mh lights for growing weed

Make sure you have enough outlets and power available in your breaker board for your grow space to comfortably accommodate all equipment. Figure out all pieces of equipment, such as lights, fans, possibly an AC or dehumidifier, and calculate how much power they’ll require. You’ll be running this equipment every day for months, so if you don’t have enough power it can be dangerous. Never overload an electrical outlet.

HIDs have been the standard in indoor weed growing for decades, but LEDs are quickly catching up to them.

As with HIDs, you can find CFLs at any local grow shop.


However, because of their low price, if you’re new to indoor growing and not sure how often you’ll do it, you may want to invest in an inexpensive HID light at first to test the water.

Power equals poundage, so if you want big yields you’ll need more wattage. Professional LEDs can start at as little as 200 watts, and go up from there. A high-watt light can double the work of several low-watt bulbs.

How big a light you need will depend on the number of plants you plan on growing, but also on the size of your space. If you’re growing in a closet or a small grow tent, you’ll only be able to fit one small light in there. If you have a wide open basement, you could invest in a bigger light as opposed to two smaller ones.

For HIDs, light occurs as an arc between two nodes inside the bulb. The gas contained in these bulbs is what makes MHs and HPSs different. HID bulbs are usually more expensive than the reflective hoods that hold them.

CFLs (compact fluorescent lamps) are fluorescent lights similar to what you’d find in a school or office building, but smaller. For growing weed, they are usually called “T5s”—the “T” stands for “tubular” and the “5” refers to its diameter, “⅝”.

You can find lights for under $100, but they may be low quality and not produce the right spectrum of light, and you can also easily spend as much as $2,000 for a large, state-of-the-art LED.

These HID bulbs usually contain sodium, mercury, and xenon, and produce a yellow/orange light, and are commonly used for flowering plants. Some growers will start plants under MH bulbs and switch them to HPSs when plants go into the flowering stage, using the same hood. These lights also require a ballast.

HID lights for growing marijuana.

These fluorescent lights are cheap and efficient and great for vegetative growth. They’re especially great for helping along germinating seeds and small seedlings because they don’t put off much heat and won’t scorch the delicate seeds. They won’t run up your electricity bill too much.

The downside to CFLs is they aren’t great for flowering plants, and growers will usually use another type of light to finish plants. CFLs just don’t generate enough intense light for plants to pack on weight.

The first thing to consider before buying a grow light is how much money you want to spend. With more states coming online with adult-use legalization, homegrowing is becoming more popular and growing technology is getting better and more efficient all the time.

Young and mature plants, or vegetative and flowering plants, respectively, like different types of light, and you can buy grow lights that target each spectrum. Commonly, growers using HIDs will use MH bulbs for vegetative plants and HPS bulbs for flowering. Some LEDs are also designed to target different light spectra.

These bulbs contain mercury and metal halides, produce a blueish light, and are commonly used for vegetative growth. They require a ballast to regulate the current. In the past, ballasts have been big and bulky, but digital ones are now available.

CFL lights for growing weed.

New LED grow lights come out all the time, but knockoffs abound. There are a lot of cheap LEDs that don’t produce the right spectrum of light for plants.

Consider how often you’ll be growing weed and how long it will take to pay off the initial investment—if you grow once a year, it’ll take a lot longer to pay off an expensive light than if you grow multiple harvests a year.

Some grow lights are more expensive than others, but also more efficient, saving money in the long run. Some lights are bulky with many parts, some light in weight, and some are better suited for young or mature plants.

There are daylight bulbs and warm white bulbs; the former better for vegetative growth, and the latter for flowering.

LEDs (light emitting diodes) are relatively new to the cannabis growing world, compared to HPSs, MHs, and CFLs, but they are quickly proving to be the way of the future. LEDs may be more expensive to buy initially, but they are far more efficient and kinder to the environment and your electricity bill. Some cities even give tax breaks to commercial growers who install or switch to LEDs because they’re better for the environment.

Grow lights allow you to control exactly how much light your plants receive, ensuring they get the same amount of light day after day, without the grower having to solve problems with bad weather or cloudy days that happen in an outdoor environment.

2.) Pick Your Bulb.

Coverage Area: 3′ x 3′ (0.9m x 0.9m) up to 3.5′ x 3.5′ (1m x 1m)

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).

MH/HPS is a great choice for growing cannabis! This type of grow light has been the “golden standard” of indoor growing for decades because they work so well at delivering fast growth, big yields and dense, potent buds! They are easy to use and there’s an abundance of information, guidance and other types of help for MH/HPS growers.

Make life easy on yourself by getting a great ballast…

Choose the right size MH/HPS light for your space.

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.

What is a “dimmable” ballast?

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.

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.

However, any reflector or hood will work with any bulb or ballast, though some are better than others.

