Hot Cotton

Heat Stress During Arizona’s Low Desert Monsoon Season


From the Coolidge, Casa Grande area west is considered the low desert production region of Arizona. It typically is characterized as having an elevation lower than 2,000 feet. The National Weather Service says monsoon season — a weather pattern that occurs in the state — officially runs from June 15 to Sept. 30. However, the presence of certain factors can influence the exact dates from year to year.

According to the Chandler, Arizona, website, “Simply put, the monsoon is the time of year, or the season, when winds shift, bringing an increase in moisture to the Southwest. The resulting storms can range from minor blowing dust to severe thunderstorms.”

High nighttime temperatures tend to be prevalent during the monsoon season when high humidity prevents them from cooling off during the evening.

Level 2 Stress Effects

“We have two different types of heat stress — Level 1 and Level 2 — based not on air temperature but on the temperature of the crop canopy,” said Randy Norton, Arizona cotton specialist. “Level 1 heat stress occurs when the crop canopy temperature gets to about 82.4 degrees Fahrenheit.

The effects of heat stress on the cotton flower can result in lack of pollen and an elongated stigma.

“Level 2 heat stress is when the crop canopy temperature measures above 86 degrees, which is highly correlated to nighttime temperatures. That’s when we begin to see a lot of effects from Level 2 heat stress. We can still get to Level 2 heat stress in the absence of monsoon, which we did this year, but for the most part, monsoonal activity correlates with our Level 2 heat stress.”

Norton said the primary impact of heat stress on cotton is with flower formation expressed in lack of pollen. Malformation of the floral parts has also been observed. The stigma, which is the female portion of the plant, becomes elongated. 

“With the lack of pollen or reduced pollen, three- to five-day-old bolls will abort off the plant because they haven’t pollinated properly,” he said. “This small boll abortion negatively impacts the yield. But the heat stress effects can vary by variety. Some varieties can handle heat stress better than others and will pollinate some compartments of the boll. This results in misshapen, ‘hooked beaked’ bolls that may not abort but will not open symmetrically, which reduces yield.”

Heat Stress Strategies

When it comes to anticipating potential heat stress, cotton farmers can utilize mepiquat chloride as an effective growth management tool to maintain the balance between reproductive growth and vegetative growth. A cotton plant that has been allowed to get out of control complicates terminating the crop and getting it defoliated and prepped for harvest.

In choosing a cotton variety for next season, farmers should look at variety performance in local variety tests to see which ones appear to have more tolerance to heat stress.

“Growers also need to look at a variety’s response to heat stress as a yield-determining factor,” Norton said. “A variety may be able to handle heat stress pretty well, but if it is not managed correctly, it will not yield. In addition to looking at local variety trials, farmers also should keep track of which varieties perform well on their farm.”

In some parts of the state, planting date management can be a technique to help avoid heat stress, Norton said. 

“Because of the Yuma Valley’s crop rotation cycle in far western Arizona along the Colorado River, cotton is planted very early,” he said. “This means the crop is going through its fruiting cycle well before the heat stress begins to develop in Arizona. The bulk of the heat stress is coming after the crop has already gone through peak bloom. It’s on the back side of peak bloom when heat stress starts to develop. 

“Not all parts of the state, even the central part, can do this because they don’t have the same type of early heat that Yuma does. The central area typically plants cotton in mid-April, which puts the crop right in the middle of potential heat stress when it is going into peak bloom.”

Some of Arizona’s cotton is double cropped with wheat that is planted in the fall. Then cotton is planted behind the wheat in late May. This means the later-planted cotton is just entering bloom during the hottest part of the summer, so the bulk of the heat stress doesn’t coincide with the bulk of the cotton’s flowering period. The late-planted cotton is avoiding heat stress on the back end of the heat stress curve.”

But because there are other risks — such as an early frost — that can affect the crop other than heat stress, Norton said Arizona cotton farmers have to carefully maintain a proper balance in all of their production practices.

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