All About Marijuana Indoor Grow Lights - International Highlife

All About Marijuana Indoor Grow Lights

The grow lights are the most important part of your plants’ life. Apart from being its only energy source, light intake dictates how quickly a plant grows and flowers, while photosynthesis, the process of creating “food” out of pure light, is one of the most magical natural processes. Like all plants, marijuana thrives under intense sunlight. Nowadays, however, even those with no direct access to the outdoors can use artificial grow lights to provide their plants with all the energy they require.

Apart from sunlight, there are three major kinds of artificial indoor grow lights for indoor grows: HID lamps (HPS and MH), fluorescent lights and LED grow light units. Each one has its own pros and cons. But before we go further with all the technical details, let’s take a look at how light works for marijuana plants.

The Light Spectrum

Light exists in a spectrum. This spectrum is measured in wavelengths that are more accurately specified by nanometres (one billionth of a meter). We know for sure that the human eye cannot perceive all light: The visible electromagnetic spectrum ranges from around 400 to 700 nm (380nm is ultraviolet light, and 750 is infrared). In fact, the visible light spectrum is only a tiny fraction of the light that actually exists all around us, but we can’t perceive. A rainbow is a perfect visualization of the visible light spectrum!

Similar to us, marijuana plants use the spectrum from 400 to 750 nm for their growing needs. This is called the PAR (Photosynthetically Active Radiation) and covers the spectral range of the sun’s light. If you run across grow lights that market as “full spectrum” they mean that they use the “full solar light spectrum” which covers all the needs of marijuana growing. As seasons change, so does the fluctuation between blue (lower nm) and red (higher nm) light. Chlorophyll, an abundant pigment in plants uses these spectrums for photosynthesis.

Bands of visible light

  • 380nm – 430nm Violet
  • 430nm – 500nm Blue
  • 500nm – 520nm Cyan
  • 520nm – 560nm Green
  • 560nm – 590nm Yellow
  • 590nm – 625nm Orange
  • 625nm – 780nm Red

When plants are in the vegetative phase, they mostly use light from the blue end of the spectrum (spring). As flowering approaches, they tend to utilize more light from the red and orange end. Light from the visible spectrum will cover the production of chlorophyll A and B, which are the cornerstone for the development of any plant.

The colors of light and Kelvin (K)

The Kelvin scale is an absolute thermodynamic temperature scale in which we measure light temperature (the color of light on a black background). Different light sources fit the Kelvin scale at different intervals. The correlation between nanometers and Kelvin is not absolute, but generally the “warmer” the light, the higher it is on the nm scale and the lower it is on the Kelvin scale.

Temperature refers to the warmth or coolness of the light source and not the actual temperature of the light itself. In simpler terms, it is the average degree of warmth or coolness of a light source, not with regards to the physical temperature, but rather, to the visual temperature of the light. Depending on the grow light you want to use, you will see a different average. Of course, the laws of thermodynamics are much more complicated, but we need to keep this about marijuana growing!

Marijuana Grow Lights

A bit of science stuff was necessary to move on to the juicy part: How does all this relate to marijuana growing? For starters, different lamps produce different kinds of light depending on their make.

As we’ve seen in previous articles there are three major categories of indoor grow lights:

  • Fluorescent Grow Lights
      • Compact Fluorescent Lights (CFLs)
      • T5 grow lights (and other fluorescent tubes)
  • High-Intensity Discharge (HID) Grow Lights
      • Metal Halide (MH)
      • High-Pressure Sodium (HPS)
  • LED Grow Lights

Metal Halide lamps are better for the vegetative stage because they produce a higher percentage of blue light, whereas High-Pressure Sodium lamps can help your plants thrive in the flowering stage. LED grow lights are more versatile but not equally intense (in most cases) A good strategy is to use both of them interchangeably or get a full spectrum grow light. That does not mean that you can not grow a plant from start to finish using HPS or MH exclusively, but the difference is quite noticeable.

Fluorescent lights are used more as supplementary lighting during vegetation, as they run cool and can be placed nearer to the plants. Their size and make can either be short and bulky or tube-like. Many people use them vertically, to supplement lighting in those hard to reach places.

A Practical Guide to Marijuana Indoor Grow Lights

When you are on the market for a grow light, you will most likely have to choose among the above options. Optimally, you need 250W of power per four plants or 600W for every 5sq.ft. (1,5sq.m). Remember that the more lights you use, the more heat will be generated. So never forget to update your ventilation and temperature control along with your light!

