MH or HPS: What Are The Differences & Which Is Better?

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MH or HPS grow light
November 22, 2021
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To grow marijuana successfully, you need a balance of a few things; temperature, humidity, water, nutrients, and light. There’s a long-standing question among cannabis growers: what’s a better source of light – MH or HPS?

All these letters do mean something. Metal Halide (MH) lights help weed plants grow during the seedling and vegetative stages. In contrast, High-Pressure Sodium (HPS) lights benefit marijuana plants in the flowering stage. This is due to the type of light that the bulb produces. 

What are the drawbacks and benefits of MH and HPS grow lights? We’re going to enlighten you on everything you need to know about these types of bulbs for your grow room. Let’s get started!

Weed plants and MH light
Weed plants and MH light

HPS and MH: what are they, and how do they work?

MH and HPS grow lights are classified as High-Intensity Discharge (HID) bulbs. Both produce light from an electrical arc in a tube filled with a noble gas and often contain a metal or metal salt. When you add heat, the mixture vibrates and turns to vapor, which then produces light. 

HID lights are the “golden standard” for growing marijuana seeds and illuminate large areas like stadiums, theaters, warehouses, and streets. HID bulbs are energy efficient and have a high light intensity. 

For your growing space that uses HID lights, including MH or HPS, you’ll need some other equipment for your setup. These include:

  • Electrical ballast: Provides electricity to start and heat the gas mixture to produce an electrical arc. You can find “switchable” ones that work with both HPS and MH. A dimmer function and a microprocessor are features for voltage monitoring and regulation.
  • Exhaust fan: Removes warm air from the indoor garden caused by the heat HID lamps generate. You can also attach a “carbon scrubber,” which eliminates the cannabis odor from the air before expelling it outside.
  • Reflective hood: Helps mitigate MH and HPS grow lights’ omnidirectional lighting by deflecting “wasted” light back towards the plants. Some models also help to dispel heat.  
  • Timer: A tool that ensures proper adherence to cannabis light cycles, essential for healthy growth and a maximum yield.

Now you know what an HID light is. So, what’s the difference between MH and HPS grow light illumination? 

HPS grow lights

High-pressure sodium lights are made with a mixture of sodium, an inert gas like xenon, and mercury. These elements are inside the tube between two electrodes. 

When electricity from the ballast heats the mixture, the xenon gas facilitates “striking” the arc, a feature both HPS and MH lights have, which then produces light. 

HPS grow lights produce a reddish-orange hue, which replicates autumn’s warm sunlight. These are used for the flowering stage, as that’s the time of year that marijuana would naturally flower. 

HPS grow light
HPS grow light

MH grow lights

Similar to HPS, MH lamps work with a gas mixture inside of an electrical arc. How they differ is from the elements inside the tube. MH bulbs contain mercury, argon or xenon, and various metal halides such as scandium and sodium iodide. 

When you add electricity, the xenon or argon gas ionizes to create the arc; then, the metals heat up to produce the light. 

In contrast to HPS, MH bulbs create bluish-colored beams, which resemble spring sunlight. This type of light aids in photosynthesis, which is vital during the early vegetative stages. It will help build solid stems and roots for the cannabis plant’s life. 

MH grow light
MH grow light

MH vs. HPS: 3 main differences between metal halide and high-pressure sodium

We’ve covered the basics of the similarities and differences of MH vs. HPS grow lights, but there’s more to know to become an expert. Understanding how the two are different will make you a better marijuana grower. 

What’s the difference between metal halide and high-pressure sodium

Here’s a deeper look into the 3 main differences between HPS and MH bulbs. 

Light Spectra 

The main difference between MH and HPS grow lights is the light spectra. Both intensity and spectra play a role in plant photosynthesis and photomorphogenesis, which are vital in the health of your cannabis plant.

Photosynthesis is associated with plant growth from direct light energy. Meanwhile, photomorphogenesis refers to the effect of light on the plant’s development, including flowering, height, and leaf size.

MH and HPS grow lights have different chemical makeup, making different kinds of electromagnetic wavelengths on the light spectrum. 

You measure the light spectrum in nanometers (nm), and you can represent it like this. 

