According to Silvertooth, five important timing criteria to follow based on crop and soil monitoring and boll development include:

1 - Boll development requires 600 heat units (HUs) to grow from a flower to a full-sized hard green boll. During that period, adequate soil moisture is needed and irrigations should be managed to support the uppermost bolls on the plants intended for harvest through this period.  An additional 400 HUs are required to develop a full-sized hard green boll into an open boll.

2 - Apply chemical at twice the number of days after the last irrigation. If the last irrigations were 10 days apart, apply the defoliant 20 days after the last irrigation.

3 – Growers can take a soil samples in advance for laboratory analysis to gain texture and other water-holding capacity information. AZSCHED simulation software available online allows the estimation of current evapotranspiration rates. Apply the defoliant when the soil reaches less than 50 percent plant available water.

4 – Apply the chemical when four nodes separate the first (top) cracked boll and the top first harvestable boll.

5 – Apply defoliant after the uppermost boll on the plant intended for harvest has reached maturity.  This is determined by cutting bolls and evaluating the seeds for adequate maturity development prior to defoliation.

In Arizona, historical HU data is available online from the UA’s AZMET meteorological network at

Defoliant application rates are also weather based. Silvertooth suggests a low chemical rate when the expected HU accumulation two weeks after application is more than 300 and the daytime high temperature averages 90 degrees F. A medium rate is suggested for 200 to 300 total HUs with an 80 degree daytime high. For a high defoliant rate, the expected HUs should total less than 200 with a daytime high of 70 degrees.

“High application rates are needed when there is less heat available,” Andrade said. “This is based on the physiological limitations of the plant and represents the general recommendation for all cotton production areas in Arizona and irrigated cotton in the desert Southwest.”

Follow the product label for rate information regarding weather conditions.

Good season-long pest control is also important to maintain quality leaves and to maximize the defoliant’s effectiveness by distribution over a large leaf area. Defoliant applied to the leaves is not translocated to other plant areas.

There are not many changes in available chemistries for defoliation. Of the defoliant products on the market, the most common active ingredients utilized in Arizona over the last six years include Thidiazuron (e.g. Ginstar, 58.8 percent), Sodium Chlorate (20.3 percent), followed by Tribuphos (e.g. Folex, 12.7 percent), and Endothall (e.g. Accelerate, 8.2 percent). This data is from reported 1080 forms from the Arizona Pest Management Center’s pesticide database.