Effects of Different Color Lights on Plant Growth

Humans and plants are generally sensitive to the same general part of the total light spectrum. This means that the same band within the ultraviolet to infrared range that we are able to perceive visibly also induces the photosynthetic response. However, plants and eyes have different levels of sensitivity within that band. Indoor plants will thrive best under specific colors.

Colors Plants Use the Most

Whereas the human eye is the most sensitive to yellow-green wavelengths, plants most strongly reflect green light, which is why we perceive them as being green in color. The colors most absorbed in photosynthesis are blue and red. Blue light promotes leafy growth, and red light enhances flowering and fruiting. As much as plants need light to create energy for themselves, they also need darkness. Light inhibits hormones that induce growth and sometimes bloom, so most growth takes place at "night," using the stored energy garnered from the day's photosynthesis.

Incandescent Light

By definition, incandescent bulbs use heat to create light. Both the old standard bulbs, with or without the white coating, as well as halogen lamps, produce light more in the red part of the spectrum. The heat can both help or harm plants, depending on their individual preferences.

Fluorescent Light

Basic bathroom "cool-white" bulbs are a good source of blue light for plants. Lights classified as "warmer" and giving off light with a distinctive pink cast exude more red light. Fluorescent "grow lights" look dark purple, incorporating both the red and blue parts of the spectrum for plants. Fluorescent lights are much more energy-efficient than incandescents, but most require a ballast in order to work, which adds initial expense.

HID Lights

High Intensity Discharge (HID) lights provide a lot of light in a small space. Several different gasses make up the phosphorescence, which affect their color. Mercury vapor produces a blue-green light, along with a lot of ultraviolet, which can cause "sunburn" in humans if it's not shielded well. Sodium vapor tends to be yellow, with a little red and not much blue. Metal halide and xenon short-arc lamps produce the whitest light, encompassing more of the whole spectrum.

Signs of Light Deficiency

In general, plants need more blue light than red. Intensity is also important; try to place plants 6-12 inches from their light source to mimic the outdoors in optimal light conditions. Different plants have different light tolerances, however; watch them for signs of how they're faring. "Leggy" plants, with long stretches of stem between leaves, aren't getting enough light, especially in the blue spectrum. Poor blooms indicate a need for more red. Faded and brown leaves, assuming that water is not the problem, are a sign that the plants are getting scorched with too much of a good thing.

Keywords: lightwaves and plants, plants and light, plant light response