The Effect of Incandescent Light on Plant Growth

Overview

Plants need light to produce their food. In a process called photosynthesis, energy from light rearranges carbon dioxide and water to form sugar molecules that plants use for energy. A waste by-product of the reaction, oxygen, is also produced, which proves very useful for animals. Not all sources of light work the same way in flora, however. Incandescent bulbs have both positive and negative effects on plant growth.

Visible Light Spectrum

Outdoors, the sun provides plants with the full spectrum of electromagnetic radiation, a portion of which is described as light. Of this light, a smaller band is "visible light." This is the part of the spectrum that the human eye can perceive. These wavelengths range from around 380 to 760 nanometers, with different lengths being seen as different colors. Plants use a subset of the visible light spectrum for their needs near the shortest and longest edges of the band, that is, at the blue and red wavelengths, respectively.

Incandescent Bulb Light Wavelengths

Incandescent bulbs do not provide even color across the visible light spectrum. In fact, their output is skewed more toward the longer wavelengths. Incandescent bulbs are not a good source of blue light, but they do produce a fair amount of the warmer colors orange and red.

Plant Reaction to Incandescent Light

Plants derive most of the energy for their needs from the blue and red parts of the visible light electromagnetic wavelengths. The fact that they do not absorb green light well at all accounts for their green appearance, as that wavelength is reflected back to the human eye. Energy from blue light goes to building most plant structures such as leaves and the stem. Red light triggers hormonal changes in plants to induce flower and fruit production. Many plants need a certain number of hours of red light exposure before blooming. Incandescent light, therefore, is good to help plants achieve flowering. Note, however, that most plants can't survive if they're in light 24 hours a day. They need darkness to stop putting all their energies into making food and instead use the stored energy to carry out other plant functions. This light-dark ratio is called the photoperiod. Different plants have different photoperiodic requirements.

Plant Reaction to Incandescent Heat

Incandescent bulbs by definition require a filament to burn brightly and produce light. Not surprisingly, this results in a lot of waste heat output. Some plants grow better at higher temperatures, and some don't, but all will wilt or burn if placed too close to an incandescent light source. Place incandescent bulbs at least a foot away from plants and watch the leaves for telltale signs of damage.

Augmenting Incandescent Light for Optimal Plant Growth

Incandescent bulbs do not provide sufficient amounts of blue lights for most plants to thrive under them exclusively. Good sources of blue light include the sun, grow lights and cool-white fluorescent bulbs. The University of Missouri recommends an incandescent to cool-white bulb ratio of about three to 10 for good quality growing light indoors.