Supplemental Lighting for California Nursery Production:

In California nursery production, light is one of the most important environmental factors influencing crop timing, plant quality, and uniformity. While the state is known for high annual solar radiation, many greenhouse operations still experience periods, especially from late fall through early spring, when natural light is not sufficient to meet the needs of certain crops. Shorter day lengths, frequent coastal fog, and prolonged cloudy periods can significantly reduce Daily Light Integral (DLI), particularly in propagation and liner production systems. 

The effects of these seasonal light limitations often show up quickly in production: stretched seedlings, uneven rooting, slower crop development, and inconsistent bench-to-bench uniformity. For growers working under tight production schedules, even small reductions in growth rate can translate into delayed shipments and increased labor costs for grading and sorting. Recent advances in supplemental LED lighting, combined with improved understanding of crop light requirements, are providing nurseries with more precise tools to manage these challenges.

Why Supplemental Lighting Matters in California Nurseries

Although California receives abundant sunlight compared with many production regions in North America, greenhouse conditions can still fall below optimal light levels for high-quality propagation. In a recent article, Bantis and Koukounaras (2023) highlight that light quantity and quality both strongly influence plant morphology, rooting, and biomass accumulation in horticultural crops. Under low light conditions, plants tend to allocate more energy toward stem elongation to reach more light rather than root development, producing weak and less uniform transplants.

In California nurseries, this is most evident during winter propagation cycles. Seasonal light availability varies significantly by region and weather pattern. According to national solar radiation mapping, outdoor Daily Light Integrals (DLIs) across California average 40 to 60 mol ∗ m^-2∗ day^-1 in summer but drop sharply to 10–20 mol ∗ m^-2∗ day^-1 during winter months (Faust & Logan, 2018).

Regional weather dynamics further suppress these light levels:

• Coastal Production: Marine fog and stratus layers attenuate photosynthetically active radiation (PAR) and reduce leaf-level photosynthesis by approximately 30% compared to clear sky conditions (Baguskas et al., 2018).
• Central Valley Operations: Multi-week Tule fog events create strong thermal inversions that trap persistent low cloud cover across the region (Gray et al., 2019), severely restricting daily solar radiation for weeks at a time.

When factoring in standard greenhouse glazing and overhead equipment which typically block 30% to 50% of incoming ambient light, bench-level DLIs during foggy winter stretches frequently collapse to 3-5 mol ∗ m^-2∗ day^-1. This falls far short of the 10-15 mol ∗ m^-2∗ day^-1 target recommended for healthy plug and liner propagation, directly extending production schedules and increasing labor costs.

For propagation and liner production, this reduction in light can have cascading effects:

• slower rooting of cuttings
• reduced carbohydrate accumulation in seedlings
• increased stretch and reduced compactness
• uneven growth across benches
• extended crop production cycle 

Improving light availability during these early stages often provides the greatest return on investment because plant quality gains carry through the entire production cycle.

DLI: The Most Useful Lighting Metric for Growers

Modern greenhouse lighting strategies increasingly rely on DLI rather than photoperiod or instantaneous light measurements. DLI represents the total amount of photosynthetically active radiation (PAR) received by a crop over a 24-hour period. Unlike PPFD (Photosynthetic Photon Flux Density), which measures light at a single moment, DLI reflects the actual light available for photosynthesis and growth each day.

For many ornamental nursery crops, recommended DLI ranges are typically:

• 10-15 mol ∗ m^-2∗ day^-1 for seedlings and plugs
• 12-20 mol ∗ m^-2∗ day^-1  for liners and vegetative crops (species dependent)

These ranges are general guidelines, as crop requirements vary by species, developmental stage, production objectives and environmental conditions. When natural DLI falls below these levels, supplemental lighting can be provided during daylight hours or used to extend the photoperiod, adding enough light to reach the target range. Recent work by Bantis and Koukounaras (2023) emphasizes that DLI-based lighting strategies allow growers to improve crop consistency while avoiding unnecessary energy use, an increasingly important consideration under California energy costs.

LEDs Are Now the Industry Standard for Supplemental Lighting

High-pressure sodium (HPS) lamps were once the dominant technology for greenhouse lighting. While effective, they are limited by high energy consumption, significant radiant heat output, and fixed spectral output. HPS lamps gradually lose light output and efficiency as they age, and replacement components become increasingly difficult and costly to source.

Light-emitting diode (LED) systems have largely replaced HPS in new greenhouse installations because they offer several advantages:

• Higher photon efficacy (more light per unit of energy)
• Longer fixture lifespan and reduced maintenance
• Lower radiant heat load on crops
• Ability to dim output and match light to crop demand
• Customizable spectral output
• Flexible fixture designs that accommodate different greenhouse structures and crop canopies

Historically, higher initial capital investment was a major barrier to LED adoption (Nelson & Bugbee, 2014). However, over the past decade, fixture costs have dropped substantially while photon efficacy (μmol ∗ J^-1) has continued to advance. Today, the initial price gap between HPS and LED systems has narrowed considerably. When combined with utility incentives/rebates available in California, lower maintenance, and substantial electricity savings, modern LED installations often achieve a rapid payback making them the most cost-effective long-term choice for nursery operations. In addition, LED systems can be integrated into automated controls that adjust output based on real-time sunlight conditions, allowing growers to supply only the light needed to meet target DLI.

