Light is important for plant growth. Without it, there is no photosynthesis, and plants cannot grow and thrive. Light is often the most limiting growth factor when growing plants indoors. Because of this, many indoor plants benefit from supplemental lighting.
Many different types of fixtures are available to provide supplemental light for houseplants, seedlings, or other indoor plants.
Factors to Consider | Incandescent | Fluorescent | LED | HID | Quick Comparison Chart | Key Terms | More Information
Factors to Consider When Selecting a Light Source
There are many options for supplemental light fixtures. Which fixture you select depends on several factors, as each light source has advantages and disadvantages. Factors to consider include:
- Wavelength of Light Emitted
- Intensity (Amount) of Light Produced
- Cost (don't forget to consider the cost of the fixture and the bulb, if applicable)
- Efficiency
- Amount of Heat Produced (hot fixtures can damage plants if they are too close)
- Expected Life Span of the Fixture or the Bulb
- Size of the Fixture (large fixtures block more natural light)
- Cost to Install
Compare each of these factors to select the best light source for your indoor plants.
Incandescent
Incandescent light bulbs are not good sources of supplemental light for plants. They produce abundant wavelengths in the red end of the visible spectrum and considerably less in the blue end. Plants need wavelengths from both ends of the spectrum to grow well. They have low efficiency, operate at high temperatures, have a short life span, and tend to have a low output (intensity).
Fluorescent
For many years, fluorescent lights have been the go-to supplemental light source for home gardeners. They operate more efficiently and produce more light than incandescent bulbs. Fluorescent fixtures come in many sizes that can cover different size growing areas. With the exception of compact fluorescent light bulbs (CFLs), they produce much less heat and, therefore, can be placed much closer to plants without causing damage. This is beneficial because the amount of light available for the plant to use (PPFD) drops off sharply as you get further away from the fixture. For this reason, plants should be positioned within 6 to 12 inches of a fluorescent light source. Bulbs should also be replaced regularly (every one to two years) as their intensity decreases sharply over time.
The wavelength of light produced by fluorescent bulbs tends to produce more wavelengths in the blue end of the visible spectrum. This abundant blue light makes them well suited for foliage growth and seed starting but harder to use to promote flowering and fruiting, which relies heavily on redder wavelengths.
When using fluorescent fixtures for supplemental light, look for grow lights (or aquarium lights), as they typically produce more PAR than traditional bulbs. If these bulbs cannot be used, look for high-output (HO) full-spectrum bulbs, which tend to have a wider array of wavelengths. Another option is to mix warm-colored bulbs (which produce more red wavelengths) with cool-colored bulbs (which produce more blue wavelengths) to get a wider range of wavelengths.
Fluorescent bulbs or tubes come in various sizes, from 12 inches to 8 feet in length. Four-foot tubes are the most common. Tubes have different diameters. T8 and T12 are common but are being phased out of use. T5 bulbs are smaller in diameter and are preferred over the other sizes as they are more efficient. However, they cannot be used in the same fixture as a T8 or T12 bulb.
How to Use Fluorescent Supplemental Light
For the home gardener, a standard fluorescent shop fixture containing two 40-watt tubes positioned 6 to 12 inches from the foliage provides sufficient supplemental light for those indoor plants that need medium to high light levels. For best results, use high-output, full-spectrum (sometimes called “daylight”), T5 bulbs. Alternatively, you can use high-output bulbs and place one cool white and one warm white tube in each fixture. Fluorescent grow bulb will provide a better mix of usable wavelengths, allowing for even better results.
LED
In recent years, LED (light-emitting diode) fixtures have become a much more prominent and affordable supplemental light source option for indoor plants. They have become the new go-to light source for many home gardeners. LED lights are efficient, producing more light with less energy. They operate with less heat than fluorescent fixtures and tend to be long-lasting. LED lights have a high output and do not fade in intensity over time, like fluorescent bulbs, making it possible to set the fixtures further away from plants. Most LED lights can be positioned 12 to 24 inches from the foliage.
LED lights sometimes provide a narrow range of wavelengths, which can reduce their ability to provide adequate PPF (wavelengths in the red and blue ends of the light spectrum). Selecting full spectrum fixtures or mixing cool and warm colored fixtures can help increase the PPF. LED lights manufactured specifically as grow lights often emit only the wavelengths beneficial for plants. This high percentage of reds and blues with very few yellows and greens gives LED grow lights (and everything growing under them) a pink or purplish color.
LED fixtures often prominently indicate their light output in lumens. This unit of measure still does not clearly indicate how well plants will grow under them. Lumens measure the amount of light perceived by the eye rather than the amount beneficial for plant growth (PPF). It is still a better indication of intensity than watts, which is how most incandescent and fluorescent fixtures denote light output. LED fixtures manufactured specifically as grow lights often indicate the PPF.
