Greenhouse growers in Connecticut face a unique combination of climatic and market pressures that make supplemental lighting more than a convenience — it is often essential. This article explains the scientific, agronomic, and economic reasons Connecticut greenhouses need supplemental lighting, describes how much light crops typically require, compares lighting technologies, and offers practical recommendations growers can implement to optimize production and control costs.
Connecticut climate and the light problem
Connecticut lies in the northeastern United States where winter days are short, solar angle is low, and overcast conditions are frequent. Natural sunlight that reaches greenhouse benches in December and January can be as little as a fraction of summer irradiance. For many crops, especially year-round production of greens, ornamentals, and vegetable transplants, natural sunlight during the late fall through early spring does not supply sufficient photosynthetically active radiation (PAR) to meet plant growth needs.
Two related factors make supplemental lighting necessary:
- Seasonal daylength reduction. Daylength falls below 10 hours in midwinter, reducing the daily light integral (DLI) plants receive even on clear days.
- Low solar intensity. Even during daylight hours the solar angle is low and diffuse light dominates; cloud cover further reduces instantaneous PPFD (photosynthetic photon flux density).
Both factors mean plants receive fewer photons per day than they need for optimal photosynthesis, growth, quality, and yield.
Key plant-light concepts every grower should know
Understanding a few technical terms helps translate lighting needs into practical actions.
PPFD, DLI, and photoperiod
- PPFD (mmolm-2s-1) is the instantaneous rate of PAR falling on a surface. Greenhouse lighting design uses PPFD to size fixtures and set intensity.
- DLI (molm-2d-1) is the cumulative number of moles of PAR per square meter per day. It is PPFD integrated over the photoperiod. Many crops have DLI targets for optimal growth.
- Photoperiod (hours of light per day) influences flowering and developmental pathways in many ornamentals and bedding plants.
A useful conversion: DLI = PPFD x hours of light x 0.0036. Example: 200 mmolm-2s-1 maintained for 12 hours produces 200 x 12 x 0.0036 = 8.64 molm-2d-1.
Typical DLI targets (crop-dependent)
- Leafy greens (e.g., lettuce, spinach): 12-17 molm-2d-1 for quality and fast growth.
- Herbs: 12-18 molm-2d-1 depending on species and flavor quality.
- Tomatoes, peppers, cucumbers (fruiting crops): 18-30+ molm-2d-1 to maximize flowering, fruit set, and yield.
- Bedding plants and flowering ornamentals: Many need specific photoperiods rather than high DLI; poinsettias are short-day, while others are long-day or day-neutral.
Because Connecticut winter DLI values can fall well below these targets, supplemental lighting is necessary to maintain production schedules and crop quality.
Why growers choose supplemental lighting: agronomy and scheduling
Supplemental lighting is used for several distinct purposes in Connecticut greenhouses:
- To increase daily photon supply so crops meet their DLI targets, improving growth rate, biomass accumulation, and crop uniformity.
- To accelerate crop cycles (shorter time to market) and therefore increase throughput and revenue per bench area per year.
- To manipulate photoperiod for flowering control: inducing or preventing flowering by providing night breaks or extended daylength.
- To maintain seedling compactness and quality in propagation houses where greater PPFD produces sturdier transplants.
- To provide consistent production when supplemental heat and light can allow winter production of high-value crops that otherwise would be uneconomic.
Each objective influences fixture choice, spectrum, intensity, timing, and control strategy.
Lighting technologies: pros and cons for Connecticut greenhouses
Selecting the right technology requires balancing energy efficiency, light output, spectrum control, heat production, lifespan, and upfront cost.
- High Pressure Sodium (HPS)
- Pros: Historically favored for high light output and relatively low capital cost per installed watt.
- Cons: Lower electrical-to-PAR efficiency than modern LEDs, yellow-orange spectrum less optimal for photosynthesis per watt, significant heat output that can disrupt microclimates, short ballast life relative to LEDs.
- Light Emitting Diodes (LEDs)
- Pros: Highest electrical efficiency in converting electricity to useful PAR, tunable spectrum (blue:red:far-red ratios), low radiant heat allowing fixtures closer to the canopy, long lifespan, dimmable and rapidly responsive, improved uniformity through fixture placement.
- Cons: Higher initial capital cost though rapidly falling, requires good design to achieve uniform coverage, spectrum choices must match crop goals.
