Indoor Grow Lights: Metal Halide in Cannabis Cultivation

As a grower, you will adhere to one of two lighting programs in indoor cannabis cultivation: LED or HID. High-intensity discharge lighting (HID) is the traditional (and arguably most intense) option. Under this umbrella are two types: High-Pressure Sodium (HPS) and Metal Halide (MH). 

Here we discuss metal halide fixtures in detail, including applications for growing cannabis (hint: it's in the veg room), lifespan, energy considerations, and other technical details worth noting.

If you have questions about all the grow light options, head over to our in depth guide to cannabis grow lights.

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Metal halide is a type of HID bulb, producing light from an electrical arc sent through a mixture of gases: vaporized mercury and metal halides. Metal halide fixtures include three parts: a ballast, bulbs, and a reflector.

All HID bulbs are extremely effective in light output, with MH typically producing 80 to 100 lumens per watt. For example, a 1000 watt metal halide bulb will produce 80,000-110,000 lumens.

In the grand scheme of your grow room budget, HIDs offer the most affordable upfront costs. However, they do produce a lot of heat, and when it comes to energy consumption, they are not as cost-effective as LEDs (find out more on whether or not LEDS are worth the cost).

In terms of lifespan and ongoing energy costs, they are more expensive than LEDs. Even compared with HPS lighting, they consume substantially more energy. 

Like all HID bulbs, metal halide bulbs have a limited lifespan (at least compared with LED fixtures). Generally speaking, you can expect to get upwards of 20,000 hours of output, but there is a steep degradation. 

Around the halfway mark (10,000 hours), you may only get 50 percent of expected lumens from the fixture. HPS bulbs have a better degradation rate than MH.

Beyond the affordability and intensity, what makes MH a preferred indoor grow light for cannabis cultivation? MH bulbs are perfect for the veg room. 

The combination of gases within the metal halide bulb, the light spectrum expressed is ideal for veg plants. That means hitting all the right notes with the PAR spectrum, hitting high levels of blues, greens, and UV.

Compared with HPS lamps, MH lights produce a blue-hued light. Although MH bulbs naturally produce fewer lumens than their HPS counterparts, they hit the spectrum needed for bushy, short vegetative growth. 

This spectrum mirrors natural sunlight during the spring and early summer months, stimulating the plants' instinct to grow as many leaves (aka photosynthesizers) as possible in preparation for flowers. 

There is an ongoing discussion about MH or HPS for veg. Yet, many growers report better results for vegging from MH in their own side-by-side comparisons. Metal halide bulbs encourage short, bushy growth, while HPS tends to encourage early stretch.

Lighting size is always a calculation based on many variables, all of which differ from one grower to the next. Determining the number of plants (or canopy size) per metal halide isn't a simple formula. 

As a rough guide to get you started, use the below metal halide chart for reference:

What can influence the numbers suggested in the above schedule? Everything and anything. 

The strain you choose follows its morphology, taking up more or less space than average. Layered on top of the strain is the pruning and training style you are following. For example, a screen of green will expand the canopy but have fewer plants. 

Another way to include the canopy coverage of an MH is to fiddle with hang height and larger reflector hoods. Hang height changes the intensity of the light and canopy penetration. Hoods capture and reflect light down into the canopy for improved coverage. Larger hoods reflect more than smaller hoods (up to a certain point). 

The primary objective for fixture height is to get the maximum production out of your plants without damaging the canopy. 

Hung too far above the canopy, and your plants don't receive enough light for optimum production. Too low, and the sheer intensity of the HID bulbs can physically burn the leaves. In between is the happy medium.

No matter how high you hang the fixture, you should also performed what is commonly known as the hand test. Hold your hand underneath the fixture at approximately canopy height. If you experience discomfort because of the heat, so will your plants. You want it to be warm, but not painfully so.

Pro Tip: It's best practice to start with your lights hung higher and slowly move them closer as the plants adapt & grow.

Although newer LEDs may rival the potential of HIDs, metal halide is still a standard across the industry for the veg room. Growers (and their plants) love the blue-light these HID bulbs emit. Most cultivators strongly feel that this blue spectrum encourages rapid, low and leafy growth.

For small scale indoor growers, they are also an affordable upfront cost. Metal halide bulbs are the go-to for many, especially those working with HID lighting, for the first stage of growing cannabis.