Artificial Light for Greenhouse Crops: LED, HPS, and How to Get It Right

Please note: Paramount Seeds is a seed supplier. We do not sell, supply, or endorse specific artificial lighting products or manufacturers. The information in this article is provided for educational purposes to help growers make informed decisions about their lighting programs.

Light is the engine of greenhouse crop production. When natural light is insufficient — whether due to season, latitude, or weather — artificial lighting bridges the gap between what the sun provides and what your crops need to perform. Getting lighting right is one of the highest-leverage decisions a greenhouse grower can make. This guide covers the fundamentals of artificial lighting, the key technologies available, and how to apply them to your specific crops.

Understanding Light in the Greenhouse Context

PAR, PPFD, and DLI — The Metrics That Matter

Before selecting lighting equipment, it's essential to understand the metrics used to quantify light for plant growth:

• PAR (Photosynthetically Active Radiation) — the wavelength range of light (400–700nm) that plants use for photosynthesis. Not all light is PAR; UV and far-red fall outside this range but still influence plant development.
• PPFD (Photosynthetic Photon Flux Density) — the intensity of PAR light measured at the plant canopy level, expressed in μmol/m²/s. This is the instantaneous light intensity your crop is receiving at any given moment.
• DLI (Daily Light Integral) — the total amount of PAR light received over a 24-hour period, expressed in mol/m²/day. DLI is the most useful metric for crop planning because it accounts for both intensity and duration. A high-intensity light run for a short period can deliver the same DLI as a lower-intensity light run longer.

DLI = PPFD (μmol/m²/s) × photoperiod (hours) × 0.0036

Understanding mol/m²/day

The unit mol/m²/day (moles of photons per square metre per day) can seem abstract at first. Here's what it actually means:

• A mole in this context is a count of photons — specifically, 6.022 × 10²³ photons (Avogadro's number). It's a way of expressing very large numbers of light particles in a manageable unit.
• Per square metre refers to the area of the plant canopy receiving that light — so DLI is a measure of light dose per unit of growing area.
• Per day means the total accumulated over a full 24-hour period, combining both the intensity of the light and how long it shines.

In practical terms: a DLI of 20 mol/m²/day means that one square metre of your crop canopy received 20 moles of photosynthetically active photons over the course of the day. You can achieve that same DLI with a high-intensity light running for fewer hours, or a lower-intensity light running longer — the crop responds to the total dose, not just the peak intensity.

A worked example: A greenhouse LED fixture delivering 300 μmol/m²/s at canopy level, running for 18 hours, delivers a DLI of:

300 × 18 × 0.0036 = 19.4 mol/m²/day

Add 6 mol/m²/day of natural winter light and your total DLI reaches approximately 25 mol/m²/day — within the optimal range for tomato and cucumber production.

Natural Light as the Baseline

In most greenhouse production regions, natural DLI varies significantly by season. In northern latitudes, winter DLI can drop to 5–10 mol/m²/day — well below the minimum for productive greenhouse vegetable crops. Supplemental lighting fills this deficit. In some year-round operations or vertical farms, sole-source lighting replaces natural light entirely.

Lighting Technologies: LED vs. HPS

High-Pressure Sodium (HPS)

HPS has been the greenhouse lighting standard for decades. Key characteristics:

• Proven track record — decades of data on crop response across all major greenhouse vegetables
• High output — HPS fixtures deliver high PPFD at relatively low capital cost per μmol
• Heat output — HPS generates significant radiant heat, which can be beneficial in cold climates (reducing heating costs) but problematic in warm climates or summer production
• Spectrum — HPS is heavy in the yellow-orange range (550–650nm); effective for photosynthesis but lacks the blue spectrum that influences plant morphology and compactness
• Efficiency — typically 1.0–1.7 μmol/J; lower than modern LED
• Lifespan — 10,000–18,000 hours before significant lumen depreciation

LED (Light Emitting Diode)

LED technology has advanced rapidly and is now the preferred choice for most new greenhouse lighting installations. Key characteristics:

• High efficiency — modern greenhouse LEDs deliver 2.5–3.5+ μmol/J, significantly reducing electricity costs per unit of light delivered
• Tunable spectrum — LED fixtures can be designed with specific red:blue ratios and can include far-red, UV, and other wavelengths to influence plant morphology, flowering, and secondary metabolite production
• Low heat output — LEDs produce minimal radiant heat at the canopy, reducing heat stress risk and allowing fixtures to be placed closer to the crop
• Long lifespan — 50,000+ hours with minimal lumen depreciation
• Higher capital cost — LED fixtures cost more upfront than HPS, but lower operating costs typically deliver payback within 3–5 years depending on electricity rates and operating hours
• Dimming capability — most LED systems can be dimmed and controlled, allowing dynamic light management based on natural light levels

Which to Choose?

For new installations, LED is the clear choice in most situations. The efficiency advantage, spectrum flexibility, and long lifespan outweigh the higher upfront cost. HPS remains viable for operations with existing infrastructure, low electricity costs, or cold climates where the heat output is a benefit rather than a liability.

