Metal Halide Grow Lights (Everything You Need To Know)

There are two reasons you might find yourself here.

One: you have questions about metal halide grow lights. If that is the case, read on. Everything will be answered.

Two: you are looking to buy metal halide grow lights. If that is the case, you won’t find help in this article. But you will find it elsewhere on this site, namely:

If you are interested in a full lighting kit that includes an MH bulb, this article will help.

If you just need some bulbs, read this review and comparison of the best MH (and HPS) bulbs.

You’re still here, so you must have some questions about MH bulbs. I will do my best to answer them all.

I will begin by explaining what they are and how they work, then I will go over when to use them and how to use them correctly to achieve the best results. Finally, I will compare them to other types of lighting, so you can be sure you are choosing the correct type for your needs.

What is a Metal Halide Grow Light?

Metal halide grow lights are a type of HID, or high-intensity discharge, lighting. These lamps contain a quartz tube filled with a noble gas. An arc of electricity is sparked between two electrodes inside the tube to create light.

MH bulbs contain metal halide. This gas burns with a cool white light that contains a large amount of light in the blue wavelengths.

Metal halide bulbs can last up to 20,000 hours, although they degrade to the point that they are no longer effective as grow lights long before that. They have a high lumen output and also generate a lot of heat.

Overall, MH bulbs are highly efficient, though they are not as efficient as HPS (high-pressure sodium or CMH (ceramic metal halide) bulbs. Note that ceramic metal halide bulbs, while they sound similar, are actually quite different. I will go into detail on the comparison toward the end of this article.

So why would we use MH bulbs, if HPS and CMH are more efficient?

Honestly, I wouldn’t. I would use CMH bulbs. But that doesn’t mean that MH bulbs do not still have a place in an indoor garden.

Why Use MH Grow Lights?

MH bulbs emit a cool white light (containing large amounts of blue). The following chart shows the color spectrum of a 6000K MH bulb.

As you can see, the spectrum contains a large amount of blue and green wavelengths and also a good amount of orange and yellow. It contains very little red.

This spectrum makes them especially effective for growing plants, i.e. for plants that do not flower or for the vegging stage of plants that do flower.

Plants that do not flower can be lettuce, various herbs, etc. Plants that flower are orchids, tomatoes, cannabis, etc.

MH lighting gets your plants to grow dense, lush canopies. It also prevents stretching during the flowering stage, which leads to a weaker branch structure.

Stretching can be caused by a lack of blue light, which is why MH bulbs are effective in preventing it. Growers flowering with HPS bulbs often see their plants stretch, since HPS bulbs emit virtually no blue spectrum light. Adding a few MH bulbs to balance out the HPS spectrum solves this problem.

Due to the lack of red wavelengths in their spectrum, MH lights are not good at flowering plants on their own, though.

This is the reason growers will use MH bulbs for vegging and HPS bulbs for flowering. Or even better, they use both bulbs simultaneously, for a much more complete spectrum.

As you’ll see below, that is why I generally recommend CMH or LED over MH or HPS. You don’t need to combine two different lights to get a great spectrum for every stage.

That brings us back to our question: why would you use MH bulbs?

These days, there are only a few reasons to go with metal halide lamps:

1. You already have an HID system and you’re not ready to switch: in this case, it makes sense to get MH bulbs to balance out the HPS spectrum, or to use for vegging, and then switch to HPS for flowering
2. You are on a strict budget: metal halide bulbs and the corresponding reflectors and ballasts, are cheaper than CMH bulbs and the ballasts and reflectors for CMH, so it could make sense to go with metal halide from a budgetary standpoint

Which MH Bulb Do I Need?

MH bulbs come in multiple wattages, with 400w, 600w and 1000w being the 3 most efficient. Among them, 600 watt bulbs are the most efficient of all.

The size bulb you need depends on the size of the area you want the bulb to cover.

The following table shows the maximum areas you can expect each size bulb to cover and the recommended distance from the plants, which should give you an idea how much vertical space each wattage would require.