High-Intensity Discharge Lights (HPS, MH)

High-Intensity Discharge Lights

These bulbs are cheap and powerful. However, they run very, very hot (which can put your garden in danger). Also, they tend to use up significantly more energy than your other choices. As we saw earlier, MH lamps are better for the early phase of your plant’s life, while HPS produce more red-orange light. If you go the HID way, it is best to combine MH and HPS for best results.

Before LED grow lights were a thing, HIDs were the primary source of lighting in indoor grows. Any grower who wants to start a quick and efficient indoor garden should seriously consider these options. However, even though the quality of newer models is consistently improving there are some drawbacks you need to keep in mind:

Cons of HID Grow Lights

  • HID lights are not very efficient, meaning they both run hot and spend a lot of energy. If you are considering getting HIDs, look at better cooling options for your grow room.
  • Due to the heat emitted by those lights, you should hang these units away from your plants. The optimal distance is 3ft (1m) for 250W fixtures, 4ft (1,2m) for 400W and so on. These grow lights are not the best choice for confined spaces.
  • As HIDs run hotter, they get burned more easily. An HID lamp will need to be replaced at least every year.
  • The spectrum of this light is limited, meaning you will have to use many of them to get decent coverage.

Full Spectrum LED Grow Lights

Full Spectrum LED Grow Lights

LED lights are omnipresent nowadays, but they have a long and complicated history. First introduced in 1962, they are the most energy efficient light type in the earth, as they create light using 60% less power than HID fixtures. Earlier LED grow lights were weak and unstable, but in the last few years, we have witnessed a massive improvement in power, intensity, and efficiency of LED grow light fixtures.

Although earlier models only ran at specific wavelengths, modern LEDs cover the full spectrum of usable light, with companies always striving to improve absorption by the marijuana plants. The technology behind LED lights is such, that allows for a much more intense light, using less energy. As a result, LED lights run cooler, last longer (about 5-10 years) and consume significantly less energy than HID units.

However, there are some drawbacks to keep in mind. Some LED fixtures (especially cheaper models) perform below par compared to their higher-quality counterparts. As a result, getting a good grow light takes some due diligence on your behalf.

The main points you should check when you are buying LED grow lights are:

  • The spectrum: Make sure that the light covers the whole usable light spectrum.
  • Check the wattage carefully. Just because the manufacturers market a grow light as a 600W model, it doesn’t mean it will perform as such. Usually grow lights work in 50-60% of their full power. Be sure that you ask for the true watts of a LED unit.
  • Check the warranty and technical support for your grow light. Contrary to other light types, LEDs are a significant investment, and serious companies know that.
  • Heat management: Does the unit cool itself with fans or heatsinks? Heat sinks are better because they remove the need for loud (and easy to break) fans.
  • The devil is in the details: Be sure to ask about the actual wattage, type of chips, recommended coverage and height of light compared to plants.

Fluorescent Lights

Fluorescent Lights

The weakest light of the bunch, fluorescent lamps come in two shapes: CFL (short, stocky) and T5 (long, lean). They are incredibly cheap, easy to get and very efficient, but on the downside, they are not at all intense. You will need to keep them close to the plants, and they should probably only be used for seedlings or as supplemental lighting. That is not to say that you can’t grow marijuana plants using fluorescent lights, but not more than a couple. Fluorescent lights come in 2700K, 3000K, 4100K, 5100K, and 6500K models. Each of them suits best to different growth phases, as we saw earlier.

Pros of Fluorescent Lights

  • Easily found in grocery and hardware stores and very cheap to buy.
  • Cool running temperatures, meaning lower fire hazard and danger of burning your plants.
  • Perfect for more confined grow areas with just a couple of plants.

Cons of Fluorescent Lights

  • Not nearly as bright or intense as HID or LED fixtures
  • You’ll need to readjust them very often as your plants grow in height.


Good quality indoor grow lights are now more affordable than ever, but that doesn’t mean you should buy the first one that you come across on Amazon. Especially when it comes to LED grow lights, you should be wary against knockoffs. The most important thing to remember is to follow the “600W per 5 square feet” rule and remember that every grow light has its own quirks. For example, HIDs need frequent replacement while LEDs much less so.

It is possible to use just one kind of grow light from start to finish. However, a combination of many different lights will help your plants, depending on their development stage. Give your plants space to grow and get models with ballasts and hoods, if buying fluorescent or HID lights.

If you are an experienced grower, share your favorite lighting setup with us in the comments! We’d love to hear your stories!

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