Light Quality: More Than Just Brightness

Supplemental lighting is not only about increasing total light, but also about controlling light quality or spectrum (Fig. 1). Research in ornamental horticulture shows that different wavelengths influence plant form, growth and development:

• Red light drives photosynthesis and biomass accumulation and contributes to the regulation of plant morphology.
• Blue light promotes compact growth, stronger stems, chlorophyll accumulation and can affect water use and irrigation needs.
• Far-red light influences leaf and canopy expansion, stem elongation and flowering.

These effects vary by crop, developmental stage, light intensity, and the proportions of other wavelengths in the spectrum. A recent review by Trivellini et al. (2023) highlighted that programmable LED fixtures allow growers to fine-tune light spectra for specific crops and production stages. This is particularly valuable in nursery systems where crops may range from compact bedding plants to vigorous perennial liners. Instead of a “one-schedule-fits-all” approach, growers can now apply crop-specific lighting strategies that improve both quality and resource efficiency.

What the Research Shows About Supplemental Lighting

Across multiple studies, supplemental lighting has consistently improved greenhouse crop performance, particularly during seasons with limited natural light. Reported benefits include:

• faster rooting of cuttings and liners
• improved seedling vigor and uniformity
• shorter and more predictable production cycles
• greater biomass accumulation
• improved transplant quality and establishment
• more consistent crop timing

In a review article, Gomez and Izzo (2018) concluded that LED lighting systems can improve production efficiency through precise control of light intensity, duration, distribution, and spectral composition. They also noted that strategies such as dynamic dimming and improved canopy light distribution can reduce lighting energy use while maintaining crop productivity. Together, these findings suggest that supplemental lighting is most effective when integrated with crop scheduling and the management of temperature, irrigation, nutrition, and other environmental conditions rather than as a standalone input.

Looking Ahead: Smarter Lighting Systems for California Nurseries

The next evolution of supplemental lighting is not just better LEDs - it is smarter control systems.

Modern greenhouse lighting systems can now:

• adjust output based on real-time solar radiation
• respond to DLI targets automatically
• integrate with climate computers and irrigation systems
• optimize lighting based on energy pricing schedules

Recent research on intelligent greenhouse control systems demonstrates that predictive and sensor-based lighting strategies can significantly reduce energy use while maintaining crop growth targets (Nelson & Bugbee, 2014; Bantis & Koukounaras, 2023).

For California nurseries, these technologies are particularly relevant because of variable winter weather patterns and high electricity costs. Rather than operating lights on fixed timers, growers can manage supplemental lighting as a responsive input; delivering light only when crops actually need it (Fig. 2).

Key Takeaways for Growers

• Shorter winter days and persistent coastal cloud cover in California can reduce DLI below optimal levels for propagation.
• Supplemental lighting is critical during early crop stages (for uniformity and timing of propagation and liner production).
• LED systems provide long-term economic and operational advantages over HPS.
• Managing supplemental lighting based on crop-specific DLI improves both crop quality and energy efficiency.
• Smart lighting controls are becoming a key tool for precision nursery production.

References

Baguskas, S. A., Clemesha, R. E., & Loik, M. E. (2018). Coastal low cloudiness and fog enhance crop water use efficiency in a California agricultural system. Agricultural and Forest Meteorology, 252, 109-120. https://doi.org/10.1016/j.agrformet.2018.01.015

Bantis, F., & Koukounaras, A. (2023). Impact of Light on Horticultural Crops. Agriculture, 13(4), 828. https://doi.org/10.3390/agriculture13040828

Faust, J. E., & Logan, J. (2018). Daily light integral: A research review and high-resolution maps of the United States. HortScience, 53(9), 1250-1257. https://doi.org/10.21273/HORTSCI13144-18

Gray, E., Gilardoni, S., Baldocchi, D., McDonald, B. C., Facchini, M. C., & Goldstein, A. H. (2019). Impact of air pollution controls on radiation fog frequency in the Central Valley of California. Journal of Geophysical Research: Atmospheres, 124(11), 5889-5905. https://doi.org/10.1029/2018JD029419

Gómez, C., & Izzo, L. G. (2018). Increasing efficiency of crop production with LEDs. AIMS Agriculture and Food, 3(2), 135–153. https://doi.org/10.3934/agrfood.2018.2.135

Nelson, J. A., & Bugbee, B. (2014). Economic analysis of greenhouse lighting: LED vs HPS. PLOS ONE, 9(6), e99010. https://doi.org/10.1371/journal.pone.0099010

Trivellini, A., et al. (2023). LED lighting to produce high-quality ornamental plants. Plants, 12(8), 1667.https://doi.org/10.3390/plants12081667