About

Dr. Paul Fisher is a Professor and Extension Specialist in Floriculture at the University of Florida's Institute of Food and Agricultural Sciences. His work involves managing university/industry groups to conduct applied research and grower training focused on greenhouse floriculture production and water conservation. His research collaborations include multi-state projects and the Floriculture Research Alliance, a university-industry consortium dedicated to improving production, propagation, and shipping of floriculture crops. Key focus areas include root zone management, production training, economics, water sanitation, and quality, with an overarching goal to increase plant resilience and reduce crop losses across production, retail, and consumer phases. Dr. Fisher supervises graduate and undergraduate students in applied research projects related to indoor propagation, greenhouse climate and irrigation control, substrates and nutrients in hydroponics, and water treatment technologies. He also teaches courses and guest lectures on topics such as nutrition for horticulture crops and hydroponics. As a leader in extension education, he directs Greenhouse Training Online, a series of professional development courses for growers in English and Spanish, and develops mobile-accessible tools to improve production efficiency. His educational background includes a Ph.D. in Horticulture from Michigan State University, an M.Appl.Sci. in Resource Management from Lincoln University in New Zealand, and a B.Sci. in Botany from the University of Auckland. Dr. Fisher has received numerous awards for his contributions to horticulture and extension education, including the 2024 Alex Laurie Award from the Society of American Florists and the 2018 ASHS Extension Division Material Award.

Research topics

• Biology
• Horticulture
• Botany
• Ecology
• Chemistry
• Traditional medicine
• Environmental science
• Medicine
• Agronomy

Selected publications

• Designs for Small-Scale Postharvest Washing of Ginger/Turmeric Rhizomes

  EDIS · 2025-09-24

  articleOpen accessSenior author

  Manually washing the rhizomes of ginger and turmeric is a labor-intensive process that adds significantly to the cost of final harvested products. Commercially available automatic cleaning equipment makes the postharvest process easier, but only at a price and scale that would benefit large-scale operations. Our objective was to design and test rhizome cleaning equipment aimed at small farmers and to compare this with manual cleaning. This publication describes tips for harvest and presents designs of cleaning equipment that are easy for a small farmer to build and maintain while reducing labor costs.

  PublisherDOI

• Microbial Quantification Using ATP and Petrifilms for Irrigation Water Treated with Cold Plasma or Ozone

  Water · 2025-06-22

  articleOpen accessSenior authorCorresponding

  Traditional methods of microbial quantification of irrigation water using colony counts from agar culture require dedicated laboratory space and trained personnel, limiting their on-site applicability. Dehydrated Petrifilm™ plates are a simpler alternative but still require 2–3 days to culture. Adenosine triphosphate (ATP) tests may offer a fast and reliable method for quantifying microbes in water. In this study, we compared (a) microbial quantification based on ATP assays with Petrifilm™-based assays, and (b) we evaluated the effectiveness of cold plasma or ozone treatments in controlling microbial growth at various oxidation–reduction potential (ORP) levels. Lake water was recirculated through an ozone or cold plasma treatment system until a target ORP of 700 mV was reached. Samples were collected at various ORP levels and plated for aerobic bacteria and yeast and mold counts using Petrifilm™ plates. The free and total ATP concentrations were measured using the Hygiena EnSURE luminometer and its accompanying free and total ATP swabs. Microbial ATP was calculated by subtracting the free from the total ATP. Cold plasma and ozone showed similar effects on microbial inactivation at 700 mV (p < 0.05). Both treatments achieved complete fungal inactivation at 600–700 mV ORP, bacterial inactivation at 600 mV ORP, and near-complete inactivation of microbial ATP at 600–700 mV. A moderate positive correlation (Pearson’s correlation = 0.39 and Spearman’s rank correlation = 0.39) was observed between the Petrifilm™ bacterial counts and microbial ATP levels, suggesting ATP quantification could complement Petrifilm™ for rapid and non-selective onsite microbial assessment of irrigation water.

  PublisherDOI

• Potential Markets for Florida Turmeric

  EDIS · 2025-04-28

  articleOpen access

  This publication examines the potential markets for turmeric (Curcuma longa) grown in Florida, highlighting opportunities in fresh produce, food and beverage ingredients, and nursery markets. Turmeric, widely used in culinary dishes and wellness products, is primarily consumed in its rhizome form, as a powdered spice. The US fresh produce market for turmeric is largely supplied by imports from countries like India, Fiji, and China, presenting price competition challenges for Florida growers. Florida turmeric producers face strong competition from imports, but by focusing on quality, differentiation, and niche markets, they can enhance profitability.

