Speaker: Dr. Brian Whipker, Professor of Floriculture, North Carolina State University
Webinar Date: March 13, 2025
Moderator: Dr. Shimat Joseph, UGA Extension Entomologist
Duration: 48:20
NARRATIVE SUMMARY
Dr. Brian Whipker, a nationally recognized specialist in diagnosing abiotic disorders in greenhouse crops, delivered an in-depth presentation on plant growth regulators for floriculture and ornamental production. With over 30 years of experience conducting research at North Carolina State University, Dr. Whipker shared practical insights from extensive rate studies and formulation screening work that has shaped PGR recommendations across the United States. His presentation focused on three strategic uses of PGRs: controlling excessive growth, diagnosing and remedying overdose situations, and implementing holding strategies when market conditions delay shipping.
The webinar opened with comprehensive coverage of ethephon-based products including Florel and Collate, emphasizing their unique mechanisms and applications in greenhouse production. Dr. Whipker explained that ethephon works by releasing ethylene gas within plant tissues, triggering responses including flower abortion, enhanced branching, and growth suppression. He highlighted the recent EPA approval of Collate 2L for substrate drench applications, representing a significant advancement for growers seeking alternatives to foliar sprays. Critical application parameters were detailed, including the importance of water pH management since alkaline conditions accelerate ethylene evolution and reduce product efficacy. Research from multiple universities confirmed that substrate drenches at 100 parts per million provide optimal growth control on most species when applied after root establishment.
Dr. Whipker emphasized timing as the most critical factor in successful PGR applications. Plants must be well-established with roots reaching the pot sides before drench applications, typically one to two weeks post-transplant. Applying too early to unstressed roots or to stressed plants magnifies negative effects including excessive stunting, delayed flowering, and uneven crop growth. He presented detailed rate recommendations for various crops including coleus at 125-175 ppm substrate drench, petunias at 50-100 ppm, and plumbago where ethephon provides superior branching compared to traditional growth retardants. Regional adjustments were noted, with higher rates recommended for warmer climates like southern Georgia and reduced rates for mountain regions.
The presentation addressed PGR overdose diagnosis and remediation strategies, distinguishing between ethephon effects and similar symptoms caused by nutrient deficiencies, pest damage, or other chemical injuries. Overdose symptoms appear approximately five days after application and include shortened internodes, dark green foliage, stunted growth, and flower delay or abortion. When overdose occurs, Dr. Whipker recommended using growth-promoting products Fresco or Fascination, which combine benzyladenine and gibberellic acid to counteract anti-gibberellin effects. Application rates of 1-5 parts per million with a recommended starting point of 3 ppm allow gradual growth resumption without causing excessive stretch.
Dr. Whipker concluded with practical holding strategies developed during the COVID-19 pandemic when shipping disruptions required growers to maintain plants at marketable size for extended periods. Multiple approaches were presented including reduced nitrogen and phosphorus fertility, low-dose PGR drenches, tighter plant spacing, and strategic pinching. Research demonstrated that phosphorus levels as low as 5-7.5 ppm adequately support most ornamental crops while effectively controlling growth, compared to the 40+ ppm delivered by standard 20-10-20 fertilizers. Cultural control methods including negative DIF temperature management and reduced irrigation were discussed, though acknowledged as challenging in warmer southern climates. The comprehensive presentation concluded with detailed discussion of coleus cultivar trials, exploring nutrient management, pH tolerance, and the diversity of modern cultivars developed through University of Florida breeding programs.
