Guides

Each guide takes one idea in depth, in plain language.

Light is food

Plants make their own food from light. In photosynthesis, pigments called chlorophylls capture light energy and use it to turn water and carbon dioxide into sugars and oxygen. Everything a plant builds, from roots to fruit, is made from those sugars. The more usable light a plant receives, up to the point it can use, the more it can grow.

Not all light is equally useful. Plants use mostly the part of the spectrum between about 400 and 700 nanometers, which scientists call photosynthetically active radiation (PAR). That range happens to be the colors we see: violet and blue at one end, green and yellow in the middle, orange and red at the other.

Four things about light that matter

1. Color (spectrum)

Chlorophyll absorbs blue and red light most strongly, so those colors are the most utilized by plants. Green, yellow and far-red light also play real roles. Read why red light and blue light are important for plants.

2. Intensity (PPFD)

Intensity is how many light particles, called photons, land on a patch of leaf each second. The measurement is PPFD, in micromoles per square meter per second. Read what PPFD is and how to use it.

3. Duration (photoperiod)

Duration is how many hours a day the light is on. It affects total growth and, for some plants, flowering. Read how many hours of light plants need.

4. The daily total (DLI)

Intensity multiplied by duration gives the daily light integral, the total amount of light a plant receives in a day. It is the single most useful number for matching light to a crop. Read daily light integral explained.

Light is also a signal

Plants do more than burn light for fuel; they read it. Specialized photoreceptors sense color, intensity and timing and tell the plant when to sprout, which way to grow, whether it is shaded and when to flower. Blue light photoreceptors help a plant grow toward the light and control the pores on its leaves. Phytochromes, which respond to red and far-red light, help a plant sense neighboring plants and the length of the night.

This is why a grow light is more than a lamp. Two lights with the same brightness can produce tall, thin plants or short, sturdy ones, depending on their color and how long they run.

Putting it together

  • Choose a full-spectrum light so plants get every color they use. See full spectrum LED grow lights.
  • Pick an intensity to suit the crop and the stage.
  • Set a photoperiod with a timer.
  • Check the daily total is in a sensible range for what you are growing.
  • Watch the plants and adjust.

My LED Grow Lights use full-spectrum white Samsung LM301b/h LEDs and include a dimmer switch, so you can control intensity as your plants change. See all of our LED grow lights.

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