Simple Guide To Indoor Grow Light Spectrum & Color Temperature

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Choosing the correct light spectrum is one of the 3 main basics of plant lighting. Sunlight and various other types of light that we see is actually made up of several different colors. For indoor gardens, it’s important to provide the proper spectrum during the right stage for best growth.

Don’t worry, I’m all about hydroponic gardening made EASY, so we will keep things simple.

The Light Spectrum Is For the Pigments

Plants use energy from light for the process of photosynthesis. In order to absorb light, plants contain pigments, molecules that absorb specific wavelengths of light and reflect all others. That reflected light is the color that you see.

There are several pigments involved in photosynthesis. Chlorophyll (a)  and carotenoids are the main ones so we will just discuss those to keep it simple.

Chlorophyll (a)

Chlorophyll a is the most abundant pigment found in plants. They do not take in very much green light, reflecting it away from the plant. This is why plants appear green.

Chlorophyll (a) plays an important role in photosynthesis, allowing plants to absorb light energy to be turned into plant energy.

Carotenoid

Carotenoids are the second most prominent pigment found in plants. They are responsible for the yellow, orange or red colors we see in plants and fruit. It is what we see in the leaves as they change colors in the fall, as the chlorophyll is less abundant.

They assist in the absorption of light, then transfer the energy to the chlorophyll. They also protect chlorophyll from photodamage.

Choosing The Right LED Grow Light Color

LED’s are considered efficient for several reasons. One of which is that they can be customized to provide the exact colors that plants need leaving out the colors they don’t. LED grow lights produce little heat making them unnoticeable on the electric bill. I think they are the best all around grow lights for hobby hydroponics.

LED grow lights are often full spectrum, meaning you don’t have to change the light as your plant enters the flowering and fruiting stages. For the best all-around growth, for all parts of a plants life cycle, your LED lighting should contain some:

  • Red LEDs (600-700 nm)
  • Blue LEDs(400-500 nm)
  • Some Far-Red LEDs (700-800 nm)
  • Some White LEDs

LED lights may also include:

  • Some Green LEDs (500-600 nm)
  • Some IR, UV LEDs


There are many LED grow lights are configured with the proper spectrum for all stages of growth, meaning you can use the same light for the whole grow. These colors mixed together is why many grow LED grow lights look purple.

Choosing The Right Color Temperature: Fluorescent & HID Grow Lights

Fluorescent and HID (high pressure sodium & metal halide) grow lights work a little differently than LED grow lights. These “traditional” grow lights do not emit the single colors that LED lights do. When looking at the light emitted by one of these grow lights, we usually see either a bright daylight or a soft yellowish hue.

Bulbs are measured by color temperature in the measurement of “Kelvin”. Below you can see the Kelvin scale.

At the lower end of the scale, the red spectrum of light is the soft hue. Red spectrum of light is needed for the flowering and fruiting stages of the plant’s life cycle. At the opposite end of the scale, you have more of the blue spectrum of light, the daylight hue. This is the light that is used for the vegetative stage.

What’s The Best Light For Seedlings & Clones?

Plants in the seedling stage need more light from the blue spectrum. For traditional lighting, you’re going to want daylight bulbs at the higher end of the Kelvin scale. You should use bulbs that are in the 5,000 Kelvin range and up for your seedlings.

Fluorescent grow lights work great for propagating seedlings and clones. Fluorescent grow lights don’t get very hot and the light is not too intense.


For HID grows, lights, metal halide bulbs can be used for propagation of seedlings and clones. but its’ not the ideal light source. HID lighting produces a lot of heat and doesn’t produce uniform light across many small plants.

What’s The Best Light Vegetative Growth?

Plants in the vegetative stage need blue light, at the higher end of the Kelvin scale. The same lighting that works best for seedlings and clones also works well for vegetative growth, it should just be more intense than it was for the seedlings. Fluorescent lighting isn’t as intense as metal halide, so I would recommend fluorescence for plants that have lower light requirements such as leafy greens and some herbs.
For vegetative growth, you should use bulbs that are in the 5,000-7,500 Kelvin range.

What’s The Best Light  For Flowering and Fruiting Stages?

As your plant enters into the flowing and fruiting stages of its life cycle it will need more of the red spectrum of light at the lower end of the scale. This light is low energy, promoting flowering, blooming and fruiting.

‘You should use more bulbs that are in the 2,000-3,000 Kelvin range for plants in the flowering/fruiting vegetative stages. If you’re using HID lighting, then you will need to switch to high-pressure sodium bulbs as those produce more red light.

Conclusion

Choosing the right light spectrum and color temperature for your plants is important. Light needs will change for the phase of growth your plants are in, so you may need to switch lights as they enter flowering, depending on the light your using. After you’ve decided on a light source, I hope this guide helps you in determining what bulbs to use and when.

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4 Answers

  1. Rhonda Williamson
    October 13, 2018 at 9:55 pm

    Your explanations are very concise and easy to understand, thank you.

    I was hoping to find a one-size-fits-all answer for what color light to use thru all stages. I don’t need seedling lights, so just blue for vegetation and red for flowering.

    I’ve read that white lights don’t provide a wide enough spectrum, but now you said that that there are other colors in white light that my plants need.

    Please help! Should I use red/blue OR white LEDs?

  2. Susan Davis
    November 29, 2018 at 4:25 am

    Thank you for your interesting site. I am brand new to all of this. I got a small African Violet plant last spring, and it produced many beautiful flowers. Then this fall I put clear plastic over its window ( to save on my heating bill), and all of the flowers died although the leaves still look very healthy. Now as winter deepens, I see that my violet isn’t getting much sun. I’m on a very tight budget and can’t afford to buy much new lighting. For the last few evenings, I’ve set my plant under a table lamp with approximately a 60 watt led bulb. Will this help my violet at all? It’s planted in soil. I hope to learn more about your method in the future. Thank you, Susan

    1. NoSoilSolutions
      November 30, 2018 at 10:27 pm

      Thanks for the feedback Susan! I’m happy you found my article helpful. LED light bulbs will work depending on the bulb. For vegetative growth, plants should be under bulbs with a color temperature between 5000K and 6500K.

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