YOUTUBE TIMESTAMPS
0:00 Introduction and Speaker Background
1:15 Overview of PGR Research at NC State
2:35 PGR Applications: Growth Control to Enhancement
3:20 Focus Areas: Control, Overdose Diagnosis, Holding Strategies
4:15 PGR Guide Introduction and Resources
5:30 Ethephon Products: Florel and Collate
6:45 Collate 2L Drench EPA Approval
8:05 Ethephon Applications in Floriculture
9:17 Application Rate Guidelines: 250-500 PPM
10:00 Timing: Vegetative Propagation Considerations
10:40 Finished Plant Applications: 1-2 Weeks Post-Transplant
11:00 Stress Magnifier Concept
12:00 Foliar Spray Timing and Flower Delay
12:40 Staggered Flowering Example
13:25 Phytotoxicity and Stress Response
14:20 Ethephon Drench Applications Begin
15:00 Water pH and Alkalinity Effects
16:00 Virginia Tech Research: Substrate pH Impact
17:00 Iowa State Timing Studies
18:00 Rate Adjustment Based on Growth Stage
18:45 New Collate Label Release
19:30 Multi-University Validation Studies
20:30 Label Access and Application Guidelines
21:15 Phytotoxicity Precautions
21:50 Coleus Applications and Results
23:00 Petunia Rate Studies: 50-100 PPM
23:50 Plumbago Growth Control
24:10 Comparison: Ethephon vs. Anti-GA Products
25:15 Rate Recommendations by Region
25:35 PGR Overdose Diagnosis Introduction
26:30 Overdose Symptoms and Timeline
27:50 Differential Diagnosis: Disorders vs. Overdose
29:00 Calcium and Boron Deficiency Comparison
29:45 Herbicide Damage Recognition
30:40 Thrips and Broad Mite Damage
31:30 Chemical Phytotoxicity Examples
32:20 Overcoming Plant Stall with Fresco
33:00 Fresco Application Rates: 1-5 PPM
33:40 New Guinea Impatiens Case Study
34:20 Foliar Spray Results: 2.5-5 PPM
35:00 Excessive Rate Effects
35:40 Poinsettia Growth Enhancement
36:00 Bract Greening from Overuse
36:35 Holding Strategies Introduction
37:00 COVID-19 Shipping Challenges
37:20 Multiple Control Approaches
38:00 Fertilizer Management: Lower Rates
38:45 Phosphorus Reduction Studies
39:30 Low-Dose PGR Drenches
40:00 Hanging Basket Tightening Examples
40:45 Tank Mix Combinations
41:30 C-Pro Cocktail Application
42:15 Cultural Controls: DIF and Spacing
43:00 Regional Climate Challenges
43:45 Q&A: Moisture Stress Limitations
44:40 Q&A: Coleus Cultivar Trials
45:15 Phosphorus Rate Findings
45:45 pH Range and Iron Chlorosis
46:15 Nutrient Analysis by Color Groups
46:50 University of Florida Breeding Program
47:20 Downy Mildew Resistance
47:50 Closing Remarks
QUESTIONS & ANSWERS
Q: What is the optimal application rate for ethephon products like Florel and Collate on most floriculture crops?
A: The sweet spot for most crops is 250-500 parts per million for foliar applications. If you exceed this range, you risk excessive growth suppression and potential flowering delays. For substrate drenches, 100 parts per million is the established sweet spot based on multi-university research. Rates should be adjusted based on application timing and regional growing conditions.
Q: When is the best time to apply ethephon drenches after transplanting?
A: Applications should be made one to two weeks after transplant, once roots have reached the sides of the container. The plant must be well-established and unstressed before application. Applying too early when roots are not developed leads to excessive growth suppression and stunting. If you delay beyond two weeks, you may need to increase rates slightly from the standard 50-100 ppm to 100-150 ppm.
Q: How does water pH affect ethephon efficacy?
A: Water alkalinity significantly impacts ethephon performance. High pH or alkaline conditions cause rapid evolution of ethylene gas from the ethephon molecule, reducing the time available for plant uptake and cutting overall efficacy. Ideally, use pure water or acidify your water source to ensure adequate contact time. Research shows that even substrate pH affects drench performance, with high pH soils releasing ethylene more quickly.
Q: What makes ethephon different from traditional growth retardants like paclobutrazol or uniconazole?
A: Ethephon works through ethylene gas evolution rather than anti-gibberellin activity. This gives it unique advantages including enhanced branching, ability to control growth without the severe stunting potential of anti-GA products, and faster recovery if overdose occurs. On crops like plumbago, ethephon provides excellent branching and size control while anti-GA products risk hitting the narrow window between insufficient and excessive control.
Q: How can you tell if plants are experiencing PGR overdose versus other disorders?
A: PGR overdose symptoms appear approximately five days after application and include shortened internodes, darker green foliage, stunted appearance, and potential flower delay or abortion. The key diagnostic is uniform symptoms across all treated plants, unlike pest damage which typically shows progressive patterns. If you know a PGR application occurred five to seven days before symptoms appeared and the entire treated area shows identical effects, overdose is likely. Compare against calcium deficiency, boron deficiency, broad mites, thrips damage, and herbicide injury which have distinctive symptom patterns.
Q: What options exist for remedying PGR overdose situations?
A: Fresco or Fascination products, which combine 6-BA (benzyladenine) and gibberellic acid, can stimulate growth to overcome anti-GA effects. Start with 3 parts per million as a substrate drench or foliar spray. If growth does not resume within seven days, make a second application at half the rate. Never exceed 10 ppm or you will create excessive stretch requiring additional growth retardant applications. The goal is to tap the accelerator, not slam on the gas.
