What Does Proper Greenhouse Lighting Look Like in Oklahoma Winters?

Winter in Oklahoma presents a predictable but challenging mix of short days, low sun angle, intermittent cloud cover, and often cold nights. For growers using greenhouses in this region, “proper lighting” is a combination of understanding what the natural light delivers, defining crop light requirements, and adding the right supplemental lighting with effective control and placement. This article lays out concrete numbers, design principles, equipment choices, and practical steps to get greenhouse lighting right for Oklahoma winters.

Winter light realities in Oklahoma: what the sun actually gives you

Oklahoma sits roughly between latitudes 34.5 and 37.0 north. In late December the clear-day solar geometry and daylength mean available natural light is substantially lower than in summer. Expect these general conditions during winter months:

• Daily daylight hours: roughly 9 to 10.5 hours around the winter solstice, increasing afterward through February and March.
• Sun angle and glazing transmission: Lower sun angle reduces direct penetration into tall benches and north-side areas. Single-layer glazing and older polyethylene may transmit 70 percent or less of incident light in winter after soiling and sun angle losses. Clear glass and double-wall polycarbonate perform better but still suffer seasonal loss.
• Weather variability: Overcast stretches can drop light to a fraction of a sunny day’s total. Cloudy days are common and reduce instantaneous PPFD (photosynthetic photon flux density) and cumulative DLI (daily light integral).

Because of those factors, many Oklahoma greenhouse operations cannot rely on natural daylight alone to meet crop light targets in winter.

Key metrics: PPFD and DLI and target ranges for common crops

Understanding these metrics is essential for specifying supplemental lighting.

• PPFD (umol/m2/s): the instantaneous photon flux density in the 400-700 nm photosynthetic range.
• DLI (mol/m2/day): the integrated daily quantity of photosynthetic photons; it is PPFD averaged over the photoperiod and multiplied by the number of seconds.

Typical target DLI and PPFD ranges by crop habit:

• Seedlings and ornamentals in propagation: PPFD 50 to 150 umol/m2/s; DLI 6 to 12 mol/m2/day.
• Leafy greens and herbs: PPFD 150 to 350 umol/m2/s; DLI 10 to 20 mol/m2/day depending on crop and cultivar.
• Fruiting crops (tomato, cucumber, pepper): PPFD 300 to 600 umol/m2/s; DLI 20 to 30+ mol/m2/day for high yield.

Practical note: In Oklahoma winters, natural DLI in a well-maintained, clear greenhouse on a sunny day may still be in the single digits (for example 4 to 8 mol/m2/day). Cloudy periods will push that lower. Most growers will need supplemental lighting to reach the 12+ DLI required for many high-yield crops.

Choosing supplemental lighting: LEDs, HPS, and the trade-offs

There are three main practical choices for supplemental greenhouse lighting: LED fixtures, high pressure sodium (HPS), and to a lesser degree ceramic metal halide. LEDs are increasingly dominant for greenhouse winter lighting; here are concrete considerations.

• Energy efficiency and efficacy: Contemporary greenhouse LEDs routinely deliver 2.5 to 3.5 umol/J in commercial fixtures. HPS fixtures are typically 1.5 to 2.4 umol/J. Higher efficacy means lower electrical cost for the same PPF output.
• Spectrum: LEDs allow spectral tuning. For balanced vegetative growth and flowering, fixtures that provide a broad white spectrum with supplemental red and some far-red options are versatile. HPS provides a red-heavy spectrum that can be fine for many crops but lacks flexibility.
• Heat load: HPS produces significant radiant heat; in winter that can reduce heating fuel needs but may create hot spots and uneven temperatures. LEDs emit less radiant heat but the electrical-to-light conversion still creates heat that must be managed with ventilation or heating adjustments.
• Initial cost and lifespan: LEDs cost more upfront but typically have longer lifespans, lower maintenance, and better dimming/control. HPS fixtures are cheaper initially but require lamp replacement and ballast maintenance.

Calculating how much supplemental light you need: a step-by-step example

This practical workflow will let you size fixtures and estimate energy use.

1. Define your target DLI and photoperiod.
2. Example: You want a DLI of 16 mol/m2/day for a leafy green bench. You plan a 14-hour photoperiod (common winter practice that uses supplemental night lighting to extend daylength).
3. Measure or estimate natural DLI. Suppose on average natural DLI in your greenhouse in January is 4 mol/m2/day.
4. Required supplemental DLI = target DLI – natural DLI = 16 – 4 = 12 mol/m2/day.
5. Convert required DLI to average supplemental PPFD: PPFD (umol/m2/s) = (DLI * 1,000,000 umol/mol) / (photoperiod seconds). Simpler approx: PPFD = DLI * 11.57 / photoperiod hours. For 12 mol/day over 14 hours: PPFD = 12 * 11.57 / 14 9.92 umol/m2/s? That looks off due to units; use practical rule of thumb: 1 mol/m2/day 11.57 umol/m2/s averaged over 24 hours. Over H hours photoperiod, average PPFD = (DLI * 1,000,000) / (H * 3600). For calculation: 12 mol/day = 12,000,000 umol/day. Photoperiod seconds = 14*3600 = 50,400 s. So PPFD 12,000,000 / 50,400 238 umol/m2/s.
6. So you need an average supplemental PPFD of ~238 umol/m2/s during the 14-hour lighting window.
7. Multiply by area to find required PPF. For a 100 m2 bench area: required PPF = 238 umol/m2/s * 100 m2 = 23,800 umol/s.
8. Convert PPF to electrical watts using fixture efficacy. With LEDs at 2.8 umol/J, electrical watts = 23,800 / 2.8 8,500 W, or 8.5 kW.
9. Energy per day during 14 hours = 8.5 kW * 14 h 119 kWh/day. Multiply by electricity cost to estimate daily cost.

