A generally cool start and end to the cotton growing season in Kansas this year has many wondering, “Will we have enough heat units to finish the crop?” Similar to corn and sorghum, cotton requires the accumulation of heat units to reach its key growth stages and mature bolls (seeds and fiber). The base temperature for calculating growing degree days for cotton is 60°F. However, unlike corn (86°F) or sorghum (100°F), the research does not recognize cotton as having a maximum temperature. The cotton crop accumulates generally 2200 to 2600 GDUs to go from planting to harvest-ready (Table 1), though research in the Oklahoma Panhandle suggests that cotton in the region may be harvest-ready in as few as 1800 GDUs on the High Plains, potentially because of higher solar radiation compared to most of the U.S. Cotton Belt [Cannon et al. (Unpublished)].
Table 1. Average range of days and heat units required to reach key cotton growth stages.
|
Growth Stage |
Days |
Heat Units |
|
Planting to Emergence |
4-9 |
50-60 |
|
Planting to 1st Flower |
60-70 |
775-850 |
|
Planting to 1st Open Boll |
102-127 |
1575-1925 |
|
Planting to Boll Opener/Defoliation |
120-140 |
2150-2300 |
|
Planting to Harvest - Ready |
130-160 |
2200-2600 |
Source: Oosterhuis et al. (1990).
Cumulative cotton GDUs are generally greatest in south central Kansas and less in central and southwest Kansas due to the combination of latitude and altitude differences (Figure 1).
Figure 1. Average number of cotton GDDs between May 1st and the average first freeze date in Kansas.
As of September 8, in the cotton-producing counties of Kansas:
- For cotton planted on May 1 (Figure 2a), GDU accumulation ranged from 1700 to 2100.
- For cotton planted on May 15 (Figure 2b), GDU accumulation ranged from 1600 to 2000.
- For cotton planted on May 31 (Figure 2c), GDU accumulation ranged from 1500 to 1900.
Figure 2. Contour maps of accumulated growing degree days for cotton (GDD) across Kansas during three different planting dates (May 1, May 15, and May 31).
The 2025 season resulted in few cotton growers planting in early May and the majority planted between May 15 and May 31. For the May 15 planting date, we see that the south central counties have accumulated 1800 to 2000 GDUs, indicating the crop may already be opening bolls in some places along the Oklahoma border. In comparison, the southwest Kansas counties’ cotton planted on May 15 would have accumulated 1600 to 1800 GDUs. For the May 31 planting date in both these areas, the crop is approximately 100 to 150 GDUs behind the May 15 planting. It is important to keep in mind that only 222 to 264 GDUs were accumulated in south central and southwest Kansas from May 1 to May 31 this year, which was up to 52 GDUs below normal and 88 GDUs less than in 2024, specifically in southwest Kansas.
Last effective bloom date
Generally speaking, the last effective bloom date in Kansas is between August 20 and August 25 (Figure 3), considering it takes about 45 days for a flower to mature into an open boll.
Figure 3. Cotton at Garden City, KS, on August 27. Significant boll load developed in the canopy. Recent blooms are likely too late to mature by the first frost.
How much heat could remain?
The average first fall freeze date in Kansas ranges from October 14 to October 21 in southwest Kansas and from October 21 to October 28 in south central Kansas. The expected number of GDDs for cotton from September 9 through the average date of first freeze is shown in Figure 4.
Figure 4. Expected average of accumulated GDUs for cotton from Sept. 8 until the average date of first freeze for Kansas.
Will we have enough heat units to finish the crop?
The answer is, “it depends.” We will continue to monitor cotton growth and development throughout the remainder of the growing season. As we approach the end of the season, cotton growers should be mindful of harvest aid application timing relative to crop maturity as well as the first frost. A follow-up article that will come out next week will discuss harvest aid applications in cotton in greater detail.
Logan Simon, Southwest Area Agronomist
lsimon@ksu.edu
Tina Sullivan, Northeast Area Agronomist
tsullivan@ksu.edu
Matthew Sittel, Assistant State Climatologist
msittel@ksu.edu
Tags: weather cotton growing degree days