Q: What are the rate recommendations for substrate drenches on coleus?
A: Research trials on over 114 coleus cultivars established that 125-175 ppm substrate drenches provide effective growth control. Southern growers in warmer regions like south Georgia should use rates toward the higher end, while mountain growers can reduce rates. Coleus shows tremendous cultivar variability in PGR response, making ethephon advantageous because it provides good control without the severe stunting risk of anti-GA products where the window between too little and too much is very narrow.
Q: Can ethephon be used on poinsettias, and are there special considerations?
A: Ethephon can enhance growth on poinsettias when plants need to reach size specifications. Substrate drenches work better than foliar sprays for this crop. However, excessive rates or multiple applications can cause bract greening instead of proper color development. Start conservatively and test on a small number of plants before full-scale application. For terminal flowering plants like poinsettias and mums, timing is critical to ensure adequate growth potential remains after application.
Q: What strategies can growers use to hold plants when shipping is delayed?
A: Multiple approaches work synergistically: reduce nitrogen rates, lower phosphorus to 5-7.5 ppm, apply low-dose PGR drenches (0.1 ppm range), tighten plant spacing to increase competition, implement hard pinching despite the delay, and utilize negative DIF temperature management if climate permits. In warm climates like Georgia and North Carolina, opening vents for cool temperatures is not feasible like northern states, making PGR and nutritional strategies more important. The key is combining several mild interventions rather than relying on a single aggressive approach.
Q: Is there a phosphorus level that effectively controls growth without causing deficiency?
A: Research on multiple species demonstrated that 5-7.5 parts per million phosphorus provides adequate nutrition while controlling growth. Most standard fertilizers like 20-10-20 at 200 ppm deliver 40-44 ppm phosphorus, far exceeding plant needs with no additional benefit. Deficiency symptoms appear at zero phosphorus, but above 5 ppm plants remain healthy with tighter, more compact growth. You can grow plants to full size then cut back phosphorus as a holding strategy, though deficiency will emerge in two to three weeks requiring periodic maintenance applications.
Q: What were the key findings from the large-scale coleus cultivar trials?
A: Trials on 114 cultivars examined phosphorus rates, pH tolerance, and nutrient differences by foliage color. Optimal phosphorus confirmed at 10 ppm. pH tolerance ranged from 5.8-6.4, with iron chlorosis developing at pH 7.0 and above. Coleus does not accumulate toxic iron and manganese at low pH like some species. Nutrient analysis by color group showed greater variation within color groups than between them, meaning generic fertilization recommendations work across the spectrum. Most modern cultivars originate from University of Florida breeding programs marketed through Dummen Orange, Syngenta, and Ball, with improved downy mildew resistance developed after the 2006 outbreak.
Q: How does moisture stress interact with PGR applications as a holding strategy?
A: While moisture stress can slow growth, it must be carefully managed. Excessive dry periods combined with PGRs can create compounding stress effects rather than gentle growth control. A little strategic water withholding works well, but severe stress damages plant quality. The goal is keeping plants healthy and marketable, not stressed to the point of unsalability. PGRs are stress magnifiers, so if plants are already stressed from low moisture, PGR applications will amplify negative effects.
ADDITIONAL RESOURCES
eGrow Alert 9.20: Control Strategies for Holding Plants
Published in response to COVID-19 shipping disruptions
Available at: www.eGrow.org
PGR Guide
Comprehensive reference guide for plant growth regulators in greenhouse production
Collaborative project with Fine Americas
Published through GrowerTalks Magazine
Label Information:
Collate 2L drench label available at Fine Americas website under ornamentals section
Drench application rates listed on approximately page six of updated label
Research Contributors:
- Iowa State University – Rate and timing studies
- Virginia Tech – Substrate pH effects on ethephon drenches
- Michigan State University – Multi-crop validation
- Ohio State University – Rate refinement
- Cornell University – Formulation testing
- NC State University – Extensive rate and crop screening
Key Research Personnel Mentioned:
- Josh Henry (NC State graduate student) – Phosphorus rate studies
- Patrick Beesey (NC State graduate student) – Petunia trials
- Jim Barrett (University of Florida) – Low-dose drench research
Plant Breeding Programs:
- University of Florida – Coleus breeding and downy mildew resistance
- Marketed through Dummen Orange, Syngenta, and Ball companies
For questions about plant growth regulator applications in floriculture, contact Dr. Brian Whipker at North Carolina State University. For information about the Getting the Best of Pests webinar series, contact Dr. Shimat Joseph at the University of Georgia Center for Urban Agriculture.