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CMH vs LED : Which is Best?

Manufacturers of both LEDs and CMH grow lights claim to produce the best plants, but which really works best for your situation? Let’s take CMH vs LED grow lights head on.

Over the past twenty years, grow lights have evolved. We have an array of modern light systems that can grow plants, like Ceramic Metal Halides (CMH) and Light Emitting Diode (LED) systems.

There are a lot of nuanced differences between the two light systems, and I’m going over as much as I can in this quick overview. We’re going to cover light spectrum, canopy penetration, cultivation size, environmental control, investment cost, and budget.

Refresher on Light

Plants typically absorb a specific portion of the light spectrum called Photosynthetic Active Radiation, or PAR for short. The number is generally represented as PAR Watts, which simply refers to the amount of energy in the PAR portion of the light spectrum, getting to the meter each second.

We also use a number called CRI to describe how similar the quality of light our grow light produces is to sunlight.

You’ll see others like PPF, and PPFD, and others, but those refer to the amount of total light and light intensity, but not specifically to spectrum.

Most growers already know to use bluer lights, like metal halides, in the vegetative stage and redder lights, like high pressure sodium lights, in the flowering stage.

LED grow lights color spectrum is made up of a variety of individual diodes with a specific color output.

Light Spectrum

Ceramic Metal Halide grow lights produce very high quality light for the plants. They have a very high CRI, which means the light they produce mimics the quality of sunlight. A great deal of that light produced is in the PAR spectrum.

Since CMH grow lights produce a full spectrum of light, it will produce some UltraViolet and InfraRed light. These parts of the spectrum are very helpful for warding off pathogens like mold and mildew, and even some pests. Plants grow more healthfully under CMH grow lights than LED grow lights.

And one more thing about CMH light spectrum, since it’s a complete spectrum, you’ll be able to see the plants clearly, without using color adjusting grow room glasses. But if you’re growing with LED grow lights, you’ll probably benefit from using grow….
On the other hand, Light Emitting Diodes (LEDs) produce light in a specific, and narrow, wavelength. The hundreds or thousands of diodes are not replaceable.

CMH vs LED: Adjusting Light Spectrum

Also, CMH grow light systems have a removable bulbs. You can replace the bulb with high quality grow bulbs that have a slightly redder (3100K) or bluer spectrum (4000K).

Many LED grow lights allow you to “adjust” the spectrum. Basically, this just means turning off the red or blue diodes on the grow light. Doing this significantly reduces light output from the grow light system.

In our CMH vs LED face off, CMH grow lights offer an adjustable spectrum with full intensity just by switching out the lamp. And LEDs can’t do that yet.

LED grow light over seedlings
LED grow lights are great in small spaces with smaller plants.

Grow Light Intensity

Grow light intensity is a really important factor to consider and also really confusing. I’ll go a little into the details, but the basic point is CMH grow lights put out way more intense light than LED grow lights ever can.
The intensity directly relates to canopy penetration.

Here’s the idea, there’s physical law about light called the “Inverse Square Law”. It states that the intensity of light is inversely proportional to the square of the distance from the light source.

Sure, reflectors and multiple light sources change things a bit, but it doesn’t change the fact that the light intensity drops steadily.

Canopy Penetration

More canopy penetration generally translates into denser, bigger buds.

CMH grow lights can penetrate 18 to 24 inches into the canopy when they are hanging three feet above. Where LED grow lights can only penetrate 6 inches deep, and must be suspended 6 inches above the canopy.

It also means that since the LED light is so weak to start off with, it won’t penetrate into the canopy. Even if you hang it six inches above the canopy. And you’ll be adjusting the hanging height of the lights really often.

But since CMH grow lights are so intense to start off with, the light will penetrate much deeper through the canopy. This means you’ll get a higher gram per watt with CMH grow lights over LED grow lights.

And you probably won’t need to adjust the light height as often.

