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Primo Maxx: To reapply or not?

Primo applied every 100 GDD at 5 fl. oz./A resulted in early-season injury (left), while Primo every 200 GDD at 2.5 fl. oz./A did not (right).

Making the decision to apply a plant growth regulator like Primo Maxx (Syngenta, trinexapac-ethyl) on ultradwarf bermudagrass (Cynodon dactylon x C. transvaalensis) is easy. In addition to growth suppression, Primo can provide enhanced color, increased density and more consistent green speed. Unfortunately, as trinexapac-ethyl is degraded, a period of accelerated growth (rebound) may occur, resulting in decreased quality. To maintain constant growth suppression and prevent rebound, sequential applications must occur at the maximum suppression point, where growth is maximally suppressed by the prior application.

Predicting this point is difficult because trinexapac-ethyl degradation rate increases as temperature increases. Basing sequential applications on heat accumulation approximated by growing degree days (GDD) proved to be a solution to this problem on creeping bentgrass putting greens.

Two field trials at Auburn University investigated GDD-based reapplication schedules for ultradwarf bermudagrass putting greens. The first trial evaluated growth following a single application of Primo (5 fl. oz./A) at eight different dates. Results indicate the maximum suppression point occurred 14 days after treatment during the cool weather of May but only after 10 days in the heat of July. Importantly, the GDD accumulation at the maximum suppression point essentially was equivalent for May and July. This indicates that GDD predict the maximum suppression point more accurately than calendar days.

The second trial tested four GDD reapplication schedules (every 100, 200, 400 and 600 GDD) and two Primo rates (2.5 and 5 fl. oz./A). The 100-GDD and 200-GDD intervals at both rates provided suppressed growth all season, but the 100 GDD at 5 fl. oz./A adversely affected quality early in the season.

Both trials indicate applying Primo every 200 GDD at 2.5 fl. oz./A will provide suppressed growth and enhanced quality all season.

Find GDD information and a GDD tracker at

Austin Brown, Jim Harris and Scott McElroy, Ph.D., are at Auburn University. You may reach Brown at for more information.

Photo: Austin Brown

This is posted in Research

About the Author: Scott McElroy, Ph.D.

Scott McElroy, Ph.D., is a professor in the Department of Crop, Soil and Environmental Sciences at Auburn University. He specializes in herbicide use in turfgrass as well as herbicide resistant weeds, plant growth regulator usage and general turfgrass agronomy research. He can be reached at

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