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.
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.
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.
What Else Do I Need?
The bottom line is this: CMH is far better and the only possible advantage of MH is cost.
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.
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.
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:
Plants that do not flower can be lettuce, various herbs, etc. Plants that flower are orchids, tomatoes, cannabis, etc.
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.
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 vs. Ceramic Metal Halide.
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 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.
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.
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.
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. LED.
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).
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.
One: you have questions about metal halide grow lights. If that is the case, read on. Everything will be answered.
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.
That brings us back to our question: why would you use MH bulbs?
MH bulbs emit a cool white light (containing large amounts of blue). The following chart shows the color spectrum of a 6000K MH bulb.
Cheap ballasts work great (I’ve grown with several!), but more expensive ballasts last longer and some of them put out more light for the same amount of electricity. It’s always a good idea to get the best equipment you can afford, but even the most entry-level equipment will get you all the way to harvest with great results!
MH stands for “Metal Halide” and HPS stands for “High Pressure Sodium.” These are two specialized types of bulbs that are great for growing plants, from a class of lights known as HID (High Intensity Discharge).
How Far From Plants? Keep 16+ inches (40cm) away from tops of plants.
The most important thing to remember is that the bulb(s) and ballast should match each other. So for example if you get a 400W ballast, it MUST be matched with a 400W bulb.
Coverage Area: 4′ x 4′ (1.2m x 1.2m) up to 5′ x 5′ (1.5m x 1.5m).
Choose the right size MH/HPS light for your space.
What is the difference between wing, cooltube and air-cooled hoods? Read the full tutorial on hoods and reflectors.
Wings are basically just a reflector with a built in spot for the bulb to connect. Wings don’t have any built-in cooling options and can tend to cause hot spots on the tops of the plants. Wings are usually the cheapest option and often have shoddy construction. Plus the edges can be sharp, not something you want at face level!
Make sure that if you have a “magnetic” ballast that you match with a bulb that is compatible with magnetic ballasts. A digital ballast can support both types of bulbs.
So you’re ready to get a MH/HPS grow light? Perhaps you want to upgrade to an MH/HPS light from your existing setup, or maybe this could be your very first grow light for growing cannabis.
Here’s a summary of what I recommend for new growers to make your life easier…
Notes: 600W is one of the most popular sizes for small-scale growers (along with 400W). A 600W grow light is the most efficient of all HID lights, which means it produces the most amount of light per watt of electricity. To prevent heat problems, I would not recommend getting a 600W light for a regular grow room without installing an exhaust setup or some other way to control temperature.
Digital ballasts cost a bit more than a magnetic ballast, yet have big advantages. A digital ballast runs cooler, uses less electricity and produces a higher light intensity. If you like taking pictures of your plants, a magnetic ballast will make dark lines all over your photos. In addition to no lines in pics, digital ballasts are compatible with more types of bulbs, while a magnetic ballast needs special magnetic ballast compatible bulbs and will burn out the wrong type of bulb.
These lights are bright! While MH bulbs give off blue light, HPS bulbs like this one give off bright yellow light, which happens to be perfect to get plants to grow buds during the flowering stage.
Coverage Area: 3′ x 3′ (0.9m x 0.9m) up to 3.5′ x 3.5′ (1m x 1m)
However, any reflector or hood will work with any bulb or ballast, though some are better than others.
Coverage Area: 2′ x 2′ (0.6m x 0.6m)
Average Yields: 300 – 600 g (10 – 21 ounces) – less for beginners, more for the pros – get the biggest yields by filling the entire grow area with a wide even canopy and remember that the strain (genetics) you grow with also makes a huge difference in your final yields.
Notes: Some (but not all) 250W MH/HPS have a “Built-In Ballast,” which means everything is actually attached together and you just plug the light directly into the wall. Some 250W HPS lights have a traditional ballast, which means that the ballast is a separate piece from the hood/reflector. In general, it’s better to get a light where the reflector is separate from the ballast – they are more efficient, cooler, and spread their light over a bigger area.
Specialty grow lights like MH/HPS grow lights can be found online as well as in person at hydroponic shops and some gardening stores. Below I will share complete shopping lists I’ve put together in order to help new growers get started with everything they need!
Get Your MH/HPS Grow Lights.
How Far From Plants? Keep 21+ inches (53cm) away from tops of plants.
2.) Pick Your Bulb.
Cool tubes are built so that the bulb is contained in a tube. They are meant to be hooked up to an exhaust (via ducting) to pull all the heat away. Cool tubes are less likely to cause hot spots on the plants than wings, and can fit in smaller spaces than any other type of hood , but they tend to be limited at reflecting light. Usually there is only a small reflector built into the top of the tube, though some have a small reflector. They do a pretty good job at reflecting light, but wider hoods tend to be more effective.
Supports MH and HPS bulbs.
A dimmable ballast allows the grower to adjust the ballast to put out less light (and use less electricity) when plants are small and don’t need full-strength light yet. This can shave quite a bit off your electricity bill and make it easier to nurture young plants. It can also be helpful to turn lights down during a heat wave. If you get a dimmable ballast, make sure your bulb can also be dimmable.
Make life easy on yourself by getting a great ballast…
Can a ballast support both MH and HPS bulbs?
