How your crop and business benefit from the transition from HPS to LED

History

Supplemental lighting is essential in modern greenhouses, particularly in regions where shorter day lengths and reduced solar radiation do not meet the requirements for productive crop growth. The necessary amount of supplemental light depends on several factors: the greenhouse’s location, time of year, crop species, the required light spectrum, and electricity costs.

Globally, greenhouse energy use is increasing due to more widespread and intensive use of heating and supplemental lighting. In northern latitudes, intensive heating is often required to maintain a favourable temperature for year-round cultivation. Supplemental lighting, especially during autumn and winter, is crucial for photoperiod control, the regulation of developmental processes such as flowering, and to ensure sufficient biomass production.

Energy efficacy

Horticultural lighting systems were first developed in the mid-19th century and became widely adopted in the industry by the 20th century. These systems primarily relied on light sources such as high-pressure sodium (HPS) lamps, metal halide lamps, and fluorescent lights. While these traditional sources successfully provided supplemental light for plant growth, they offered limited flexibility in spectral tuning to optimize specific plant responses.

In the 1980s, the National Aeronautics and Space Administration (NASA) began investigating light-emitting diodes (LEDs) for plant growth in space, which later contributed to an industry-wide transition from gas-discharge to solid-state lighting systems.

The ongoing shift from HPS to LED lighting has significantly improved energy efficiency, primarily due to the higher photon efficacy of LED chips (3.8–4.5 μmol J⁻¹) compared to HPS fixtures (1.9 μmol J⁻¹) in converting electrical input into photosynthetically active radiation (PAR). The energy performance of LEDs is expected to continue improving, further increasing their advantage over conventional horticultural lighting systems.

LED transition

In the Netherlands, both the greenhouse area using artificial lighting and the applied light intensity have increased rapidly in recent years. This trend is reflected by a decrease in energy use for heating and a corresponding increase in energy use for lighting. Transitioning from HPS to LEDs can reduce lighting-related energy costs by approximately 10–25%.

In contrast, adoption of LED lighting in the United States is increasing, with LEDs accounting for 16% in 2016, and 78% in 2024. The transition to LEDs is especially critical for high-light crops such as cannabis, which require long photoperiods and high light intensities. In such cases, the potential for energy savings is greatest, although the total percentage of savings is ultimately constrained by the proportion of total energy use attributed to lighting.

Reduced radiant heat output

LEDs emit negligible amounts of near-infrared radiation, in contrast to HPS lamps. However, because HPS lamps contribute significantly to radiant heat, a transition to full LED systems requires strategic adjustments to compensate for the loss of thermal input to the greenhouse environment (Fig. 1).

In the Netherlands, for instance, 25–30% of the heating demand was historically met by high-intensity discharge (HID) lighting. In contrast, Nordic countries are well-positioned for a smooth adoption of LED lighting, thanks to the availability of renewable energy sources such as geothermal energy. Utilizing geothermal heat can reduce production costs by up to 35%, enabling commercial greenhouse production in colder climates that would otherwise be economically unfeasible. In such settings, where heating is both inexpensive and environmentally sustainable, the benefits of LEDs’ lower electrical demand outweigh the costs of additional heating.

Many greenhouses also use combined heat and power (CHP) systems to produce electricity, heat, and CO₂ by burning natural gas. In HPS-equipped greenhouses, the excess heat from CHP generators often needs to be ventilated, inadvertently increasing the carbon footprint. In contrast, an LED-based greenhouse can more efficiently match its electrical demand with the thermal output of a CHP system, minimizing waste and emissions. Consequently, greenhouses relying on CHP will see substantial energy savings when transitioning to LED lighting.

Finally, LEDs offer customisable light quality, precise photoperiod control, and high-intensity output with minimal radiant heat. This enables tailored light schedules to optimise photosynthesis, plant morphology, and overall yield.

Figure 1: Energy savings in greenhouses by transition from high-pressure sodium to LED lighting (Katzin et al. 2021)

Effect of radiant heat on other environmental parameters

It is important to note that the use of HPS lighting, particularly in crops requiring high light intensities and long photoperiods, significantly increases air temperature within the greenhouse. During summer, this can result in suboptimal air temperatures for crop growth and further elevate humidity levels due to increased plant transpiration. As both air temperature and humidity rise, ventilation rates must increase, which may result in the loss of supplemented CO₂. Additionally, energy costs may increase to maintain optimal conditions for temperature and humidity. In winter, excess radiant heat from HPS can also drive increased ventilation, again due to elevated humidity from transpiration.

Moreover, the type of light source and the associated radiant heat directly affect plant tissue temperature. In extreme cases, where irrigation and air movement are limited, plant temperatures may exceed air temperatures by 6 to 12°C, depending on the light source. Higher plant temperature accelerates developmental rates, which may have negative effects when optimal plant temperature thresholds are surpassed.

Under typical growing conditions, the use of LEDs results in a leaf temperature reduction of approximately 1.3°C compared to HPS lighting, though this difference is often less than expected when transitioning to LED systems.

Figure 2: Analysis of environmental effects on leaf temperature under sunlight, high pressure sodium and light emitting diodes (Nelson and Bugbee 2015)

Additional benefits of LEDs

LEDs not only provide higher energy efficiency, but they also enable increases in light intensity and photoperiod duration that are often unfeasible with HPS systems. In greenhouses, especially during summer, using HPS at high intensities can elevate air temperatures to suboptimal levels, negatively affecting crop yield and quality. This is particularly problematic when blackout screens are required to prevent light pollution, as is mandated in many countries, further limiting the ability to vent excess heat.

Additionally, LEDs offer greater flexibility in environmental control. Light intensity can be dynamically adjusted in response to real-time electricity prices, allowing for more cost-effective energy use. Moreover, the light spectrum can be tailored to steer specific aspects of plant growth and development, a level of control not possible with HPS fixtures.

Sources:
Holweg, M. (2025).  Photobiology of medicinal cannabis: Pharmaceutical compounds and crop morphology (Doctoral dissertation, Wageningen University and Research).

Katzin, D., Marcelis, L. F., & van Mourik, S. (2021). Energy savings in greenhouses by transition from high-pressure sodium to LED lighting. Applied Energy, 281, 116019.

Nelson, J. A., & Bugbee, B. (2015). Analysis of environmental effects on leaf temperature under sunlight, high pressure sodium and light emitting diodes. PloS one, 10(10), e0138930.

Cannabis Business Times ‘2024 State of the Cannabis Lighting Market’