- Ceramic Metal Halide and Fluorescent (including T5)
- Pros: Used in propagation benches for their broader spectrum and lower initial cost for small installations.
- Cons: Lower efficiency and shorter lifetime than LEDs for large-scale supplemental lighting.
In Connecticut, LEDs are increasingly the practical choice for new supplemental lighting systems because the energy savings, long life, and spectrum control reduce total cost of ownership and improve crop outcomes, especially when utilities offer incentives.
Design and control considerations
Good results require more than just buying fixtures. Consider these practical elements.
Uniformity and mounting
- Aim for uniform PPFD across production areas; coefficient of variation (CV) for PPFD should ideally be < 0.3 (30%) to avoid areas of under- and over-exposure.
- Mount height depends on fixture optics and crop stage. LEDs can be placed closer to canopy, but avoid scorching or excessive localized light if fixtures have narrow beams.
- Use reflective surfaces, hanging patterns, and interlighting for tall crops to improve canopy penetration.
Controls and sensors
- Dimming and scheduling are critical. Dimming saves energy during shoulder seasons and allows precise DLI control.
- Quantum sensors and PAR meters let you measure PPFD and confirm DLI. Use datalogging to track integrated DLI and adjust schedules.
- Daylight-harvesting or photocell control can reduce artificial light when natural light rises, saving energy while maintaining target DLI.
Integrate with HVAC
- LEDs emit less heat than HPS; reduced waste heat can lower greenhouse temperature and heating fuel offset. But in winter, lost heat may increase heating demand. Design HVAC accordingly.
- Consider zoning lighting and heating so high-light sections can receive more heat if needed to improve crop quality.
Economic and operational considerations
Supplemental lighting is an investment that should be evaluated on crop value, energy cost, and expected yield improvements.
- Calculate expected additional yield (or crop cycles) from achieving target DLI and balance against energy cost per mol of photons delivered and fixture amortization.
- LEDs usually offer lower operating cost per mol of PAR delivered and longer life, often paying back through energy savings and reduced maintenance.
- Seek utility rebates and state incentives; many utilities in the region provide programs for energy-efficient lighting installations that materially improve payback.
- Schedule lighting strategically: provide full supplemental lighting during darkest months and use daylight-harvesting/dimming during shoulder seasons to reduce cost.
Practical examples and rules of thumb
- For leafy green production in Connecticut winter, aim to provide enough supplemental lighting to reach a DLI of roughly 12-15 molm-2d-1. If natural DLI is 4 molm-2d-1, the supplement must provide about 8-11 molm-2d-1.
- To convert desired DLI to PPFD for a given photoperiod: PPFD (mmolm-2s-1) = DLI / (hours x 0.0036). For example, to reach 12 mol with 14 hours of light: PPFD = 12 / (14 x 0.0036) 240 mmolm-2s-1.
- For fruiting crops like tomato with DLI targets of 18-25 mol, supplemental lighting must be substantially higher, and growers commonly use interlighting combined with top lighting to penetrate dense canopies.
Installation checklist for Connecticut growers
- Conduct a light audit: measure winter DLI across benches on representative days to determine supplemental requirement.
- Choose fixtures based on efficiency (mmol/J), spectrum, canopy penetration, warranty, and total cost of ownership.
- Design for uniformity: create a layout and mounting heights that minimize PPFD variation.
- Install controls: timers, dimmers, and sensors (quantum sensors and photocells) to maintain targets and harvest daylight savings.
- Model energy cost vs. revenue gains: estimate payback period and investigate utility incentives.
- Train staff on measuring DLI, adjusting schedules, and maintaining fixtures.
Final recommendations
Supplemental lighting is not an optional luxury for many Connecticut greenhouse operations — it is a strategic tool that enables consistent, high-quality production year-round. Growers should:
- Start with measured data: quantify current winter DLI and specify crop targets before purchasing fixtures.
- Prioritize LEDs for new installations because of their efficiency, spectrum control, and lower long-term costs.
- Use controls and sensors to avoid over-lighting and to exploit natural sunlight when available.
- Consider photoperiod manipulation for flowering control as a complement to DLI management.
- Evaluate the economics by calculating additional yield value against energy and capital costs and seek available incentives.
By combining accurate measurement, crop-specific targets, efficient fixtures, and smart control strategies, Connecticut greenhouse operators can overcome winter light deficits, shorten crop cycles, increase yields and quality, and do so with a responsible approach to energy use and expenditure.