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MH and HPS Grow Lights for Marijuana

The key to growing marijuana indoors successfully is replicating the plant’s ideal natural outdoor environment while reducing the risks associated to growing outdoors, like unpredictable weather and pest infestation. In most conversations, cannabis seed growers agree that the most important factor in growing indoors is the lighting used.


Growers have a wide range of lighting options for their indoor grow operations in Utah or Rhode Island, but the question that continues to rise above the rest is whether to use metal halide (MH) or high-pressure sodium (HPS) lighting. Before deciding, it is essential to understand the light cycle required by marijuana plants during their symbiotic growth cycle.

Light Cycling

Most marijuana plants are photoperiodic, which means their growth stage is determined by the number of hours of light they receive. It is well known among growers that these plants require thirteen to eighteen hours of light in the vegetative stage and equal parts light and dark to enter the flowering stage. Barring automatic flowering plants, most feminized and regular seeds produce photoperiod plants whose lifecycle depends on a well-structured light/dark schedule.

Vegetative Stage

During the vegetative stage, the plant focuses its energy on growing bigger and producing leaves. This stage has a direct effect on the yield and quality of the marijuana it produces later. Plants with thick, tough stalks can hold up the weight of dense buds without cracking.

To get the most robust plant possible, most growers recommend providing plants with eighteen hours of light and six hours of dark from the seedling stage through the vegetative stage. It is important to provide the plant with blue spectrum lighting during the vegetative stage. Blue light wavelengths trigger chlorophyll production, which assists the plant in growing tall and producing leaves.

Flowering Stage

During the flowering phase, the lighting needs to be set to 12 hours dark/ 12 hours light. If there are too many plants sharing one light, the flowers will not develop to their full potential. It is recommended that a grow room has at least thirty-thousand lumens per ten square feet.

A marijuana bud comprises a stem, sugar leaves, and a calyx. When the plant does not receive adequate lighting, it will produce more leaves, making the buds less dense. If the plant receives an abundance of light, it will focus its energy on calyx production, increasing the plant’s yield. Growers should provide their plants with red spectrum lighting during this stage, as red spectrum wavelengths trigger flowering.

Too Many Hours of Light

Many new growers from Virginia to New Mexico assume that leaving the plants in the light twenty-four hours a day is the best way to ensure they do not enter the flowering stage too early, but that is not the case. Plants that receive twenty-four hours of light are forced to grow quickly, which can leave them weak and leggy.

When growers do not use timers on their lighting system, they can accidentally give their plants too much light. If this happens when the plants are in a vegetative stage, it will not harm them much. If this happens when they are in the flowering stage, the plant’s yield and quality may be impacted, and may even cause the plant to become a hermaphrodite.


Metal Halide (MH) Lights

MH lights are a variety of high-intensity discharge (HID) light that is commonly used to grow marijuana indoors. They are a workhorse light that has been used heavily by marijuana horticulturists since the 1960s.

How Does a Metal Halide Light Work?

Metal halide light bulbs create light by sparking electricity between two electrodes nestled inside a quartz tube filled with mercury and metal halides. The spark vaporizes the metal halide mixture to produce a cool, white light that contains blue wavelengths. While metal halide lights can last up to 20,000 hours, they are ineffective as grow lights long before they reach that many light hours.


What MH Light Should You Use?

MH bulbs are produced in a range of wattages, with 400-watts, 600-watts, and 1000-watts being the most efficient. The wattage required depends on the size of the grow area. A 400-watt light can cover a 3-foot by 3-foot room, a 600-watt bulb can cover a 4-foot by 4-foot room, and a 1000-watt bulb can cover a 5-foot by 5-foot light. It is important to keep lights at the right distance from plants, as well. If they are too far away, the plant will not receive the right light wavelengths, causing it to stretch and not produce well. If the light is too close, the plant could suffer light burns.

