Paul Brown, UA Extension specialist and research scientist based in Tucson, discussed the impact of planting dates on cotton plant emergence and the vulnerability to heat stress and pests. He warned against planting seed too early in cool soil yet not too late due to the threat of heat stress which develops during the summer monsoon season.

Cotton, a warm-season crop, does not develop good root structure in soil below 60 degrees. Research suggests that planting into soil 60 degrees or lower can slow germination and increase the susceptibility to disease, Brown says. Very cold soil can result in poor stand development.

Chilling injury can destruct or abort the tap root and cause the development of lateral roots. Complications can develop later in the season.

“The first day water is put to seed is the critical day for performance in cool soils,” Brown said. “The metabolic processes associated with early germination are very sensitive to temperature during the first 24 to 48 hours after moisture reaches the seed. If the soil temperature is too cold, these mechanisms get out of whack.”

The industry-preferred planting standard is 65-degree soil or higher. Sixty-plus degrees is acceptable, 55 degrees is marginal, and 50 degrees is dangerous.

Brown manages the UA-owned Arizona Meteorological Network, an automated system of 28 solar-powered rural and urban weather stations which gathers information including soil temperatures at 4-inch and 20-inch depths beneath the soil surface.

By March 1 in Buckeye (West Phoenix), soil temperatures at 20 inches are usually 60 degrees or higher. The 4-inch temperature should drive planting date decisions.

“The surface is the key factor since it’s close to the current climate,” Brown said.

On average, soil temperatures at 4 inches reach the acceptable range for planting during the last week in March in Buckeye. However, early spring weather can fluctuate year-to-year. The direct measurement of soil temperature is the best way to make planting decisions.

One way to estimate when soil temperatures are approaching an acceptable level is to monitor the minimum air temperature each morning. Soil temperatures at the seeding depth typically remain in the acceptable planting range when minimum temperatures remain in the upper 40s or higher.

Another way to gauge when soils near the proper thermal range is to monitor heat unit (HU) accumulation.

The official UA-recommended planting window for the Buckeye area is: full season varieties – March 17 to April 14 – 400 HU to 700 HU since Jan.1; medium maturity – March 17 to April 21 – 400 HU to 800 HU; and short season – March 17 to May 14 – 400 HU to 1,000 HU.

Typically, growers in the Buckeye area begin planting at about 500 heat units.

“The earlier you plant the better as long as the soils are acceptable for seed germination,” Brown said.

Related Arizona crop production charts are available March 1 at the AZMET website, — earlier in the Yuma area. 

Brown is working with the UA’s Pedro Andrade and Randy Norton to develop a phone app that will deliver the latest planting information directly to Smart cell phones.

While planting too early can have season-long drawbacks, planting too late also creates risks. Planting after mid April in central Arizona can reduce lint yields tied to summer weather including the monsoon season and heat stress.

“Heat stress is based on canopy temperature,” Brown said. “Heat stress can result in malformed reproductive structures on cotton flowers. The result is aborted or small, misshapen bolls which can reduce yields.”

In Buckeye, the median first heat stress date is about July 12. Early optimal planting dates which fall within the HU-based planting windows can help minimize exposure to heat stress.

“The concept is to try to avoid heat stress during primary bloom,” Brown said.

As to the weather forecast for this spring, Brown says forecast maps from the National Oceanic and Atmospheric Administration suggest warm and dry weather in the West and Southwest.