This old song seemed like the perfect title for talking about artificial lights for houseplants. Even if there are windows in a room, the amount of light is very low. Fortunately, there are relatively cheap LED options available. Light is made up of the different colors, or wavelengths, of the visible spectrum – Red, Orange, Yellow, Green, Blue, Indigo, Violet. A plant’s pigments capture light energy and use it to make food. Chlorophyll production peaks in the red and blue spectrums, but accessory pigments help plants capture additional wavelengths of light. As such, there are optimal wavelengths of light that will better support plant growth. This is known as Photosynthetically Active Radiation, or PAR. We measure PAR by using the Photosynthetic Photon Flux Density, or PPFD. This measures the amount of photons of light in the correct range hitting a specific area, and is measured in micromoles per square meter per second (µmol/m2/s). A PAR meter is an expensive way to measure how much usable light is available for your plants.
LED specialty lamps that have a good spectral output (PAR) are easily acquired. I would caution the buyer to read the reviews to help judge the quality. Good lights will generally include some description of PAR and/or PPFD. Light quickly diminishes with distance, as it essentially becomes less concentrated. Look in the product information for how close the plant should be to the light for maximum effectiveness. Generally, keep the plant 12-18 inches away from the light source. If it is too close, or the light is too bright, plants can be burned, drop leaves, or become spotted. A cheaper option is to use existing light fixtures and replace the bulbs with either full spectrum or plant grow bulbs. The ideal plant grow bulbs look pink, as they have red and blue LEDs. Full spectrum bulbs tend to have a better mix of wavelengths to make the light appear more natural to us, but still have good PAR. CRI, or Color Rendering Index is a measure of a light’s ability to accurately show colors compared to natural light. The closer to 100, the better the light’s appearance.
We can also judge a bulb by “color temperature” reported with a Kelvin number. For example, a 2700K bulb is commonly referred to as a warm white. This is reminiscent of the light given off by a traditional incandescent bulb, and shows heavier output in the red end of the spectrum. A 6000K bulb would be called a “daylight” bulb, and has a heavier output in the blue end of the spectrum.
Many people confuse quality of light with brightness, which is measured in lumens. Light can be very bright, but of poor quality. Too bright, and it may burn the plants. Also, buying a light that throws out a lot of green light does not do much, even if it has a high output (lumens). Lastly, Watts are used as a measure of energy consumption. While it is true that higher wattage may mean a greater output of light energy, the efficiency of LEDs means that a lot less watts are needed to put out the same number of lumens as compared to an incandescent bulb.
So, then, this begs the question: What light is best for growing plants? To answer, I am going to assume that the plants are going to be displayed in the living area and viewed by people regularly. In this case, I would try and find a 5000K bulb with a CRI close 97 or 98, and a PPFD rating that matched the requirement of my plants for the given distance from the light fixture.
The following is a quick list of what is useful and what is not useful as a metric for plant growth:
PAR - Photosynthetically Active Radiation. It is the ideal measurement for plant growth.
PPFD - Photosynthetic Photon Flux Density. It is a useful metric to judge output at a particular distance from the fixture.
Kelvin - Kelvin refers to “color temperature” – lower number more red (ex. 2700), higher number more blue (ex 6500). It is not detailed enough as a metric for plant growth.
CRI - Color Rendering Index. It is useful for how true-to-life the plants will look.
Lumens - A unit to measure brightness. It is not particularly useful as a metric for plant growth.
Watts - How much energy is used to run the bulb/fixture. It’s not useful as a metric for plant growth, but lower wattage costs less to run equipment.
Warm vs Cool - Warm indicates the red end of the spectrum while cool indicates the blue end of the spectrum. See Kelvin. It is not detailed enough as a metric for plant growth.
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