This example shows why fixture efficacy, photoperiod choice, and accurate measurement of natural DLI are critical to cost control.

Practical fixture placement, uniformity, and controls

Lighting is not just quantity; distribution matters.

• Mounting height and spacing: Use manufacturer PPFD maps to space fixtures for 10 to 20 percent uniformity targets. Lower mounting height increases PPFD but reduces footprint per fixture and increases nonuniformity. For bench crops, 1 to 2 foot mounting heights above canopy are common for high PPFD; for tall crops, higher mounting is needed.
• Uniformity target: Strive for an average PPFD uniformity ratio (min/avg) of at least 0.7. Poor uniformity creates slow spots and spotty growth.
• Dimming and daylight integration: Use dimming drivers and daylight sensors. On sunny winter days, natural light may be enough part of the day; dim LEDs accordingly to save energy. Control systems can integrate PAR sensors to maintain target PPFD or DLI.
• Scheduling and photoperiods: Long-day crops may need 14 to 16 hours; short-day crops should avoid accidental night lighting. Timers and motion-proof wiring are essential.
• Photoperiod management tools: For flowering control, blackout curtains or light-proofing may be necessary to prevent light leakage at night, especially for short-day ornamentals.

Interplay with heating and energy management

Lighting choices affect greenhouse thermal management in winter.

• Heat contribution: HPS will reduce heater runtime but can overheat near lamps. LEDs produce less radiant heat, so heaters must be sized accordingly.
• Night heating vs daytime lighting: In many Oklahoma greenhouses, it is cheaper to raise daytime supplemental light and let nights remain at a slightly lower setpoint than to heat fully. But crop-specific minimum night temperatures must be maintained.
• Insulate and reduce losses: Double-layer inflation, thermal curtains at night, and sealing air leaks reduce required supplemental lighting and heating energy by conserving the light you already have and reducing temperature swings that force higher light intensity for stress mitigation.

Monitoring and measurement: tools every grower needs

• A handheld PAR meter (PPFD meter) is indispensable to measure canopy PPFD at representative positions during supplemental light operation and sunny/cloudy days.
• Data logging of PAR and temperature: If you can log hourly PAR you can calculate real DLI and adjust schedules to hit crop targets precisely.
• Energy monitoring: Measure electrical draw of lighting circuits to validate design and to find savings.

Crop-specific winter strategies for Oklahoma growers

• Lettuce and microgreens: Moderate DLI targets (10-18). Use broad-spectrum LEDs and aim for 12-16 mol/day with a 12-16 hour photoperiod. LEDs with 2.8-3 umol/J keep electricity use manageable.
• Basil and herbs: Many herbs perform best with DLI 12-18. Pay attention to leaf quality and flavor, which can decline under low winter light. Consider supplemental light early in the day to reduce night respiration losses.
• Tomatoes and cucumbers: High DLI crops are the most expensive to grow in winter. If winter production is required, expect substantial supplemental lighting costs; use high-efficacy LEDs and maximize daylight capture using white exterior surfaces and clean glazing.

A practical winter lighting checklist for Oklahoma greenhouses

• Measure natural DLI for at least a week in December-January using a PAR logger.
• Define crop-specific DLI and PPFD targets based on cultivar and yield goals.
• Calculate supplemental PPFD and total fixture PPF using a realistic photoperiod.
• Choose high-efficacy LED fixtures (2.5-3.5 umol/J) sized and spaced using manufacturer PPFD maps to meet uniformity targets.
• Install dimming controls and PAR sensors for daylight-adaptive control.
• Use thermal curtains and improve glazing cleanliness to reduce required supplemental lighting.
• Monitor canopy PPFD, DLI and energy use weekly and adjust schedules.

Final practical takeaways

• Oklahoma winters usually require meaningful supplemental lighting for most greenhouse crops; assume natural DLI will be insufficient for high-yield crops.
• Use DLI and PPFD to size lighting rather than guesswork. Measure actual DLI in your greenhouse.
• LEDs are the most cost-effective long-term choice for winter supplemental lighting because of their high efficacy, controllability, and lower maintenance.
• Controls (dimming, sensors, schedules) and good greenhouse thermal management reduce operating costs and improve crop uniformity.
• Start small and iterate: install sensors and a modular lighting array you can expand, then tune schedules to hit crop-specific DLI targets while watching energy costs.

Proper greenhouse lighting in Oklahoma winters is a mix of careful measurement, realistic crop targets, high-efficacy fixtures, and disciplined control. When those elements are combined, growers can produce high-quality crops efficiently even during the darkest months.