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: which high intensity discharge (HID) light is better – metal halide (MH) or high pressure sodium (HPS)?
Which grow light is best – HPS grow lights or MH grow lights – is perhaps the most common questions among those new to indoor gardening or HID (high intensity discharge) grow lights. Ironically, most HID grow light enthusiasts recommend using both MH and HPS grow lights. MH grow are suited best for vegetative growth and HPS grow lights are superior for flowering. While either type can be used for the full growing cycle, when combined, using an MH grow light for vegetative growth and flowering under an HPS grow light is the most recommended HID grow light setup because utilizing the strengths of both types at the right times produces the best results. If you’re forced to make a choice between the two types, your decision should be based mostly on the type of plants you’re planning to work with.
To understand more about the differences between HPS grow lights and HM grow lights, keep reading!
Metal Halide Grow Lights
Metal Halides are another specific type of HID grow light. Instead of giving off a reddish light like their HPS counterparts, MH grow lights supply plants with a blue-white light. Also referred to as “full-spectrum”, the output of metal halide lights is more evenly distributed across the visible light range like midday sunlight. The balanced spectrum of MH produces lush, compact vegetative growth that will support big yields when plants are ready to flower, whereas plants that receive too much red light during the vegetative stage can become spindly and develop fewer flowering sites. In the same respect, this also makes an MH light a better option for those growing kale or other vegetative crops such as lettuce or basil.
Two main types of MH grow light bulbs are commonly available to growers: 1) standard metal halide bulbs, which must be operated on metal halide ballasts or digital ballasts, and 2) metal halide conversion bulbs which must be operated on standard high pressure sodium ballasts or digital ballasts. Metal halide conversion bulbs that operate on HPS ballasts or digital ballasts have the same type of full-spectrum output as standard MH bulbs and usually have the added benefit of a higher lumen rating.
Metal halide lamp construction lends itself to color customization much more so than HPS lamp construction, therefore growers can find a wider variety of light outputs available with metal halide bulbs. There are specialized “Blue” MH grow lights that produce more light energy in the blue spectrums but less in other spectrums, and there are also MH grow light bulbs with increased red spectrum that are designed to mimic the output of HPS lights. HPS-like MH bulbs are still simply not as effective as having a full-fledged HPS light to flower your plants under though due to the significant difference in light intensity.
High Pressure Sodium (HPS) Grow Lights
High pressure grow (HPS) are a specific type of HID grow light often associated with a reddish / orange-ish output. If you have ever seen a grower’s indoor grow light and noticed it had a red/orange appearance, it is likely an HPS grow light. You may have also seen HPS lights used in parking lots and street lights because they provide a lot of illumination and they’re brighter than fluorescent lighting. While HPS lights are also used for general lighting applications, high-quality HPS grow lights are engineered specifically to grow plants and put out even more light than standard HPS lighting.
There are basically two main factors that contribute to HPS being a better option for flowering than MH. First, the reddish output of HPS more closely resembles the natural sunlight a plant would absorb in the autumn (flowering) seasons, which drives the plant’s maximum flowering production. In addition, HPS lights provide higher light intensity (lumens) than MH. This can be around 25-40% more lumens/watt than MH lights of the same wattage, and when it comes to flowering, the more light – the better the results (yield, quality, etc.)
Today’s best HPS grow lamps (bulbs) also offer a refined output or enhanced spectrum with higher blue range output to assist with vegetative growth. HPS grow light bulbs also maintain their brightness better and last longer overall than MH, although they still become less efficient towards the end of their lifespan and should be replaced before they burn out completely. The general recommendation is to replace HPS grow lamps after 12-18 months of regular use.
Metal Halide Vs HPS: So Which Bulb Should I Get?
The best answer is both.
You’ll get the best results if you run both bulbs all the time, but this is quite expensive, so it is often recommend using MH bulbs during seeding and vegging phases and using HPS bulbs during the flowering phases.
If you only use one bulb throughout the grow cycle, you’re better off going with HPS. It will result in spindly plants (due to lack of blue light, which helps plants grow strong), but using MH during blooming is worse.
MH bulbs are one of the best lights for vegging cannabis, but using them during flowering has a fairly large negative impact on yields, because flowering plants really need that red light.
While both high-pressure sodium and metal-halide bulbs have worked well to grow marijuana for many decades, the truth is that their spectrum is not ideal. Combining the two makes it much better, but it is still relatively inefficient compared to some of the more modern options.
When it comes to HID lighting, ceramic metal halide (usually shortened as CMH) is much better suited to plant growth. CMH bulbs can have varied spectra, but they are all much more complete than HPS or MH. They give you a great wavelength distribution that is most similar to natural sunlight out of all HID options.
CMH bulbs are also more efficient than HPS or MH, they emit less heat, and the last longer.
And then, of course, there are LED grow lights. Since this article is about HID lighting, we won’t go into LED too much, but we should mention it, because the technology has improved so much that these lights are now competing with HID lights, even CMH.
In the past, the cost of LEDs was always too high, but that is no longer the case. There are some great budget LEDs on the market, like ________, ___________, and ___________.
LEDs do still cost more than HID, but not much more. You’ll easily make back the difference within a year in cost savings, because they use far less electricity and emit far less heat, so you won’t have to spend nearly as much on cooling. They also last a lot longer, are safer, easier to operate (no bulb changes) and are better for the environment. This article has much more on LEDs vs HPS.
Metal Halide Vs HPS: Final Thoughts
In the end, the best answer to the HPS vs. MH debate is perhaps to go with CMH or LED instead if your budget can allow for them. That’s probably not the answer you were looking for, in which case, follow the advice above.
A mixture of HPS and metal halide bulbs is best and using MH for the vegetative stage and HPS grow lights for flowering is a solid combination that many professional growers use. If you only use one for the entire grow, make it high pressure sodium.
MH only won’t work well for flowering. As a bonus, HPS bulbs tend to last longer, too. Always check every bulb after buying it to see if it is any good. Learn how to tell if a high pressure sodium bulb is bad.