HPS or LED? Why not both with a hybrid lighting system

Jillian Whitehead

Growing Advice, Hybrid Lighting, Lighting Strategies, Lighting Technologies | February 10, 2020

What is a hybrid system?

Choosing between a traditional lighting system such as High Pressure Sodium (HPS) or Metal Halide (MH) lighting and newer Light Emitting Diode (LED) technology can be a complicated decision. Factors such as energy use, ROI, maintenance costs and how they affect the growth of your plants should be considered.  Many growers are looking to switch over to LED lighting technology for reasons such as energy savings or fine tuning spectral recipes. However, given the young state of the industry and high initial costs it can be overwhelming and cost prohibitive to change over. There are many benefits and advantages to both LED and high intensity discharge (HID) technologies and one of the best ways to take advantage of them can be to implement a hybrid lighting system.

A hybrid lighting system is where a growing facility installs both LED and HPS lighting (or even two different styles of lamps such as combining HPS and MH) in one grow space. In the case of an LED/HPS hybrid application, this combination of the two technologies can provide the ideal balance between diffuse light distribution and spectral efficiencies. It also allows the flexibility to take advantage of the radiant heating from the HPS lighting, but also to extend the supplemental lighting period longer into the spring by using the LED lights only when outside temperatures and daylight hours increase. A hybrid system also reduces the initial capex and can allow a grower to further manage costs by maintaining their existing infrastructure.

Lighting Overview

High pressure sodium (HPS) lighting has been the mainstay in commercial horticultural applications since its introduction.  The spectrum is rich in red wavelengths that provide an effect of daylength perception in plants promoting both growth and flowering in long-day plants like cannabis. But this spectrum is comparatively poor on the blue end of the spectrum, which inhibits stem elongation. HPS lamps also produce a significant amount of radiant heat in the plane of illumination.  This radiant heat is very effective at transferring heat to crops like tomatoes, peppers or cannabis, that require high temperatures for growth and development.  The heat emitted from the lamps can also help to offset the cost of heating with natural gas or electricity.

Metal halide (MH) and ceramic metal halide (CMH) luminaires have been traditionally used by growers for the vegetative growing phase due to their broader spectrum and high emittance of blue light.  The blue light limits the flowering response and increases chlorophyll content, thereby helping increase the photosynthesis rate of the plant.  In addition, the blue and UV wavelengths have been shown to enhance the plant defense mechanism by initiating metabolic activity responsible for terpene and cannabinoid synthesis.

When comparing spectral efficiencies, HPS lamps emit a lot of light that is not usable for plants, making the HPS spectrum far less efficient than LEDs where the spectral output can be targeted to only those wavelengths that are useful to the plant.  In general, LEDs have a higher energy efficiency, longer life expectancy, and greater spectral efficiency.

One other advantage is that LEDs can help control the temperature of your growing environment as they do not emit energy as radiant heat.  This is because LEDs dissipate heat away from the illumination field in the form of convective heat, unlike HPS lamps which emit heat in the direction of illumination in the form of radiant heat.  Since heat is directed away from the crop, lights can be placed very close to the crop canopy without any burning.  However, because of the lack of heat, this also means in cooler months of the year heating costs will be higher with LEDs, especially in northern climates where studies have shown that overall energy costs remain the same between LEDs and HPS lights when the extra heat is needed.

LED and HPS hybrid system

There are many advantages to having a hybrid HPS and LED lighting system including cost savings, greater spectral efficiencies and light distribution.  When it comes to light spectrum HPS lamps have been confidently used as a growing spectrum for decades. Lately though, there is a lot of talk on light recipes for LED technology. But there has not been any solid evidence to support which combination of wavelengths will be best for crop production and for the most part, it is largely strain/variety dependent. There has also been research to suggest that blue light will enhance the synthesis of terpenes, anthocyanins and THC/CBD but again, it will largely depend on your genetics and will require trialing on your end to find out if you get the results you desire. So, unless you are growing only one strain or variety per grow area, having specific light recipes for your crop does not make sense.

Plants need the right temperature to grow and develop properly. Many research trials over the years have confirmed the relationship between temperature and plant growth.  In applications with HPS lighting, the radiant heat from the lamps heats up the crop hastening development. LEDs, as we mentioned earlier, do not emit heat that is transferable to crop temperature and therefore extra heating is needed in the colder months.  A hybrid system will help to balance the heat load in your growing facility – giving the plants the temperature they need to develop while also helping to offset heating costs in the colder months. And conversely, in spring and fall months, where light levels need to be increased but you don’t want to increase the indoor temperature, LEDs can be turned on instead to increase the DLI and/or photoperiod while keeping temperatures low.

Another aspect to think about, is how each light source distributes light. LEDs emit light in a single direction and light intensity diminishes the further away you are from underneath the light source. And because of the directional distribution of light from LEDs, shadows can be exacerbated if placed too close to the canopy.  On the other hand, HID lighting provides a more diffuse lighting pattern because they use reflectors to direct the light.  Plants are able to use diffuse lighting more efficiently than directional light due to the way the light penetrates into the canopy. Diffuse light also typically results in more growth than light directly from above.   The way HPS lighting should be installed is so that the light overlaps so shadows are reduced, and light penetrates in a horizontal direction.   Having a mix of both HPS and LED lighting provides an ideal distribution of both diffuse and directional light.

Therefore, if you combine the already tested and true HPS luminaires with the more efficient LEDs, plants will be able to take and use wavelengths that they need (when they need them), while also providing radiant heat and diffuse light from the HPS luminaires in addition to being able to extend your light use with LEDs into the spring months without the added heat load.

A hybrid lighting system provides the bet of both technologies and can be used in both greenhouse and indoor applications

HPS and MH Hybrid Lighting System

One other hybrid system we wanted to touch on, was the combination of HPS and MH lighting since a hybrid lighting solution can also mean adding different spectrums of light within the same growing space.  In this case we are talking about a higher red HPS luminaire mixed with a higher blue MH or CMH luminaire.  A properly designed hybrid light system using MH and HPS lighting can allow the grower to improve crop productivity and quality.

In flowering rooms where a grower might want to experiment with adding more blue to try and increase terpenes or cannabinoids, MH lamps might make more economical sense than installing LEDs which can be 3-5 times the price. Some growers also like having the mixed spectrums for vegetative growing where plants receive a balanced spectrum for growing but not enough red to initiate any flowering response.

Conclusion

LEDs are now about 40% more efficient than the best HPS technology, however, the initial costs are still 3-5 times higher than a 1000W HPS.  To install only LEDs you will have to invest heavily and keep that installation for at least 5-10 years to see a return on investment. In that time, we expect LEDs will continue to become more efficient, so the question becomes when is a good time to invest.  Overall maintenance costs are small and similar between both LED and HID technology, and since heating from the HPS lamps are of great advantage to crops like tomatoes or cannabis, as well as producing more diffuse light, it makes sense to still use HPS luminaires in your grow space. HPS lighting provides the best return on investment, lowest initial and lifetime costs, and is a mature and proven technology that is easy to maintain. Adding the LED lights allows you to increase your energy savings, have more control over spectrum and morphology and may also increase polyphenols, terpenes and other secondary metabolites such as cannabinoids.  Learn more about how a hybrid system can work in your growing facility by contacting us here or start by requesting a custom light plan.