For more on hanging height, read this guide.

What Else Do I Need?

In order to run a metal halide bulb, you will also need a ballast and a reflector. The reflector can be a simple mogul socket with a cord (note that you can not use a standard light socket for these bulbs), an open reflector, or an air cooled reflector.

The ballast needs to match the wattage of the bulb, or at least be dimmable to the bulb’s wattage.

If you do not already have these components, I highly recommend buying a grow light kit, like the ones reviewed in that link. This ensures that all the components are not only compatible, but were actually designed to work together.

If you wish to buy the components separately, I recommend getting a digital dimmable ballast. Those will auto-detect your bulb type, making your life much simpler, while also extending the bulb’s life. There is no real reason to get an older, more manual, ballast these days.

When it comes to the reflector, keep it simple. Get cool hoods for a small garden, if you have the budget and are fine attaching ventilation to the reflectors. Otherwise go with a wing or an umbrella-style reflector.

The same post about grow light kits also has an extensive section on the different types of reflectors. It should help you figure out which one is right for you.

Metal Halide vs. LED

There is really no contest here anymore. If you do not already have a lighting system, I strongly recommend LED lights over metal halides.

MH lights have always had one big advantage over LED grow lights: they were significantly cheaper. That huge price difference has decreased a lot, so that you can now get a good LED grow light for only slightly more than an MH system (bulb with reflector and ballast).

See how much LED light you can now get for just a few hundred dollars.

With the price difference approaching zero, the one big reason to get MH grow lights over LED is vanishing. Especially when you consider all the advantages of LEDs.

They cost far less to operate, since they use less power and create less heat, meaning you don’t need as much cooling equipment. The lower operating costs generally make up for the higher initial price within a year.

Apart from that, LED grow lights are far easier to use (no additional equipment needed; just plug them in and grow) and they last much longer, too.

But what about that ugly pink/purple glow?

These days, you can get plenty of full-spectrum white LED grow lights that emit light similar to natural sunlight (and, in the best cases, supplement it with red and blue). Not only do your plants look natural under these lights, the white LED spectrum is actually better for both growth and yield.

Metal Halide vs. HPS

I have a whole article on HPS vs. MH bulbs that covers this in detail. All I’ll mention here is that MH bulbs are better for vegging due to their blue-heavy spectrum and HPS bulbs are better for flowering due to their red-heavy spectrum.

That said, plants veg better if they have more red light than MH provides and they bloom better if they get more blue light than HPS arrives, so you would ideally want to use a combination of both bulbs during the whole grow cycle.

Alternatively, you would use one of the two bulbs (HPS would be better if used for the whole grow) and supplement it with light that provides the missing wavelengths. In the case of HPS, supplement it with some other blue-heavy light, like cool white LEDs or fluorescent bulbs.

Metal Halide vs. Ceramic Metal Halide

Ceramic metal halide and metal halide sound very similar, but they are actually quite different. You can read about those differences here.

The bottom line is this: CMH is far better and the only possible advantage of MH is cost.

The price difference is not large enough to justify going with MH over CMH, so I would only recommend metal halide if you already have an existing system and are getting replacement bulbs.

Ceramic metal halide bulbs have an almost ideal spectrum for every stage of growth, with light in every color and lots of red and blue. In fact, the only lights with a better spectrum are certain LED fixtures.

CMH bulbs also generate less heat and they output more light per watt consumed. Finally, they degrade much slower than MH bulbs and thus have a longer usable lifespan of around 24,000 hours on average.

Metal Halide Vs. Fluorescent

Cool white fluorescent lights also have a spectrum with a large blue wavelength component. The spectrum is actually more complete than the MH spectrum. The difference is power.

Fluorescent lights simply can’t deliver the same output that MH can. They are far less efficient. As a result, fluorescent bulbs are great for vegging small gardens or for seeding or cloning. If you need power, then go with MH.

Photo Credit

Title photo by Plantlady223 – Own Work, CC BY-SA 4.0, Link