How to use HPS/MH Grow Lights For Growing Weeds?

First of all, you should know that an HID (High Intensity Discharge) Grow Light System isn’t the right choice for all cannabis growers. Just like any other cannabis grow lights, MH/HPS grow lights have pros and cons.

Many people choose HID grow lights because they are ultra-powerful grow lights, including the incredibly popular “Metal Halide” (MH) and “High Pressure Sodium” (HPS) grow light combo for growing cannabis.

Yes, this type of system has been used for decades and gets great results, bigger yields, and tends to be easier to maintain than just about any other grow light system for growing marijuana. That’s why the MH/HPS combo is often referred to as the “golden standard” grow lights for growing cannabis…

The different parts of an HID grow setup

Every grow setup that features HID lights should have the following parts:

• Light bulbs: These will be either MH/HPS light bulbs, depending on the stage of growth your marijuana plants are in, or else they will only be HPS lights for the entire life cycle of your plants.
• Ballast: This connects the HID lights with a regular light bulb socket. It is necessary for an HID grow setup because these lights aren’t compatible with normal sockets. This will be either magnetic or digital.
• Hood: This serves as a reflector to help hone all of the light in on your plants below. It is located above or around the light bulbs and is necessary for efficient use of the light energy. Some hoods are also used for the containing and dispelling of heat.
• Ventilation: This will be necessary to get rid of the hot air in your growing area. It generally includes a fan and ducting to keep the temperature at a healthy level for your plants. It may also include odor control, such as a carbon scrubber or filter.
• Timer: This works with the lights according to your settings. It will switch them on and off automatically so that you don’t have to.
• Grow tent: This helps make the walls reflective and waterproof, help with exhaust, protect the plants from leaks in light or from dust, and adds extra security to your growing operation. I highly recommended this.

Example MH/HPS Setups

250W HID

1-2 oz per month
Electricity: $69/month

400W HID

1.5 – 3.5 oz per month
Electricity: $96/month

600W HID

2.5 – 5 oz per month
Electricity: $128/month

Although there is a larger size MH/HPS grow light available (1000W), at that size it starts going outside the scope of a “hobbyist” grower as far as ease and yields. In addition to needing a lot of extra cooling, a 1000W HPS grow light is less efficient compared to a 600W grow light (as far as how much light is put out for electricity used). I think most hobbyist growers would be happier with a 600W, or even two 600W lights, over a 1000W.

MH vs. HPS grow lights

HPS grow lights(High Pressure Sodium)

You know when you fly into a city at night and the lights on the ground all have an orange reddish color? That color is a result of pretty much every street light in the world using an HPS light.

HPS lights can be used throughout the entire growth cycle of your weed plants, but the orange light they produce makes them the best HID lights to use during the flowering stage and they’ll encourage the growth of bigger buds.

HPS lights produce a lot of light, but they also generate a fair amount of heat so including an exhaust fan into your grow setup is a wise move if you’re using a more powerful HPS light.

Interestingly, the amount of light produced by HPS lights doesn’t increase in proportion to the power they use. For example, a 600 Watt HPS light produces 7% more light per watt than a 1000W HPS light.

MH grow lights(Metal Halide) 

MH grow lights produce a light that’s more blue in color and this makes them the best HID lights for young wed plants or plants in the vegetative stage. The blue color encourages the rapid growth of leaves and stems.

MH lights can also be used throughout the growth cycle but they won’t produce anywhere near the same yields than an HPS light. Most growers using MH lights will switch to an HPS light when they trigger the flowering stage.

Combining HPS and MH lights for the best of both worlds

MH grow lights are best for the vegetative stage and HPS grow lights are best for the flowering stage, so an ideal solution is to use both! Many growers do this and if you buy the correct ballast, fixture and reflector it will support both types of lights so you can swap them out. Almost all the options on our list of the best HID lights support swapping between HPS and MH bulbs.

Choose Your Ballast

A quality ballast should be digital and work with either MH or HPS bulbs. Most ballasts also have a dimming function that allows you to adjust the percentage of light your bulbs emit. A ballast designed to regulate a particular bulb wattage will ONLY work with matching bulbs so make sure you choose the right version for the size of your grow space and the bulbs you plan to use.

Choose Your Bulbs

Your bulbs will need to match your ballast. If your ballast supports 600W Metal Halide, you need to get 600W Metal Halide bulbs. If your ballast supports 1000W High Pressure Sodium, you need to get 1000W High Pressure Sodium bulbs.

Some ballasts support both Metal Halide and High Pressure Sodium, and will clearly state that on the box.

Some ballasts are digital and dimmable, which means you need to get bulbs that support digital/dimmable features.

Some bulbs claim to be “enhanced” or be ultra-efficient. While there is not a whole lot of difference between the different bulbs, you will tend to get slightly better results from the higher quality bulbs, and they tend to be more durable/long-lasting.

Therefore, like with the ballast, I recommend getting the best bulbs you can afford. These bulbs generally last 1-2 years of normal use, so you will not have to replace them for quite a while. They do get dimmer over time, so it’s a good idea to always replace all your bulbs every 3-4 grows, even if they haven’t burnt out yet.

Choose Your Reflectors

There are three main types of HID reflectors that you should think about buying: winged reflectors, cool tubes, and air-cooled hoods. Winged reflectors have lots of surface area and, therefore, are great for the reflecting part of their job; they aren’t as good with keeping cool, however. Cool tubes are better at keeping cool, but have a smaller reflector and, therefore, aren’t as good on the reflecting side of things.

Air-cooled hoods are the best of both worlds, as long as you take advantage of their handy design feature to easily be hooked up to an exhaust system. They will trap the heat right near the bulb so it doesn’t reach your plants, and then will be sucked out through your exhaust system. Generally speaking, air-cooled hoods (the wider, the better) are the most popular and most effective choice for indoor marijuana growers with an HID light setup.

Setting up and using your MH/HPS lights

Quick Guide – Distance From MH/HPS Lights To Plants

Setting up an air system

To calculate the strength of the inline fan for cannabis, you will need to figure out the appropriate “CFM” (cubic feet per minute) rating for your grow area. You only need to multiply the cubic area of the space (length x width x height) by 2 if the exhaust path is efficient (no bends or turns, but a short, straight route), or by 3 for an inefficient one. Then use the resulting number to choose an appropriate fan that has that same number or higher.

You should also consider the intensity of the lights you are using. Make sure to do the research to determine the total the appropriate fan for the wattage of the light and the area of your grow space. When in doubt, use a larger exhaust fan than you think you need.

The best type of fan for indoor growing areas is an internal circulation fan to keep a constant flow of air around the area. This will help the temperature remain stable and will bring in fresh air with CO2. Fans placed beneath the lights are perfect.


The strength of your exhaust system may also depend on the type of lights you are using. If you are using a 400W HPS grow lamp, for instance, the temperature of the room could be raised by as much as 15 to 30 degrees Fahrenheit.

Now that you know exactly how to get started… Have a happy growing!