Metal Halide or MH light is a gas discharge lamp which produces light by creating an electric arc in a pressure chamber full with mercury and metal halides. This electrical arc excites the gas mixture inside the pressure chamber and it starts to glow. The light produced by the gas mixture is directly linked with the elements that are in that bulb but overall the metal halide lamp produces a white (full spectrum) light with a spike in the blue region of the visible light spectrum. Metal halide lamps have been long used for regular cannabis growing and growers use them also with autoflower strains. Basically you can grow cannabis fully from start to finish with this light but the buds will be fluffier than growing with HPS or LED lights as this light source does not produce sufficient RED light that plants need more in the flowering phases. Because of this phenomenon metal halide lamps are most widely used for seedling and clone growing and basically are good all the way to the start of the flowering cycle.
Metal Halide bulb usage
Metal Halide lights can be good for almost any grow operation and you can grow your crop under this light from start to finish but usually the best use for metal halide lamps is in the pre-flowering stages of growth or as a supplement used together with HPS to produce even more full spectrum light. MH bulbs are also sometimes used at the last stages of plants growth because the Metal Halide bulbs produce a bit more UVB radiation and that apparently helps the resin production so you get crispier buds. But for the sizes and shapes There are MH bulbs ranging from 150W to 1500W and larger bulb tend to get more efficient so a grower with a 150W Metal halide bulb will get less light Watt per Watt than grower with 1000W MH bulb. But apart from the lumen output differences all the sizes are the same and you can grow your cannabis plants in closet with that 150W bulb or get a 1000W light and grow huge plants for commercial purposes.
Another thing that may interest potential MH buyers is the orientation or the bulb as not all bulbs can be placed horizontally or vertically. There are special symbols on the sockets that are H (Horizontal) and BU (Base Up), BD (Base down) and you should take these into consideration because the bulb is specially designed to be places in that orientation and you can only rotate it for only 15 degrees to any side (15 degrees is really small amount so basicly they need to be straight!) but if you put a Horizontally designed bulb vertically you can diminish the life expectancy of that bulb so be careful!
Color temperature and Ultraviolet UVB radiation
Metal Halide bulbs usually have a color temperature around 4000 K but you can get many different types of these bulbs ranging from 3000K up to 20000 K and each bulb has its own light spectrum. The typical 4000K bulb is ideal for plant growth as it closer resembles the Sunlight than for example the HPS bulb. These MH bulbs each has a bit different color temperature because the gas mixture inside the arc tube can vary slightly and you can have +- 100 K difference from bulb to bulb and usually they are rated after 100 hours of work time that is called the seasoned time. At this seasoning the light have to work for those 100 hours to get all the parts to start working properly and at that stage the faulty bulbs usually break or does not start but that is all done by manufacturers!
Basically at the 4000 K mark the light is almost neutral and higher up the light gets more of the blue tint and lower than 4000 K the light gets more yellowish so you need to take this in mind and check for the exact color temperature of your intended bulb.
Metal Halide bulbs also create more UVB radiation that is known to help plants produce more resin because it is in essence harming them and that little damage that the UV radiation does will allow the plant to produce more resin to protect itself from that sort of light.
Tip: Don’t stare at the Metal Halide bulb because the UV radiation can harm your eyes and if the outer glass shell breaks turn of the bulb immediately and be extremely cautious because the outer glass absorbs most of the harmful Ultraviolet radiation.
Wattage & lumens
Just like the HPS lights Metal Halide lights have the typical 150, 250, 400, 600 and 1000 Watt lamps but their lumen output is considerably lower than the HPS grow light bulbs.
Here are the average lumen output form different Metal halide grow lights:
- 150 W -> 12,000 lumen = 80 l/W
- 250 W -> 21,000 lumen = 84 l/W
- 400 W -> 36,000 lumen = 90 l/W
- 600 W -> 60,000 lumen = 100 l/W
- 1000 W -> 110,000 lumen = 110 l/W
This chart is for the educational purposes only because the actual lumen to Watt ratios can be different for different manufacturer bulbs and you need to check your bulb for the actual amount, but basically this is how they should be. As you can see bigger wattage lamps are more efficient but not by much and you will get a lot of heat from the higher wattage bulbs so consider your growing conditions before buying a metal halide bulb for your autoflowers as you will need to ventilate the excess heat from the growing area and that will also take electricity that will again reduce the total efficiency of these lights!
Metal Halide lamps run hotter than HPS lamps because their efficiency is lower than and a light bulb can convert energy only to light or heat and if the lamp has lower light to watt (lumen/Watt) ratio then it will run hotter producing more heat from the same energy input. Metal Halide lamps are about 10 to 20 % more inefficient than HPS lamps ho they run hotter by the same amount and you will definitely need to think how to get rid of the excess heat or be happy that you can heat your cold grow room.
The costs of running a MH grow light would be almost the same as HPS light so you can check out the HPS article for more information but both of these bulbs would need replacing every year or about 5000 hours and they cost almost the same.
MH lifespan and How to save bulbs
Metal Halide lamps usually have fairly long lifespan of about 15000 hours that could result in almost two year continuous growing but no one tells you that with time the lumen output decreases tremendously. Usually when the MH light reaches the 7000 hour mark it has lost about 50 % or its efficiency so if a new bulb would throw out 60,000 lumens than the one year old bulb could have just 30,000 lumen output. This is a big problem for indoor growers as these bulbs need to be replaced every year. If you won’t replace your metal halide lamp then you will be losing money and yield because for the same amount of Watts you will get quite a lot less light and growth.
The way how you can prolong the MH bulbs life is the same as prolonging the HPS light bulbs and that is why I will not go into details here.
The biggest advantage is the blue spectrum light
Halides tend to produce a shorter, more compact plant
They produce ultraviolet light that is beneficial to plant growth and resin production
Very energy efficient 60 to 120 lumens per watt
Produce a lot of heat.
In the flowering phase plants will not get as much red light as they should.
A bit more expensive than High pressure Sodium bulbs.
Ceramic metal halide bulbs
Apart from the regular Metal Halide lights there are also Ceramic Metal halide lights that closer resemble the actual sunlight and they have both the blue and the red spectrum color outputs. These lights are a variation of the Metal Halide lights and they do their discharge in a Ceramic tube where elements like mercury , argon, metal-halide salts are vaporized and dissociated into atoms that produce the light. Because these lights have more elements in their discharge tube they can have different color temperature output and thus give your cannabis a more even light that more closely resembles the natural sunlight and will grow taller and bushier plants. With this light you won’t need to replace you MH bulb with a HPS when the flowering starts because the CMH bulb has enough red light to fully mature your herb.
These Ceramic Metal Halide bulbs have the same efficiency as HPS bulbs but they have the disadvantage that they cannot be used with electronic ballasts so you will lose some efficiency due to the older generation of ballast technology.
If you liked this post then you can also check out grow lights like LED, HPS, T5 and CF right here at the autoflowering-cannabis.com blogs Light section and I would be thankful if you shared this post trough twitter or Facebook!