Determining when, how, and if cotton plants need a chemical plant growth regulator can be a daunting chore with numerous factors in play. 

Weather, pests, variety, and management practices all play roles, said Benjamin McKnight, Texas A&M Extension cotton specialist, College Station. 

McKnight said plant nutrient availability may influence cotton growth and development and PGR requirements.  

He said variety selection plays a crucial role.  

“Early maturing varieties typically need a less aggressive PGR approach compared to some of the later maturing varieties,” he said. “Most seed companies provide some PGR management guidelines for their varieties.” 
He said squares and bolls are “some of the most effective plant-growth regulators. When plants are loaded with squares and bolls, a lot of that photosynthate goes to those developing reproductive structures, inhibiting vegetative growth of that plant.” 

Insects and weather 

Physical damage to the plant influences vegetative growth as well.  

“Insects feeding or adverse environmental conditions may cause fruit shed and necessitate earlier PGR applications,” McKnight said. “Weather conditions are important. Know what the long-term forecast looks like. Be aware of conditions conducive for excessive growth, such as a lot of rainfall and warmer temperatures.” 

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McKnight noted that fruit retention and growth habits will vary from one variety to another. Again, he suggested checking with seed dealers.  

“PGR programs are much more common for producers with irrigated production systems. Programs tend to be less aggressive in dryland systems, especially coming out of years like 2022 and 2023 in Texas when hot, dry conditions were not conducive to vegetative growth in the plant.” 

Nitrogen factor 

McKnight said excessive nitrogen fertility leads to enhanced vegetative growth and the need for more aggressive PGR management. “Coupled with abundant soil moisture, excess nitrogen can lead to excessive growth, which in turn can slow the initiation of fruit and delay maturity.” 

He offers three basic strategies to manage PGR applications. 

“The first is to use higher rates, less frequently — an 8- to 16-ounce rate approaching first bloom with follow-up applications one to three weeks later. Rates range up to 24 ounces, depending on conditions and how plants are growing.” 
A second option is using lower rates over multiple applications.  

“That was a more common approach when we were making a lot more insecticide applications,” McKnight said. “It was convenient since we were already spraying. We could make several PGR applications at low rates, along with insecticide sprays, during the season.” 

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The third option is applying a high rate towards the later stage of bloom. 

“I don't think one size fits all,” McKnight said. “That’s   especially true in Texas, considering the weather we usually deal with.” 

PGR application timing

He said most PGR products have a maximum single-season application, particularly mepiquat chloride products. A 24-ounce rate of 4.2% concentration is the max rate per application, up to 48 ounces per year. “PGR products have accompanying labels and we always encourage growers to familiarize themselves with that label.” 

Flexibility 

McKnight said producers need to be somewhat flexible and adjust to changing conditions.  

“If producers get into a situation where they have high temperatures, rainfall scarcity, drought-limited or no irrigation, and they are growing a shorter compact variety with a heavy fruit set, or experiencing recovery from plant injury, they might want to be a bit more conservative. “With rapid growing conditions, on a vigorous tall variety, the situation will be different. Some varieties are inherently robust with aggressive growth habits. Producers need to stay on top of those with PGR anytime conditions are conducive to boll rot.” 
He said in situations with poor fruit retention and late planting, producers might lean towards a more aggressive program. 

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Timing PGR applications

Timing PGR applications depends on several factors, some out of producers’ control. McKnight offers three basic approaches to initiate PGR applications. 

The first is evaluating height to node ratio. “Consider three different categories of growth — mid square to bloom, early bloom, and mid bloom.” 

He refers to a chart that considers plant height and node count. “Divide plant height by that node count. Where that number falls on the chart indicates growing conditions in the field. 
“With aggressive growth at early bloom, anything greater than two indicates a need for some PGR.  
 “A mark of two around mid-bloom would indicate growth about normal. So, keep an eye on it.” 
Nodes above white flowers are a common means of determining PGR applications.  

“Go to the highest first position white bloom as the beginning point,” McKnight said. “Go up the stem, count the nodes above that point all the way to the terminal. As the field gets into early bloom, most situations show seven to eight nodes above white flower as desirable. 
“Anytime we get nine or more, that indicates potential for aggressive PGR action. If plants show six nodes above white flower, that indicates the plants are a bit behind schedule and will not need a PGR yet.”  
A third option is an in-season method. “Between the 4th and 5th node is the most recently fully elongated part of the main stem,” McKnight said. “Look at the internode area between the 4th and 5th node and measure with a ruler or a tape measure. From 2 1/2 to 3 inches is a large internode spacing that indicates the plant is in active vegetative growth.” 

More on Nitrogen 

McKnight referred to comments on nitrogen management and suggested that sampling to depth could be a valuable tool in managing excessive growth and PGR applications, as well as saving on fertility costs. 

He said a typical recommendation for a 1,000-pound-per-acre cotton yield (two bales) is about 134 pounds of nitrogen. For one bale, use half that rate, 62 pounds. 

He noted a Burleson County deep sampling trial for available nitrogen at different depths shows 12 pounds of plant available nitrogen at a depth of zero to six inches. At 24 inches, sampling showed 44 pounds of plant available nitrogen and 96 pounds at 48 inches. 
“The study highlights the importance of soil testing to gauge how much nitrogen we have. In a lot of cases, we're over applying nitrogen. In a field that had corn the previous year, in dry conditions, producers may have a lot of residual nitrogen in the soil, so it's important to take that into consideration instead of just fertilizing.” 

Knowing what’s there not only saves nitrogen but also may make PGR management a bit easier.