Light SpectrumNanometers
Invisible ultraviolet electromagnetic radiation10–400 nm
Visible light 400–700 nm
Invisible infrared radiation700 nm–1 micrometer

As mentioned above, MH makes a cool bluish light, while HPS makes a warm reddish light.  

The blue light spectrum is 400 to 500 nm and triggers physiological and developmental responses, including germination, photosynthesis, CO2 and chlorophyll absorption, plant density, height, and overall growth. 

The red light spectrum is 600 to 700 nm and answers the “MH or HPS for flowering?” question. HSP bulbs are appropriate because this light spectrum helps stem growth, photomorphogenesis, and leaf nutrient content. 

Lumens per Watt

MH and HPS grow lights have different lumen per watt or efficiency. Therefore, MH is much less efficient compared to HSP lamps. 

Lumens are the measurement of visible light power. Watt is the measurement of energy consumed. Thus, lumens per watt is how well a light source converts energy to illumination. 

For example, if the bulb is at 250 watts, the average lumen per watt over the bulb’s life is 58 for MH and 87 for HPS. That means that HPS is producing more light with the same energy as an MH bulb. 

When discussing MH vs. HPS, the high-pressure sodium lights win. They’re not the most energy-efficient on the market, though. Take a look at HPS vs. LED; the LED bulbs are better in this aspect

Lifetime use

All light bulbs burn out eventually, and MH and HPS grow lights follow that characteristic. 

HPS grow lights tend to maintain their brightness for longer compared to MH ones. HPS bulbs tend to last about 24,000 hours, and MH lasts between 6,000 to 15,000 hours. Even the best MH lamps only last about 60% of the life span of HPS ones.

Metal halide vs. HPS: 3 pros and 2 cons of each 

Although MH and HPS grow lights are both HID bulbs, each has its own sets of pros and cons. What’s the difference between metal halide and high-pressure sodium?

Let’s explore the advantages and disadvantages of MH and HPS grow lights to help you determine which is better for your grow tent. 

HPS grow lights pros and cons

High-pressure sodium lights are a must in all grow rooms. There are some great qualities and also some other things to be mindful of when using HPS bulbs in your indoor garden.

HPS grow lights ProsHPS grow lights Cons
Optimal for flowering cannabis plants

Bright light, which helps with visibility

High concentration of light
Adverse effects during vegetative stages

Need to replace every 12 to 18 months
Cannabis plant under HPS grow light
Cannabis plant under HPS grow light

Pro 1: Optimal for flowering cannabis plants

HPS or MH for flowering? HPS bulbs’ wavelengths are perfect for the flowering stage of cannabis plants. The warm light mimics sunlight in late summer and fall when naturally a marijuana plant would be in its flowering stage. 

Pro 2: Bright light, which helps with visibility

Nothing’s worse for a pot cultivator to get to the last weeks of growing when you can finally get to use your leaves, and there’s a problem. Marijuana plants are vulnerable to mold, pests, and other pathogens

Compared to MH, HPS lights’ brightness allows you to get a good look at all the parts of your marijuana plants. In addition, it helps when pruning and trimming off mature leaves. 

Pro 3: High concentration of light

If you don’t have autoflower weed seeds, the concentration and amount of light cannabis plants get during the later stages of life directly correlate to the yield

In contrast to MH, HPS bulbs ensure a sufficient amount of light getting to all parts of the flowering weed plant for a healthy and high yield. 

Con 1: Adverse effects during vegetative stages

HPS or MH for veg? Although you might think HPS bulbs will be appropriate for your weed plant’s entire lifespan, that’s not the case. When you use HPS, not MH, you’ll have spindly weak plants during the vegetative stages. 

Con 2: Need to replace every 12 to 18 months

If you grow marijuana consistently, you can expect to replace the bulbs every 12 to 18 months. If you don’t grow full time, it’s about 24,000 hours. 

Pro tip: Always have a few spare HPS bulbs around, so if one burns out, you can replace it quickly without damaging your cannabis plant. 

Metal halide grow lights pros and cons

You might think that HPS bulbs are superior to MH when comparing metal halide vs. HPS. That’s not true. MH lamps play a vital part in growing vigorous, healthy cannabis plants. Similar to almost everything in life, with the good comes some bad. 