The only case where LEDs are a better option here are low ceilings, and LEDs work great for under canopy lighting. I believe if you’re working with ceilings less than 8 ft in height, it’s a good idea to use LEDs.

Project Duration

If you haven’t already thought about how long you’ll use your lights, you may want to consider it now.

I’ve helped lots of growers over the years. Most of the time people keep their grow lights for two or three years before swapping them out. Warranties expire, new technology comes out, the grow lights themselves eventually don’t perform as well as they did when they were new.

That said, you can replace the CMH grow bulb every six months to a year and it will perform like new. Assuming you’re cleaning your reflector too!

LEDs are supposed to last eight to nine years before their “usable life” is over. But often times a diode here or there will fail. And in that time the system itself will become outdated. Even with a long lifespan, LEDs light output will steadily fade over that time period. Nothing lasts forever.

The math on a long term investment looks good on paper for LEDs, but it doesn’t reflect most real world scenarios.

Cultivation Size

Let’s talk about the size of your grow. Because of the lack of overall canopy penetration, you will need to keep your LED lights 6 to 18” above your canopy.

This could be a good thing if your ceiling height is less than eight feet and you really need to be careful with your heat management.

But, like I mentioned above, if you have a larger and taller space, using CMH technology is a better choice.
An increased ceiling height allows for better heat management, better air-flow capability and more even light coverage for fuller and taller flower strains.

Then there is technology like DimLux Half Force, that turns the lights on and off, every three hours, in a checkerboard pattern throughout the grow room. This system works well because the lights send out so much light that they overlap. Even when every other light is turned off. Basically, there is enough light intensity for the plants to grow at near its maximum potential. Regardless if the light directly above them is only on half the day.

LEDs cannot do this. They are so much close to the canopy, and the LED light output lacks intensity. LED grow lights are also unidirectional, so the light doesn’t spread out like a CMH grow light can. So all the LEDs must be on all on at the same time.

Maxi Controller Datalog
A controller, like the Maxi Controller (pictured), can take do much more than just turn the AC on and off. The Half Force feature will make your grow use half the energy.

Environmental Control

Here’s where LEDs really shine. They don’t put out as much heat as a CMH grow light.

That means you’ll use less air conditioning to cool a grow room filled with LEDs than the same one covered by Ceramic Metal Halides.

The verdict on environmental control isn’t so clear if you use DimLux Half Force. This is a major amount of energy saved. The lights are only on half the time. So both the grow lights and the air conditioners cooling the room are working much less.

Investment Cost and Budget

Finally, everyone’s favorite factor to consider, BUDGET!

The initial cost of an LED is 2 to 4 times the cost of a comparable CMH fixture. LED fixtures, especially the ones that include the UV spectrum, are very expensive in terms of up-front cost.

CMH grow lights are much more affordable in comparison.

When it comes to power consumption and the power bill, LED’s only save the grower about 15% on the power bill compared to CMH fixtures.

However, the LED grower will use less power for cooling, but that savings depends on other factors in the grow space.

The additional cost a CMH grower will realize is replacing bulbs. You will be buying replacement CMH bulbs every twelve months of use.

Additionally, it’s recommended that you change between the 4100K CMH bulbs in the vegetative phase to the 3100K CMH bulbs for flowering.

So you will probably want to account for a little added cost of the extra bulbs when you’re using CMH technology. But you will optimize your light cycles and extend the timeframe to replace bulbs.

Closing Thoughts on CMH vs LED

Obviously, you’re comparing CMH vs LED. Like I wrote above, both LEDs and CMH grow lights are effective at growing plants. CMH grow lights are great for commercial cultivations and hobbyists with enough space. They produce bigger denser buds, and they offer a rich list of options. On the other hand, LEDs are perfect for hobbyists with smaller spaces like grow tents. And they’re great for people growing in spaces with lower ceilings. Also, LEDs are great for under canopy lighting.

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