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Metal Halide vs. High-Pressure Sodium Grow Lights

Last Updated: June 15, 2022By Jason Levin

There are so many options when it comes to picking the right lighting to grow the best weed. If you plan to grow marijuana plants, and are familiar with some of the common lighting methods, you may wonder whether if it’s really necessary to set up metal halide (MH) lights for the vegetative stage and then switch to high-pressure sodium (HPS) grow lights when it is time for the cannabis to flower. Can’t one type of light suffice for the entire grow process

We’ll explore these two lighting systems and analyze whether you need both. Perhaps just one can suffice for the vegetative and flowering stages of marijuana plants.

 metal halide light bulb by EYE Hortilux

(Blue Metal Halide by EYE Hortilux

What are metal halide grow lights?

The debate on HPS or MH for flowering ought to start by understanding what each of these lights are. Let’s start with metal halide grow lights. They are a variety of lights that give off a high-intensity discharge of white light that often leans towards a bluish hue. 

Notably, MH lights emit a broad spectrum of light, similar to how sunlight appears at about midday. This full-spectrum is balanced and often favors the luxuriant growth of plants, which means that the vegetative growth will be vigorous under this type of grow light. However, since the light is broad-spectrum, it is possible to have these same lights during the flowering stage. If you do this, your results may be less than optimum since the lights may not be bright enough to provide the light intensity demanded by marijuana plants while they flower.

On a side note, the attributes of MH grow lights make them ideal for growing vegetative crops, such as kale, basil, and lettuce, since these plants will grow lush and green under this type of light.

Why do people believe MH lights are better suited to vegetative growth while the “popular” belief is that HPS lights are best used during the flowering phase? The answer to this lies in the seasonal changes during the year. The rays of sunlight appear to be redder during summer and early autumn, and that is the time when plants, including cannabis, flower. Since MH lights have more light on the blue side of the spectrum, similar to spring and early summer, these lights are best used during vegging.

Double Ended HPS by EYE Hortilux

(Double Ended HPS by EYE Hortilux)

What are high pressure sodium grow lights?

HPS lights give off a reddish or orange appearance. This type of light is also used in many public spaces, such as parking lots. It emits intense light that provides better illumination.

Many people believe that HPS lights are better for cannabis in the flowering stage because the red appearance of the light mimics the appearance of sunlight in autumn. Furthermore, the superior light intensity of HPS lights makes them ideal for the flowering phase of marijuana growth since the plants need a lot of light to grow large and heavy buds.

 

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What does science say about HPS or MH for flowering?

We will talk about the science of these two types of grow lights from two perspectives. That way, you can decide on the most suitable lights for the flowering stage of your marijuana.

Perspective One: 

The first perspective centers on the appearance of sunlight from here on Earth. Technically speaking, sunlight doesn’t change because the light we see comes from the same source. Instead, what we see seems different during different seasons because of the effects that cause light to scatter

For example, the sky looks blue because of the way light scatters. The sunset looks dazzling because of how sunlight is scattered during that time of day to produce the different colors we see. These color differences don’t mean the light from the sun has changed. They represent the angle at which this light strikes certain objects and how it changes as the day progresses, hence the different colors we see.

From this perspective, the changes in the color of sunlight are just apparent and not actual differences in the light we receive during different seasons or times of the day. This could suggest that HPS and MH lights of equal power can do a good job when your marijuana is flowering.

cannabis leaf lit up by different lights

Perspective Two: 

The second perspective to approach the debate on HPS or MH for flowering is the nature and genetics of cannabis plants themselves. All flowering plants can be described based on how they respond to the changes in the light they receive during the different seasons. This concept is known as “photoperiodism.” It can loosely be broken into two words “photo” (meaning light) and “periodism” (meaning time segments, in this case, the different seasons of the year).

Some plants begin flowering once they “realize” that the days grow longer while the nights become shorter. These plants are called long-day plants, and they include carnations, potatoes, and spinach.

Another group of plants, such as marijuana, begins flowering once they sense that the days are getting shorter and the nights are longer. Such plants are called SDPs (short-day plants). These shorter days are typically prevalent in autumn and winter. We also have day-neutral plants which don’t seem moved into flowering by either longer days or shorter daytime durations.

It is interesting to note that plenty of research on photoperiodism focused on just one plant called Arabidopsis thaliana. This particular plant is a long-day plant because it flowers in spring and summer when the days are longer than the nights.

Marijuana, on the other hand, is a short-day plant, so it may be unfair to conclude that if A. thaliana flowers well under red spectrum light, cannabis also will. The best way to know is to grow your own with a weed grow kit

 

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What’s the overall takeaway on HPS or MH for flowering?

There isn’t a rigorous study that considers how well marijuana grows and flowers under different lighting conditions. We need to consider environments in which blue-spectrum dominant light exists vs. environments in which red-spectrum lighting dominates. For this reason, the jury is still out on this one. That means you must use your discretion to decide whether to use only one lighting setup or change as your plants transition from vegetative growth to flowering.

However, many growers get good results in the vegetative state when using metal halide lights. They also get good flowering when the lights are switched to the red-dominant lighting provided by HPS lights.

You might want to experiment with both types of lights and find the approach that gives you the best results. Or better still, get lights that benefit both blue-spectrum and red-spectrum light at the same time. For example, enhanced HPS lights can also emit blue light, and there are also specialized MH lights that give off red-spectrum light. These “hybrid” lights will save you from the pricey prospect of buying and setting up two different lighting systems for the same marijuana cultivation cycle. Technology is ever evolving and new lights are being developed. (Hint: LEDs!!) 

 

cannabis basking in sunlight

Sunlight is always best

Regardless of which you choose, every grow light is ultimately trying to replicate the sun. Cannabis plants grown under the sun will always be different from marijuana grown under a light. 

If you need to supplement your natural light, try Light for Pot,  our LED grow lights are easy to use and highly effective for growing a few plants indoors. LEDs are newer technology which means lower energy bills. Combine that with our complete grow kit system for an all-around solution to your home growing cannabis needs. 

Related: How to Grow Cannabis Using LED Lights

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