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.

Average Yields: 200 – 400 g (7 – 14 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.

Average Yields: 125 – 250 g (4 – 8 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.

What does “digital” or “magnetic” mean when it comes to HID bulbs?

Some companies use the term “Deluxe” to describe some of their higher end bulbs. Deluxe bulbs are generally considered to be “spectrally enhanced” which means they provide a little bit more usable light to your plants for the same amount of electricity. They are meant to enhance growth without using more energy, and they tend to cost quite a bit more. Some manufacturers claim their Deluxe bulbs are 10% brighter than their non-Deluxe counterparts.

Air-cooled hoods are great for capturing hot air and keeping them away from the plants. They are also able to be hooked up to an exhaust system. One of the advantages of air-cooled hoods when growing cannabis is they have a big, smooth reflecting surface to reflect a great deal of light down at the plants (without also sending heat down at the plants). This allows the plants to get more light from the same 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.

Coverage Area: 2′ x 2′ (0.6m x 0.6m)

How Far From Plants? Keep 10+ inches (25cm) away from tops of plants.

Average Yields: 75 – 150 g (2.5 – 5 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.

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.

Can a ballast support both MH and HPS bulbs?

What is the difference between wing, cooltube and air-cooled hoods? Read the full tutorial on hoods and reflectors.

What are “XXL” hoods?

When getting an MH/HPS grow light, you need to choose whether you want to get an “All-In-One” kit which has all the individual parts packaged together, or whether you’d like to get the 3 pieces separately.

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!

Some ballasts only support one type of bulb, while others are compatible with both types of bulbs. For most new growers, I recommend getting a ballast that supports both MH and HPS bulbs so you can interchange them easily between the vegetative and flowering stage. It is possible to grow a cannabis plant from seed to harvest on either just MH or just HPS. If you’re going to choose just one, I recommend getting HPS since it’s more efficient (puts out more light for the amount of electricity used) and will get bigger harvests in the flowering/budding phase. But it is also possible to grow from seed to harvest with just an MH bulb, as long as you don’t mind smaller buds. Every ballast will clearly state what types of bulbs it supports, so no guesswork needed.

How Far From Plants? Keep 16+ inches (40cm) away from tops of plants.

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!

What is the difference between a “digital” and “magnetic” ballast?

After veg and moving to flower, most growers also agree that switching to a light source with more of the red spectrum is better for flowering. The old standard has been to use either CMH or fluorescents in veg and switch to HPS into flower. This is the tried and true lighting recipe for indoor cannabis growers. By far the majority of cannabis produced in North America today is grown in this way. With the onslaught of LED lighting manufacturers jumping into this new “green rush” of opportunities we are seeing this standard lighting recipe being challenged more and more.

With all of these factors above, how do we answer our initial questions?? Quite frankly, there is no simple answer! Every grow is different – from different: strains being grown, growing climates by geography, style of growers, indoor versus greenhouse, etc. All of these factors will weigh heavy on what is better for you!

Does this article answer our very first question? Let’s break it down and see. Upon further investigation, the crops in question were grown entirely under each light source. Not in a veg room under one type of light and then moved to flower. For this reason, the HPS plants stretched in veg and were not off to a healthy start!

Lighting and Cannabis Qualities.

What about growing in a greenhouse? From our graph above showing the spectrum of sunlight compared to that used for photosynthesis is there any benefit for using a light source where you can dial in the spectrum to add more blue? The natural light received by your crop in a greenhouse will cover all of your light spectrum needs. So the big question for greenhouse grows should center more around the ROI comparisons between LED and HPS — not about light spectrum needs. This is not something you will hear from light manufacturers of LED lighting. Your LED-only supplier will have you believe you NEED their spectrum specifically!

There is a big misconception in the industry stemming from the LED suppliers that the ability to have your LED lights so much closer to your crop will help increase production lower into the crop. This could not be further from the truth! When your light source is closer to the top of the canopy than the shadows created from the top layer of leaves are much bigger than if the light was further away. (Use a flashlight and make a shadow on a wall with your hand. Watch how the shadow grows as you get the light closer to your hand). Also, light penetration into the crop comes from the crossover between fixtures.

This is congruent with other findings that some added blue in flower will cause this. But the reason we see these amounts vary is because every strain reacts differently to different spectrums. Keep in mind that the cannabis plant grew in nature all over the world at different longitudes with different spectrums. With all of the variations of cannabis strains in today’s market made from these ancestral plants from all over the world, each will perform differently under different spectrums.

