Benefits of HPS and CMH Grow Lights for Your Greenhouse or Growroom

By Chris Bond
Published: December 15, 2022
Key Takeaways

These lights might be considered old school by some, but they are effective grow lights that still have a place in certain growrooms.

Anyone new to growing cannabis, and even some more experienced growers, can easily get confused regarding all the different artificial lighting options out there. This is especially true when each system lays claim to being the best for your inside grows. While recently there has been much press given to LED technology and its advances (and rightly so), or for T5 fluorescent bulbs and their ease of use, high pressure sodium (HPS) and ceramic metal halide (CMH) lighting should not be dismissed. In some instances they are not the best solution and are indeed obsolete, but in many instances they are still a viable option that should not be overlooked or considered to be the old way of doing things.

Let’s take a look at what exactly these lights are, their characteristics and advantages, and why they might still be a good option for your greenhouse or growroom.


High Pressure Sodium Lights (HPS)

High pressure sodium bulbs have been a tried-and-true source of artificial lighting for plant growth for most of the last 60 years. They remain the most common lighting source in commercial propagation operations of all types, not just cannabis. They provide high-intensity light on the red end of the spectrum, which is akin to the morning, evening, and autumn sunlight. This is ideal to support the flowering and fruiting phases of plant development.

The first sodium lighting was actually developed closer to 100 years ago (1932) in Holland, but it was a low pressure sodium bulb, producing yellow light (though very efficiently). The General Electric Corporation (GE) is credited with introducing the high pressure sodium bulb in 1964, which quickly dominated as the primary source for light on the red end of the spectrum. Both high and low pressure bulbs generate light by way of an electric arc produced through vaporized sodium metal, along with other gases and metals.
HPS bulbs are still considered to be the best source for red lighting.


They are also highly efficient; the bulbs last for a very long time. Bulb life can last up to two years each. As a proportion of the amount of electricity used and spent to operate them, they are still the most efficient light type on the market for your crops’ flowering phase when compared to yield. Because they have been on the market for so long, they are readily available, and often in easy-to-use kits that contain ballasts, reflectors, and everything needed just to plug in and go.

No lighting source, save for the sun, is perfect. High pressure sodium bulbs have their drawbacks. Even though they are quite ubiquitous in the world of greenhouse lighting, they can still be on the expensive side. They also cannot just get tossed into the garbage when they are spent. This is because they contain mercury and must be disposed of properly, adding to their cost. Because they generate light on the red side of the spectrum, they do not provide the blue light needed by plants (for the vegetative stages of growth). Unless plants are trained or pruned to grow only under red light, they can get tall and gangly. This isn’t to say it can’t be done; when properly pruned to be shorter and bushier, yields can be increased growing under only red lighting.

Ceramic Metal Halide Lights (CMH)

Metal halide (MH) lighting is the preeminent source of blue lighting for plant growth. It is ideal for plants in their vegetative stages. This type of bulb generates light by passing an arc of electricity through a mix of various gases. The mix of these gases are what give the MH light its color, intensity, and heat.

Ceramic metal halide is a type of MH that uses a ceramic material for its arc tube (just like in HPS bulbs). Metal halide lighting originally used quartz as its arc tube material, but ceramic can withstand much higher heat and provides a more balanced spectrum of light, making it the preferred type of MH lighting system.


Just like with HPS, there are pros and cons to these types of bulbs. They do offer an optimal source of blue light for plants, which is still among the best options available as it is the type of light that most closely mimics the bright summer sun. They are also very long-lasting bulbs, retaining as much as 85 percent of their intensity after one year of daily usage. They are very easy to replace when their efficiency has dropped to a sub-optimal level (bulbs should be replaced when their output drops below 85 percent). They are cost-effective and widely available in most garden and hydroponic stores.

CMH and MH bulbs burn very hot. This can raise the temperature in a growroom above an acceptable level. An exhaust system is often required in conjunction with using these types of lighting systems. The initial setup of these systems can be costly.


Choosing LED or HPS Lights for Commercial Growers
Choosing the Right Light for Your Grow Tent
Lighting Uniformity in Horticulture


For most growers, it is not an either/or scenario. Since both types of bulbs offer different advantages, many growers opt to incorporate both types of these HID lighting systems into their growing operation. They tend to favor MH during the vegetative phase of growth and then favor HPS lighting during the flowering phase. That said, some growers do opt for one or the other for their entire grow and then adapt their cultivation methods accordingly.

Both types of bulbs and lighting systems are classified as high-intensity discharge (HID). HID lighting systems are characterized as being highly efficient and cost effective. These systems are popular among many growers because they offer consistent and reliable results, they are easily scalable, and are easy to operate. HID systems like HPS and CMH provide light penetration into the plant canopy more thoroughly and efficiently than any other lighting source currently available. With the exception of some LED systems, they provide higher yields as a measure of electricity used than any other system.

Another advantage that should not be overlooked is the amount of information readily available regarding HPS and CMH systems. Because they have been in use for so long and have been researched extensively, there is a treasure trove of knowledge out there that can take a lot of the guess work out of your grow. This means the trial and error work has been done in regard to lighting, and you can focus on experimenting with other variables like nutrients and different strains of plants. HID systems are not the best fit for everyone and do not work in all situations though.

HID systems should only be used in areas with high overhead clearances. Ideally there should be at least five feet (1.5 m) of space between the top of the plant canopy and either an HPS or CMH light. Growers utilizing grow tents, or other small spaces, should opt for a different type of grow light. They are also not the cheapest system to set up. They can be cost-prohibitive for the hobby or small-scale grower. This is not only because of the needed ballasts and reflectors, but also because of the exhaust system needed to remove the excess heat that HID lighting systems generate.

These systems also take more time to properly set up compared to other options like LED and some fluorescent systems. Some HID bulbs, especially HPS, are also not easily sourced locally. These tend to be only available in hydroponic or specialty garden stores, but most often need to be purchased through online sources.

So, there is nothing wrong with wanting to use HPS or CMH for your crops. There are a lot of scenarios where they are still a good option and in some cases, a much more practical one. They were at one time in the not so distant past two popular options, so there are still millions of these lights in service. Though it is sometimes seen as the opposite of progress, there is still sound logic in the maxim “if it ain’t broken, don’t fix it,” especially when talking about HPS and CMH bulbs.


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Written by Chris Bond | Certified Permaculture Designer, Nursery Technician, Nursery Professional

Profile Picture of Chris Bond

Chris Bond’s research interests are with sustainable agriculture, biological pest control, and alternative growing methods. He is a certified permaculture designer and certified nursery technician in Ohio and a certified nursery professional in New York, where he got his start in growing.

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