DLI Targets by Crop

Each greenhouse crop has a target DLI range for optimal production. Supplemental lighting should be designed to bring total DLI (natural + artificial) within these ranges:

Tomato

• Minimum DLI: 20 mol/m²/day
• Optimal DLI: 25–35 mol/m²/day
• Maximum photoperiod: 18 hours (tomato requires a dark period; continuous light causes leaf curl and physiological disorders in most varieties)
• Notes: Tomato is highly responsive to increased DLI up to approximately 30–35 mol/m²/day. Above this threshold, returns diminish and heat stress risk increases. Intercanopy lighting (placing fixtures within the canopy rather than overhead only) significantly improves light distribution in tall tomato crops.

Browse our Greenhouse Tomato varieties.

Cucumber

• Minimum DLI: 20 mol/m²/day
• Optimal DLI: 25–30 mol/m²/day
• Maximum photoperiod: 18–20 hours
• Notes: Cucumber is a high-light crop with strong yield response to increased DLI. Fruit set and development are particularly sensitive to light levels. Intercanopy lighting is beneficial in long-season cucumber production.

Browse our Greenhouse Cucumber varieties.

Lettuce

• Minimum DLI: 12 mol/m²/day
• Optimal DLI: 17–22 mol/m²/day
• Maximum photoperiod: 20 hours (some varieties tolerate continuous light; check variety specifications)
• Notes: Lettuce is a lower-light crop compared to fruiting vegetables, making it well suited to sole-source LED production. Tip burn risk increases at very high DLI or under poor airflow — blue light and airflow management are both important. Far-red light can be used to accelerate growth and increase fresh weight at harvest.

Browse our Greenhouse Lettuce varieties.

Pepper

• Minimum DLI: 20 mol/m²/day
• Optimal DLI: 25–30 mol/m²/day
• Maximum photoperiod: 16–18 hours
• Notes: Pepper is sensitive to photoperiod — some varieties are day-length sensitive for flowering. Confirm variety requirements before setting photoperiod. High DLI supports fruit set and color development, particularly in colored bell pepper production.

Browse our Greenhouse Pepper varieties.

Microgreens

• Optimal DLI: 12–20 mol/m²/day depending on species
• Maximum photoperiod: 16–18 hours
• Notes: Microgreens have lower light requirements than mature vegetable crops. LED is the preferred technology for microgreens due to low heat output and the ability to place fixtures close to the crop. Avoid excessive intensity, which can cause bleaching and tip burn in sensitive species like basil and amaranth.

Photoperiod Management

Photoperiod — the duration of the light period — influences both photosynthesis (through DLI accumulation) and plant development (through photoperiodism). Key principles:

• Most greenhouse vegetables are day-neutral for flowering — they flower based on age and development rather than day length. However, photoperiod still affects growth rate and DLI accumulation.
• Avoid continuous light for tomato — most tomato varieties develop leaf curl and interveinal chlorosis under 24-hour lighting. Maintain a minimum 6-hour dark period.
• Use dynamic lighting control — modern LED systems with light sensors can automatically adjust supplemental light output based on real-time natural light levels, maintaining a target PPFD setpoint and maximizing energy efficiency.
• Night interruption lighting — for photoperiod-sensitive crops, a brief period of low-intensity light during the night can substitute for a full extended photoperiod, reducing energy costs while achieving the desired developmental response.

Spectrum Considerations

Light spectrum influences plant morphology, development, and secondary metabolite production beyond its role in photosynthesis:

• Blue light (400–500nm) — promotes compact growth, thick leaves, and stomatal opening. Insufficient blue light leads to etiolation (stretching) and weak stems. A minimum of 10–15% blue in the spectrum is recommended for most crops.
• Red light (600–700nm) — the most efficient wavelength for photosynthesis. Most LED fixtures are red-dominant for this reason.
• Far-red light (700–800nm) — not PAR, but highly effective at accelerating growth through the Emerson enhancement effect. Adding far-red to a red/blue spectrum can increase photosynthesis efficiency and accelerate crop development. Particularly valuable in lettuce production.
• Green light (500–600nm) — penetrates deeper into the canopy than red or blue; contributes to photosynthesis in lower leaves. Full-spectrum white LEDs include green and are preferred by some growers for worker comfort and visual crop assessment.

Practical Lighting Design Considerations

• Uniformity — aim for less than 20% variation in PPFD across the growing area. Poor uniformity creates uneven crop development and complicates management.
• Mounting height — LED fixtures can be mounted closer to the crop than HPS due to lower heat output. Intercanopy LED systems in tall crops (tomato, cucumber) dramatically improve light distribution to lower leaves.
• Reflective surfaces — white or reflective greenhouse cladding, floors, and walls significantly improve light utilization efficiency.
• Energy management — lighting is typically the largest operating cost in a supplemental lighting program. Invest in smart controls, light sensors, and dimming capability to minimize energy waste.
• Heat management — even low-heat LED fixtures add thermal load to the greenhouse. Account for lighting heat output in your climate control calculations.

The Bottom Line

Note: Paramount Seeds does not supply artificial lighting products or equipment. For lighting procurement, consult specialist greenhouse lighting suppliers and request photometric plans specific to your greenhouse dimensions and crop requirements.

Artificial lighting is a precision tool — not a switch you flip and forget. The right technology, the right DLI target, the right spectrum, and the right controls all work together to deliver consistent, high-quality crops year-round. For most new greenhouse operations, LED is the technology of choice, and DLI management is the discipline that turns that technology into results.

Contact the Paramount Seeds team 

Browse our full Greenhouse Seed catalog for varieties selected for performance under supplemental lighting conditions.