  PublisherOA PDFDOI

• Macronutrient Solubility in Response to the pH of Soilless Container Substrates

  Journal of soil science and plant nutrition · 2025-03-06 · 11 citations

  articleOpen access

  Abstract This study investigated the complex interactions between pH and nutrient availability in soilless substrates, focusing on how different nutrient solution formulations, substrate components, and liming materials influence macronutrient solubility. The objective was to develop a more sophisticated representation of pH-nutrient solubility relationships compared with charts currently used by horticulture professionals. Macronutrient concentration was measured in response to substrate-pH using Ca(OH) 2 and Mg(OH) 2 in four peat-based substrates (70% peat mixed with 30% perlite, pine bark, coconut coir, or vermiculite by volume). A range of lime rates and fertilizer formulations were applied, and resulting pH and macronutrient concentration in a water extract solution was analyzed. Nitrate-N and ammonium-N concentrations decreased at higher pH levels. Phosphorus concentration decreased, especially when limed with Ca(OH) 2 . Potassium availability was affected by substrate type and cation exchange capacity. Calcium solubility increased with Ca(OH)₂ but decreased with Mg(OH)₂ at elevated pH levels, whereas magnesium solubility showed a reverse pattern. Sulfate-sulfur concentration remained stable across treatments. Chemical equilibrium simulations with Visual MINTEQ highlighted the formation of insoluble Ca-P compounds with Ca(OH) 2 as the lime source, and low solubility Mg-P compounds with Mg(OH) 2 as lime, hindering available P. This study illustrates several key trends in pH effects on macronutrient solubility and plant availability that have practical implications for horticulture fertilizer management and emphasize the real-world complexity beyond widely-used pH solubility charts. The review of published plant studies highlights that tissue nutrient concentrations do not always correlate with nutrient solubility. This discrepancy is influenced by factors such as root-secreted compounds and pH modifications caused by nutrient uptake by plant roots.

  PublisherOA PDFDOI

• Potential Markets for Florida Ginger

  EDIS · 2025-04-22

  articleOpen access

  This publication explores the potential markets for ginger (Zingiber officinale) grown in Florida, focusing on opportunities in fresh produce, food and beverage ingredients, and nursery markets. Ginger is widely used in a variety of culinary dishes and wellness products, primarily consumed as the rhizome, or ginger root. The US fresh produce market for ginger is dominated by imports from China, Peru, and Brazil, making price competition challenging. Florida ginger producers face significant competition, but by focusing on quality, differentiation, and exploring niche markets, they can enhance profitability.

  PublisherOA PDFDOI

• Micronutrient Solubility in Response To Root Zone pH for Soilless Plant Culture: Simulation of Chemical Equilibria

  Journal of soil science and plant nutrition · 2025-12-01

  articleSenior author
  PublisherDOI

• Comparison of Water and DTPA Extractants for Testing of Greenhouse Soilless Substrates

  Communications in Soil Science and Plant Analysis · 2024-07-29

  articleOpen accessCorresponding

  The main purpose of this study was to improve the accuracy and reliability of nutrient analysis on soilless substrates by comparing two different saturated medium extract (SME) methods, using either water (SMEW) or DTPA (diethylenetriamine-pentaacetic acid, SMED) as extractants. A series of analyses were performed to compare ion levels quantified from either SMEW or SMED. Substrate pH, electrical conductivity (EC) and nutrient data were analyzed from a survey of substrate components (coconut coir, peat, bark, perlite, vermiculite), and blended greenhouse propagation substrates using the SMEW and SMED methods. In addition, a series of SMEW and SMED extracted samples were sent to three commercial soil testing laboratories for nutrient analysis. The objectives of these analyses were to (1) validate that published relationships between SMEW and SMED methods were consistent with correlation curves for pH, EC and nutrient levels across a wide range of soilless substrates, (2) compare statistical variability of results from SMEW and SMED methods and (3) compare variability in results between different laboratories. The results validated that previously published standards for interpreting SMEW extractions can be used for SMED method for macronutrients and EC due to an approximately 1:1 relationship for test results between extraction methods. Up to 2 orders of magnitude higher levels of micronutrients were extracted for iron (Fe), manganese (Mn), zinc (Zn), and copper (Cu) using SMED compared with SMEW, with low correlation between extractants. Concentrations of ions from SMED extractions were less variable than SMEW for micronutrient analysis but were comparable for macronutrient analysis. Results from this study were combined with published research to provide suggested ranges using SMED for soilless substrates.

