Simple Guide To Indoor Grow Light Spectrum & Color Temperature

Simple Guide To Indoor Grow Light Spectrum

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

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) , chlorophyll (b) and carotenoids are the main pigments involved in photosynthesis in plants.

Choosing The Right LED Grow Light

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 less heat compared to HPS lighting making them almost unnoticeable on the electric bill. LED’s have become best all around grow lights for hobby hydroponics.

Most all LED grow lights full spectrum, meaning you can use the same light as your plant transitions into the flowering and fruiting stages. Some lights will have switches that add different spectrum of light when ready and some lights run all spectrums all the time. 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

These colors mixed together is why many grow LED grow lights look purple. It’s becoming more common now that LED grow lights includes higher proportion of green which makes the light appear more bright white. This white light makes it easier to inspect your plants and also makes for better pictures of your garden.

Below is my MarsHydro grow light which gives off a visible purple light and my Viparspectra grow light which gives off a visible white light.

bright white led grow light

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 bright daylight or a soft yellowish hue.

The bulbs of these grow lights are measured by color called “Kelvin”. Below you can see the Kelvin scale.

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. For vegetative growth, you should use bulbs that are in the 5,000-7,500 Kelvin range.

As your plant enters into the flowering and fruiting stages of its life cycle it will need more of the red spectrum of light at the lower end of the kelvin 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.


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 you’re using. After you’ve decided on a light source, I hope this guide helps you in determining what bulbs to use and when.

9 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.

  3. Liz
    October 5, 2019 at 5:15 pm

    I am new in the plant world and gonna try growing flowers from seed over the winter. I purchased LED 4000 lumen/5000k cct daylight. Will this work to grow strong healthy seedlings?

  4. Mot
    October 8, 2019 at 8:38 pm

    I have LED grow lights with 3 colors that each have an on off switch. My question is, for flowering plants should I use just red or red and white or red, white and blue?

  5. Jeff C in Louisiana
    April 30, 2021 at 10:32 am

    Thank you for the tip about tearing off a bit of the pod to cover the seeds. My intuition was that dropping seeds into the holes was suboptimal. I used an opened paper clip to tear new smaller holes and poked my seeds into those.

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