Let’s take a closer look at the advantages and disadvantages of Metal Halide growing lights. 

Metal halide grow lights ProsMetal halide grow lights Cons
Great for the vegetative stage

Can customize to be cooler or warmer

High source of white CRI light
Can’t use during the flowering stage

Long warm-up time
MH cannabis grow lights
MH cannabis grow lights

Pro 1: Great for the vegetative stage

MH or HPS for the vegetative stage? During the seedling and vegetative stages, the blue light from MH bulbs is perfect. It reflects a bright spring day when buds are starting to pop. 

This type of light helps with photosynthesis and the development of strong roots, stems, and leaves. 

Pro 2: Can customize to be cooler or warmer

You can purchase MH lights that have two modes, cooler and warmer. The first setting is better in the earliest parts of the growing process, and the latter one is good to use towards the end of the vegetative stage. 

It’s a less abrupt transition when the plant starts the flowering stage, and you use the HPS lights. 

Pro 3: High source of white CRI light

Compared to HPS, MH lights have white CRI light, making a correct representation of the color. This is essential in growing marijuana because diseases or nutrient deficiencies present themselves as discoloration. 

Pro tip: Look up the characteristics of a cannabis plant before growing, so you know what to expect regarding color. 

Con 1: Can’t use during the flowering stage

HPS or MH for flowering? With the strength of HPS lights, MH bulbs can’t compete due to the light spectrum they produce. Unfortunately, MH lights are too weak to be used during the flowering stage. 

If you want a successful harvest, you’ll need MH lights in the beginning and HSP ones for the later stages. 

Con 2: Long warm-up time

As mentioned above, MH bulbs aren’t as energy efficient as HPS lights. One of the reasons is because it takes a long time for them to heat up. With some, it can take up to 20 minutes to get to its full brightness. 

MH or HPS: which one is the best

So which is better: MH or HPS?

MH and HPS grow lights have their own characteristics, and it’s hard to say which one is the best. To grow large-yielding concrete plants, we recommend having both. There are tons of benefits of MH and HPS during different stages, and just one won’t produce successful cannabis plants. 

That being said, you can find some bulbs that function both ways. For example, there’s one setting for MH and HPS. These will be ideal if you don’t want to change the bulbs every couple of months when the marijuana plants start maturing. 

Weed HPS grow light
Weed HPS grow light

Which is hotter: HPS or MH?

MH lamps are hotter than HPS lights. This is a critical element you need to keep in mind when heating and cooling your grow room. Make sure you consider this if you’re having problems with temperature in your indoor garden. 

Can I replace metal halide with high-pressure sodium?

We encourage you to change from MH to HPS during the growing cycle, but you can’t interchange the bulbs with the same ballast. You’ll need to have two separate ones for HPS or MH. 

Is HPS or MH better for veg?

MH or HPS for the veg stage? MH bulbs are better for the vegetative stage because of the type of light that the lamp produces. The blue beams resemble springtime, and it helps with photosynthesis. 

Is metal halide good for flowering?

No, you shouldn’t use MH lights for flowering because the light intensity isn’t strong enough to let the cannabis flower properly. If you use MH instead of HSP lights during the flowering stage, you’ll have weak and small leaves. 

HPS or MH for flowering? HPS lights are a must. 

Are metal halide lights energy efficient?

Compared to the HPS, MH lights are less energy efficient. So if you’re looking at all types of lights in general, they’re not the best, nor the worst, when looking at energy efficiency.

Marijuana plants under MH grow lights

Key Takeaways about HPS and MH

If you’re ready to invest your time, money, and energy into becoming a cultivator, you want excellent results. Having both MH and HPS grow lights will create the best weed plant for quality and size. Of course, there are pros and cons with each type of bulb, but overall they’re both essential. 

If you’re a novice cannabis grower, we recommend investing in high-quality lights. You won’t be disappointed in the results of your first harvests. Even if you’re an experienced marijuana planter, you should look at your setup and make sure it’s up to date. Buy new bulbs if needed. 

Now you know everything you need to know about MH and HPS grow lights, get online, and purchase cannabis seeds to test out your new lights. What’s the wait? Get growing today!