One of the reasons you still hear people claiming that LED is not ready for commercial cannabis grows is because the reflectors from an HPS fixture mounted 4 ft from the canopy will provide a wide angle of light so the crop will receive light from the fixture directly above as well as the lights from the next row over and even 2-3 rows away. This light is coming in from different angles hitting the crop beneath the canopy leaves that are causing shadows from the light above. This does not happen if you are using an LED fixture that is a big panel of LEDs or made up of multiple LED bars where they have to be mounted 9-12 inches from the canopy. You get no crossover from the fixtures but only shadowing from the top of the canopy.

Another area you should always do your own math on is the BTU calculation for your grow room. Yes, LEDs produce less heat than HPS and this can be the deciding factor. As the cost to cool your grow room can be one of your highest production costs. Your AC load is based on the amount of BTUs produced from your fixtures. This is simple math as 1 watt equals 3.412 BTUs. No matter what 1000w HPS fixture you use the BTUs from that fixture will be 3412!

Most of these “new to horticulture” LED manufacturers are making claims that you should be using a full spectrum LED in flower to truly maximize plant terpenes and THC levels. But is this really true? See the graph below comparing what parts of the light spectrum is used for photosynthesis compared to the full spectrum light from the sun.

HPS or LED: A Key Factor to Consider.

LED technology has only been available on a commercial level in the horticultural industry for around three years. So realistically, they would only have had time to trial a limited number of spectrums that may yield good results for one or two strains/cultivars. We simply have not had the technology long enough to perform the testing needed!

In countries like Canada, where the federal government has legalized recreational cannabis, there is the ability to do more research. The University of Guelph is one organization that is leading the way in this research. As he states in this article from HortiDaily — Michael Dixon, Director of Environmental Control Research at Guelph says: “We have found that the optimal LED spectral recipe changes with every strain of cannabis.” This is a very important statement for growers to consider! Will you be growing just one strain in your grow the entire time? If not, what spectrum do you choose? Yet if the added blue light in flower only increases your THC levels by 4% is there a justifiable ROI in the cost of the added light spectrum?? If so, can you do that by just adding a few MH lamps? Or does the ROI make sense to go all LED? (1000w DE MH lamps are now available with the PL Light NXT 1000w DE fixtures).

Do this math with your LEDs as well. The PL Light TopLED is 320 watts so the BTUs will be 1092 BTUs. While that looks like a big savings it will take at least two times the amount of our LEDs to get to the same light level as a 1000w HPS. On the other side of this argument is that the LED fixtures do not provide the radiant heat as HPS and your crop roots will perform better with warmth. It is proven that there is a direct correlation between heat and crop production. Plants grow slower, producing less fruit the colder they are. In northern climates many growers rely on the heat from HPS to warm their crops.

There is a lot of information out there on the internet from various growers and lighting suppliers. You can find a contradiction to almost every point you find! Most growers agree that for young plants through the veg stage of growth you will want to have a light source with a good amount of blue light or even white (full spectrum) lighting. The main reason for this is to reduce internode stretching to produce a stockier, healthier plant.

Cannabis Research in Canada.

“What lighting source should I use for my licensed cannabis grow?”; “I am comparing LED to HPS for my cannabis operation. Which source do you recommend?” These are questions people ask me almost on a daily basis. We have covered this question in past posts on our blog, like this one by our very own head of marketing: LEDs vs Traditional Lighting but most cannabis growers know that this crop is different than most and many growers have experimented with lighting technologies on a small scale to find they can sometimes increase terpenes and THC levels just by changing their light sources. To begin, let’s unpack the information that exists on this complex topic.

Disclaimer Nothing in PL Light Systems including, but not limited to, written, visual, or editorial content, or external links on the website and digital journal, shall in any way be considered as promoting any illegal or illicit activities within your jurisdiction.

The best advice I have is to get a comparison of your ROI done by a versatile company. One that can offer both technologies where you can have light plans designed to the same levels with both technologies. Don’t trust just what the LED guy says or the only HPS guy says. Look at both, do your own math. Know what works for the strains you are growing and measure your own ROI. The easiest way to do this is going to this link and ask your local PL Light Territory Manager for advice as we are the “Lighting Knowledge Company” and whether you decide to use LED or HPS, you will get a true and honest comparison of both technologies.

When comparing both technologies, always compare light levels of the entire grow and not just fixture to fixture. I cannot stress enough that there is no such thing as a 1-for-1 replacement of HPS by LED! That simply does not exist in the marketplace. Yes, there are LED fixtures that exist when measured directly beneath the fixture. The output looks equal to or even greater than an HPS. But, when you move the PAR sensor any direction from centered beneath the LED the light level decreases much more under the LED than any HPS fixture!

These claims are not totally bogus however. We are now seeing real results that a bit more blue light in flower or at the very end of the flowering stage does increase terpenes/THC levels. Depending on where you search you will find these amounts vary from minimal 3-5% to unreal amounts of 26-38% as in this article from HortiDaily supplied by an LED manufacturer.