Here’s a quick reference guide to use as a starting point:
There are a couple factors you must consider when deciding how far away to keep your grow lights from your plants. The most important is to make sure it’s not causing too much heat where your plants are.
Therefore, like with the ballast, I recommend getting the best bulbs you can afford. These bulbs generally last 1-2 years of normal use, so you will not have to replace them for quite a while. They do get dimmer over time, so it’s a good idea to always replace all your bulbs every 3-4 grows, even if they haven’t burnt out yet.
About Sirius Fourside.
Consider that you want a direct line to the exhaust, with as short a distance as possible, so that hot air is quickly and easily released outside of your home.
You now know everything I wish I’d known before I started growing with HID lights. If you’re serious about growing cannabis, HID grow lights are going to give you the best yields, most ease of use, and (in my opinion) the most enjoyable growing experience.
What’s the difference between Metal Halide and High Pressure Sodium? MH lights are generally used in the vegetative stage of growth (when young plants are growing just stems and leaves), while HPS lights are considered optimal for use during the flowering stage when cannabis plants are producing buds.
Cannabis plants like having a lot of fresh air all the time, and they will reward you for your diligence in setting up a proper exhaust system.
Even with a good exhaust system, it is recommended that growers still install small oscillating fans in their grow space to blow air above and below to the plant canopy to improve air circulation and help prevent any hot spots or humid pockets of air.
* Of course fans don’t come in all these sizes, so round up to the closest fan size available.
Use a staple gun to reinforce anywhere you use duct tape, including where the ducting goes through the hole you made in the foam.
Some bulbs claim to be “enhanced” or be ultra-efficient. While there is not a whole lot of difference between the different bulbs, you will tend to get slightly better results from the higher quality bulbs, and they tend to be more durable/long-lasting.
Yet it’s worth it when you can grow rows of high-yielding marijuana plants like the ones displayed here to the right. These plants were grown using an HID grow light system. The High Pressure Sodium (HPS) lights are what makes flowering plants look golden in pictures, though the plants look normal under regular light.
< 15,000 lux – sparse or “stretchy” growth – plant isn’t getting enough light 15,000 – 50,000 lux – good amount of light for healthy vegetative growth 45,000 – 70,000 lux – optimal amount of light for cannabis plants in the flowering (budding) stage 70,000 – 85,000 lux – a lot of light, some strains thrive at this light level, but some plants lose their top leaves early under this light intensity, especially plants that are not resistant to heat/light (like many indicas) > 85,000 lux – at this light intensity, you’ve hit the plant’s “saturation point” which means your plant can’t use all the light (be careful of light bleaching!)
In my experience and from talking to all the growers I know who’ve experimented with different hoods, many growers seem to agree that air-cooled hoods, especially extra wide air-cooled hoods, get the best yields and results when growing marijuana.
Learn how to make a stealthy “secret window” exhaust system like this below…
A 1000W ballast is slightly less efficient than a 600W lights and will cover an area of about 4′ x 4′ (1.2m x 1.2m) up to about 5′ x 5′ (1.5m x 1.5m). The further your light from the plants, the less powerful the brightness but the greater and area the light will cover. You will optimize your brightness in a smaller space by keeping the light as close to the plants as possible without burning your plants. 1000W lights need a significant amount of cooling power as the bulbs get very hot in a small amount of space. In big grow operations in the US where indoor space is very limited, some growers opt to use several 1000W grow lights, as even plants along the edges get strong amounts of light, and this reduces the total number of grow lights to adjust up and down as rows of plants grow taller. Sometimes 1000W lights are placed on automatic movers so they can cover more plants and prevent these powerful lights from accidentally causing light burn.
Either MH or HPS lights can be used for the entire grow and will get good results, but modern ballasts often allow a grower to interchange MH and HPS bulbs, so many growers choose to use what’s been proven to get the best results and grow with both types of bulbs. If you want to use MH and HPS bulbs with the same ballast, it’s important you look for this feature to make sure you don’t accidentally put the wrong bulb in the wrong ballast. It’s actually simple in practice, and I’ll walk you through everything in the choose your ballast section below.
Choose the Placement of Your Grow Tent & Exhaust Carefully.
Optimally, you want the exhaust to pull air from your hot lights and directly out a window, in a short, straight line if possible.
Think of your ballast as a power source for your HID bulbs.
A great way to check for heat is to use the “hand test.” After your grow light has been turned on, place one of your hands (palm facing downward) near the top of your plants and let it sit there for 10 seconds. If the light feels too hot for your hand, it’s too hot for your plants and you will need to either move the light up or find a way to better control heat. Plants that are allowed to be too hot for too long will begin to show signs of heat stress.
400W – covers 3′ x 3′ (0.9m x 0.9m) area up to 3.5′ x 3.5′ (1m x 1m)
I also used duct tape to secure the foam to the window because it’s cheap, easy to use, and is very durable when combined with staples.
Now on to the pros and cons of HID grow lights like MH and HPS…
I like using foam + corrugated plastic because it’s inexpensive and easy to cut the hole for the exhaust. Having the foam/plastic combo in the window increases the suction in the room so more hot air is vented out and blocks light (no light gets in or out). As an added bonus, the foam dramatically reduces the noise of the fan (so you can just barely hear it even when you’re standing right outside the window).