  PublisherOA PDFDOI

• Paclobutrazol Residues in Recirculated Water in Commercial Greenhouses

  HortTechnology · 2024-03-20 · 3 citations

  articleOpen access

  Reusing irrigation water has technical, environmental, and financial benefits. However, risks are also associated with the accumulation of agrochemicals, in addition to ions, plant and food safety pathogens, and biofilm organisms. In this project, we measured the concentration of paclobutrazol (a persistent and widely used plant growth regulator) in recirculated water in greenhouses producing ornamental plants in containers. Solutions were collected from catchment tanks at nine commercial greenhouses across seven states in the United States in Spring and Fall 2014. Paclobutrazol was detected in all samples, with differences observed by season, greenhouse operation, paclobutrazol application method, and irrigation method. Across operations, the residual concentration of paclobutrazol was higher in spring for most greenhouses (ranging from 0 to 1100 µg·L −1 ) compared with the fall (ranging from 0 to 8 µg·L −1 ). The spray-drench application method resulted in the highest residual concentrations (up to 35 µg·L −1 ), followed by substrate drench (up to 26 µg·L −1 ) and foliage spray (concentrations under 3 µg·L −1 ). Residual concentrations were higher with overhead irrigation (up to 35 µg·L −1 ) compared with subirrigation systems (up to 15 µg·L −1 ). Our results indicate that paclobutrazol is likely to be a growth retardant risk in greenhouse operations recirculating water. A clear understanding of the risks associated with recirculated water intends to support the development and implementation of risk management strategies to ensure and promote safe use of recirculated water in greenhouses. Overall, the most effective preventative strategy is to ensure the use of the minimum amount of the a.i. necessary per unit of space and time.

  PublisherOA PDFDOI

• Substrate Moisture and Temperature Effects on Limestone Reaction Rate in a Peat-Based Substrate

  Communications in Soil Science and Plant Analysis · 2024-08-13 · 1 citations

  articleSenior authorCorresponding

  Lime reaction rate in a container substrate is influenced by temperature and moisture, which are factors that vary between batches of substrate during manufacturing, storage, and crop production. The effects of temperature and moisture on the duration required to achieve a stable substrate-pH are useful information for substrate companies and growers, as well as a key component when modeling lime reaction over time. The pH of a 70 peat : 30 perlite (by volume) substrate was quantified over time under a range of different storage temperature (1.9 to 33.3°C) and substrate volumetric water content (VWC, 0.168 to 0.568 L of H2O/L of substrate). The lime source was a horticultural dolomitic carbonate limestone screened to the fraction that passed through a 100 US mesh but was retained on a 200 US mesh (0.075–0.15 mm) incorporated at 2.67 g·L−1 of substrate. Experiments provided two data sets for calibration and validation. Lime reaction rate increased with increasing substrate temperature and substrate moisture level. The duration required to reach a target substrate-pH value of 6.0 was used to indicate 90% of maximum pH effect from lime. Duration varied from 4 days with the combined high temperature (20.6 and 33.3 °C) and high VWC (0.468 and 0.568 L H2O/L of substrate) to 53 days with low temperature (1.9 °C) and low VWC (0.168 L H2O/L of substrate) for the calibration data set. At an example low VWC of 0.168 L of H2O/L of substrate, the duration required to reach substrate-pH 6.0 at 1.9, 7.8, 10, 20.6, and 33.3°C was 53, 38, 34, 23, and 17 days, respectively. Similarly, if the temperature was held constant at 33.3°C, reducing VWC from 0.568 L H2O/L to 0.128 L H2O/L would decrease lime reaction rate from 100% to 21%, requiring five times the duration to reach an equilibrium pH. Results can be used to compare the relative effects of moisture and temperature on lime reaction rate for substrate manufacturers and growers.

  PublisherDOI

• Potential to improve current mist irrigation control practices by young plant operations in the U.S.

  Acta Horticulturae · 2024-11-01 · 2 citations

  article1st authorCorresponding
  PublisherDOI

Frequent coauthors

• William R. Argo

  40 shared

• Sarah A. White

  35 shared

• Alexa J. Lamm

  University of Georgia

  34 shared

• Laura A. Warner

  32 shared

• Jinsheng Huang

  29 shared

• Peyton Beattie

  28 shared

• Royal D. Heins

  Utah State University

  18 shared

• Erik S. Runkle

  Michigan State University

  17 shared

Awards & honors

• Alex Laurie Award from the Society of American Florists (SAF…
• ASHS Extension Division Material Award (Website) for Greenho…
• Contributor to “Light Management in Controlled Environments”…
• UF Term Professorship Award for faculty achievement (2017)
• ASHS Kenneth Post Award for best graduate student research p…