High Pressure Sodium (HPS) Lights

HPS lights are another type of HID light that has a higher spectrum of yellow and red light than MH lights. HPS lights are often used in streetlights, giving them their well-known orange hue. HPS lights resemble natural sunlight more than MH lights do, making them a more desirable lighting option. However, HPS lights are low on blue light wavelengths, so they do not produce the best lighting for a marijuana plant’s vegetative stage.

Benefits of Metal Halide Lights

Metal halide lights have a long history of being used in horticulture for several reasons, including:

Perfect Spectrum for the Growing Stage

Since MH lights emit a high amount of blue light, they are ideal for marijuana plants in the vegetative stage. These lights produce plants that have large, bushy canopies and promote strong stalks capable of bearing weight during flowering. MH lights also emit a large amount of orange and yellow lights, giving them a wide spectrum that is beneficial in all stages of growth.

Prevents the Plant from Stretching During the Flowering Stage

While MH lights do not produce enough red light to be used alone during the flowering stage, using them as a supplement to a red-heavy lighting system prevents the plant from stretching. When there is an insufficient amount of blue light, the plant reaches for the nearest blue light source but is not receiving enough of it to strengthen the stalk, making it bend under its own weight.

They are an Inexpensive Lighting Option

MH lights, and the ballasts and reflectors they require, are less expensive than ceramic metal halide (CMH) or light emitting diode (LED) lights. Those who are looking for a reliable lighting system on a budget would do well with MH lights.


How Does a High-Pressure Sodium Light Work?

HPS lights include a frame-supported narrow arc tube located inside of a bulb. The tube contains highly pressurized sodium, mercury, and xenon and is made of aluminum oxide ceramic. HPS bulbs emit light by creating an electric arc that vaporizes the sodium to create light with a large amount of red and yellow wavelengths.

Benefits of High-Pressure Sodium Lights

HPS lights have several benefits that make them an ideal lighting option for indoor grower operations, including:

They are Brighter than MH Lights

While they lack the right output of blue light to fully support the growth stage of a marijuana plant, HPS lights have a much higher light intensity than MH lights. HPS lights have as much as 40% more lumens per watt of light than MH lights, which increases the number of flowers a marijuana plant produces.

Perfect for the Flowering Stage

Since HPS lights produce light that is heavy on red wavelengths, they are ideal for a marijuana plant’s flowering stage. Not only does their red light support the production of flowers, but their high-intensity light improves flowers’ quality and quantity.

What HPS Light Should You Use?

HPS lights produce a lot of heat, so it is important to adjust the settings of the grow room accordingly. The lights are directed at the marijuana plants, so they need to be strung up higher than MH lights. However, their superior light intensity means each light covers a wider area than MH lights do, with a 400-watt bulb covering 16 square feet.

Using both Metal Halide and High-Pressure Sodium Lights

While both MH and HPS lights have a range of benefits, neither produce a broad enough spectrum of light for all stages of marijuana growth. That is why many growers use both lights to mimic the different stages of sunlight that a marijuana plant needs. It is best to use MH lights by themselves during the growth stage and use both MH and HPS lights during the flowering stage to prevent stretching.

Do not Forget Plant Spacing

Growing indoors requires precise lighting to deliver the right wavelengths in the right intensity at the right time to optimize growth and flower production. Equally important to finding the right lighting is the space between each plant in relation to the next. Plants that are too crowded prevent each other from accessing light, creating weaker plants that produce fewer flowers. Plants should be far enough from each other to allow light to penetrate the canopy and reach all parts of the plant.

The Bottom Line

The answer to the question of whether to use MH or HPS lights is simple. Oklahoma growers, and those across the USA, should use both lights in different ratios at different stages of plant growth to produce the most robust marijuana plants that can support many dense buds. When used together, these lights help growers replicate the plant’s optimal natural light conditions. Also the lighting options discussed above will have essentially the same impact on 420 seeds (which are high in THC) or CBD seeds. If you have heard of 710 seeds, this name comes from the upside-down spelling of OIL. In order words, our 710 seeds are high-potency strains great for producing cannabis oil or other concentrates. Remember, whatever stage you are at with your indoor grow setup, you can always stock up on seeds by shopping at weed-seeds.com. Happy growing!


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