By Arnold Schumann, Laura Waldo and Laura Cano-Castro
Florida citrus growers know the challenge all too well. Huanglongbing (HLB) disease continues to suppress yields, and the economics of open-field production grow tighter every season. Citrus under protective screen (CUPS) has emerged as one of the most promising strategies for restoring profitability. Originally designed to exclude the Asian citrus psyllid, the insect vector of HLB, the screen houses create a distinctive microclimate that fundamentally changes how citrus trees grow and produce fruit.
The production numbers tell the story. CUPS grapefruit trees grow roughly twice as fast as open-field counterparts. By year three, trees are producing 300 to 400 boxes per acre, climbing to around 800 boxes per acre by year five. Pack-out rates exceed 90%, and annual net fruit revenue can reach $20,000 per acre, putting growers on a path to break-even within seven to 10 years. What exactly is happening inside the screen house that drives such performance?
The CUPS enclosure uses 50-mesh high-density polyethylene (HDPE) screen with monofilament threads just 0.28 millimeters (mm) in diameter. The openings (roughly 0.78 by 0.26 mm) are small enough to block psyllid entry, but the screen also reshapes the growing environment in several important ways.
Wind speed inside the CUPS drops by about 75%, which lowers evapotranspiration (ET) by approximately 25%. Trees face less water stress and can direct more energy toward growth and fruit production. Daytime temperatures run a few degrees warmer than ambient, while nighttime lows stay essentially the same as outdoors. The dewpoint inside CUPS tends to be a few degrees Fahrenheit higher at night, reducing evaporative cooling during cold events and providing a measure of passive freeze protection.
SUNLIGHT TRANSFORMATION
Perhaps the most significant microclimate benefit involves the way the screen transforms sunlight. Researchers at the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) Citrus Research and Education Center (CREC) used spectrometry to measure solar radiation inside and outside the CUPS. The screen transmits about 78% of visible light, resulting in a 22% reduction in total photosynthetically active radiation (PAR).
At first glance, less light seems like a drawback, but the screen is not simply blocking light; it is redistributing it. Analysis across multiple spectral bands confirmed that the screen acts as a neutral attenuator and diffuser rather than a color-band filter, meaning it does not selectively alter wavelengths critical to photosynthesis.
Here is where it gets interesting. Diffuse light (the soft, scattered component of sunlight) increases by 190%, nearly 3x inside the CUPS compared to outside. Meanwhile, direct PAR drops by 55%. Each translucent HDPE thread functions like a tiny cylindrical rod lens, refracting the narrow beam of direct sunlight into a broad angular pattern. The entire screen roof essentially becomes a massive light-diffusing panel, delivering soft, near-uniform illumination to the canopy regardless of the sun’s position.
WHY DIFFUSE LIGHT MATTERS
This shift from direct to diffuse light has profound consequences for tree physiology. In an open grove, direct sunlight saturates the outer canopy leaves while the inner canopy stays in deep shade. Additional photons hitting outer leaves are wasted or cause sunburn, while inner leaves operate well below their photosynthetic potential.
Under CUPS, this dynamic is reversed. Outer-canopy leaves receive less direct light, reducing saturation and sunburn risk, while the increased diffuse light penetrates deeper into the canopy, reaching leaves that would otherwise be shaded. A larger proportion of the tree’s total leaf area is actively photosynthesizing at closer to optimal levels. This improvement in whole-canopy photosynthetic efficiency is likely the single most important factor explaining the dramatically higher yields in CUPS. Combined with freedom from HLB, lower ET stress, higher Brix, improved flavor, higher planting densities, storm protection and passive freeze protection, the CUPS microclimate delivers a package of advantages that open-field production simply cannot match.
KEY TAKEAWAYS
The UF/IFAS CREC research continues to show that CUPS is more than a psyllid-exclusion tool. It is a protected-agriculture system with a microclimate that actively enhances citrus production. The screen’s ability to convert harsh, directional sunlight into gentle, canopy-penetrating diffuse light may be the most underappreciated factor in its success, and growers evaluating CUPS should weigh this benefit alongside pest exclusion and storm protection.
With mature grapefruit yields exceeding 800 boxes per acre on just 160 to 180 pounds of nitrogen (an efficiency of 4.5 to 5.0 boxes per pound of nitrogen) and promising early results from specialty cultivars like Minneola tangelo (see sidebar), CUPS offers a viable path toward profitable citrus production in an HLB-endemic world. The screen house microclimate is a powerful, science-backed advantage that Florida growers can put to work.
Acknowledgments: The authors gratefully acknowledge the support of grower stakeholders and cooperators, UF/IFAS Extension agents, and laboratory and support staff. This work was funded by the Citrus Research and Education Foundation, the U.S. Department of Agriculture (USDA)/Florida Department of Agriculture and Consumer Services Specialty Crop Block Grant Program, the UF/IFAS Citrus Initiative and the USDA Emergency Citrus Disease Research and Extension Program. This material is based upon work supported by the USDA National Institute of Food and Agriculture (award number 2018-70016-27387).
Arnold Schumann (schumaw@ufl.edu) is a professor, Laura Waldo is a lab manager/biological scientist, and Laura Cano-Castro is a Ph.D. graduate assistant — all at the UF/IFAS CREC in Lake Alfred.
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