which occurs when soil temperatures reach a minimum of 65°F at the planting depth. After successful germination and emergence, the seedling enters a period of vegetative growth, establishing its root system and producing true leaves. This initial phase of growth is relatively slow as the plant channels energy into its underlying structure.
The first visible sign that the plant is preparing for flowering is the appearance of small, triangular structures known as “squares.” A square is the botanical term for the developing flower bud, encased by protective leafy structures called bracts. These first squares become noticeable on the lower part of the plant between four and five weeks after planting, often at the sixth or seventh node on the main stem.
It takes approximately 21 to 25 days for a pinhead-sized square to develop fully and open into a flower. This squaring period, which begins around 35 to 47 days after sowing, precedes the plant’s first bloom.
The Short Life of the Cotton Flower
The first true flower, or “white bloom,” appears between 60 and 70 days after sowing, about three to four weeks after the first square formed. When the flower first opens, it is a creamy white or pale yellow color, indicating it is receptive to pollination.
The cotton flower is short-lived, enduring for only a single day. Pollination, which is largely self-pollination, occurs within hours of the flower opening. The most distinctive feature of the cotton bloom is its rapid color change, which serves as a natural indicator of successful fertilization.
Within 24 to 48 hours of opening, the flower’s petals change color, turning a dark pink or reddish-purple hue. This transformation signifies that fertilization has taken place. The fertilized flower will then wither, dry up, and fall off the plant, a process that happens within three to seven days, preceding the formation of the cotton boll.
Boll Development and Maturation
Immediately following the withering and dropping of the fertilized flower, the ovary begins to swell and develop into the cotton boll, which is the plant’s fruit. This green, capsule-like structure is the protective vessel where the cotton fiber and seeds develop. The boll undergoes a rapid growth period, reaching its full size within about three weeks after the flower first appeared.
The development of the fiber inside the boll is a two-stage process: elongation and thickening. During the first 25 days after the bloom, the individual fibers, which are extensions of the seed coat, rapidly elongate to determine the final length of the cotton staple. Over the next 25 days, the fibers thicken as layers of cellulose are deposited on the inner walls.
The entire maturation process, from the initial white bloom to the boll’s final opening, requires a total of 50 to 70 days under optimal conditions. The process culminates in “boll opening,” where the mature boll dries out and naturally splits along its sutures, revealing the fluffy, white lint that is ready for harvest.
Environmental Drivers of Flowering Time
The specific timing of the cotton bloom is not fixed by calendar days alone but is highly dependent on the accumulation of heat. The plant’s development is measured by “accumulated growing degree days” (GDDs), a measure that quantifies the amount of heat energy available to the plant above a base temperature of 60°F. Cotton plants must accumulate a specific number of GDDs to transition from one developmental stage to the next, including the onset of flowering.
A faster accumulation of GDDs due to warmer temperatures will accelerate the plant’s growth, causing it to reach the flowering stage earlier in the season. Conversely, cooler weather will slow down the developmental timeline. The availability of soil moisture can also affect the timing of the bloom; a plant under severe water stress may shed its squares or delay flowering to conserve resources.
Growers can influence the timing by selecting different cotton varieties, as some are bred to be earlier-maturing, requiring fewer GDDs to reach the bloom stage. The exact day the first flower appears is a function of the cotton plant’s genetics interacting with its local environment.