How to Use LED Supplemental Light
For the home gardener, a standard, four-foot LED shop fixture positioned 12 to 24 inches from the foliage provides sufficient supplemental light for those indoor plants that need medium to high light levels. For best results, use full-spectrum fixtures with a high output (at least 3,0000 lumens). LED grow lights will provide a better mix of usable wavelengths and a good amount of light, allowing for even better results.
HID
High-Intensity Discharge (HID) fixtures are commonly used in commercial greenhouses and less frequently used in the home. These fixtures have a much higher output than other light fixtures and provide more light in the wavelengths that benefit plant growth (PAR). This makes them ideal for flowering and fruiting plants that require high and very high light levels. Their high light output means they can be positioned farther away from the plants than other fixture types and still provide adequate DLI. This is good since these fixtures are large (making them more likely to block any natural light) and they produce a lot of heat (which can damage plants if placed too close). While HID fixtures use more energy than fluorescent or incandescent, they are more efficient when you balance the PAR produced with the energy used.
Two major types of HID fixtures are used for supplemental light: metal halide and high-pressure sodium. Metal halide lamps are bluer in color, which makes them well suited for vegetative growth, but not as good for promoting flowering and fruiting as high-pressure sodium bulbs, which are redder in color. Plants grown exclusively under high-pressure sodium bulbs can get leggy.
Commercial-grade LED fixtures are quickly replacing HID fixtures as they can produce light more efficiently with wavelengths and outputs equivalent to HID lights. That, along with their high cost to purchase, maintain, and operate, is why home gardeners do not use HID light fixtures extensively.
Supplemental Light Source Quick Comparison Chart
Use this guide to determine the best light source for you based on several factors. In general, the more "+" in the column, the better it is as a supplemental light source for a home gardener.
| LED | LED Grow Light | Fluorescent | Fluorescent Grow Light | HID (Metal Halide) | HID (High-Pressure Sodium) | Incandescent | ||
|---|---|---|---|---|---|---|---|---|
| Intensity | +++ | ++++ | ++ | ++ | ++++ | ++++ | + | +=low ++++=high |
| Wavelength (PAR) | ++ | ++++ | ++ | +++ | ++++ | ++++ | + | +=low ++++=high |
| Wavelength (Blue) | ++ | +++ | +++ | +++ | ++++ | +++ | + | +=low ++++=high |
| Wavelength (Red) | ++ | +++ | + | ++ | +++ | ++++ | +++ | +=low ++++=high |
| Cost to Purchase | ++++ | ++ | ++++ | +++ | + | + | ++++ | +=expensive ++++=inexpensive |
| Cost to Operate | ++++ | ++++ | +++ | +++ | + | + | +++ | +=expensive ++++=inexpensive |
| Efficiency (energy converted to light) | ++++ | ++++ | +++ | +++ | +++ | +++ | + | +=low efficiency ++++=high efficiency |
| Cool Operating Temperature | ++++ | +++ | +++ | +++ | + | + | ++ | +=high temp ++++=low temp |
| Distance from Plants | +++ | ++++ | ++ | ++ | ++++ | ++++ | + | += close ++++=far |
| Lifespan of Light Source/Bulb | ++++ | ++++ | + | + | +++ | +++ | ++ | +=short ++++=long |
| Small Fixture Size | ++++ | +++ | +++ | +++ | + | + | ++++ | +=large ++++=small |
| Special Wiring Required | ++++ | ++++ | ++++ | ++++ | + | + | ++++ | +=yes ++++=no |
Key Terms & Ideas
- Daily Light Integral (DLI) - the amount of PAR delivered over an area in a 24 hour period of time (mol/m2/day). DLI basically tells you how much photosynthesis can occur.
- Footcandle - A unit of measurement that indicates light intensity defined as the number of lumens over one square foot. (1 fc = 10.76 lux)
- Intensity - Refers to the amount of light produced or emitted. Intensity is measured in several different ways but the most useful for growing plants is PPF and PPFD.
- Kelvin - A unit of measurement that indicates the approximate color of the light source. Lower numbers are redder in color and higher numbers are bluer.
- Lumens - a measure of the amount of light given off as perceived by the human eye.
- Lux - A unit of measurement that indicates light intensity defined as the number of lumens over one square meter. (lux = lumens/square meter)
- PAR (Photosynthetically Active Radiation) - The specific range of light wavelengths plants use to photosynthesize.
- Photoperiod - The physiological response of a plant (such as flowering) to the length of the night or a dark period.
- Photosynthetic Photon Flux (PPF) - The amount of PAR being provided by the light source. (Essentially, the amount of light being provided that is useful to the plant.) Measured in micromoles per second (µmol/s).
- Photosynthetic Photon Flux Density (PPFD) The amount of PAR being provided over a surface area. (Essentially, the amount of light useful to plants that actually reaches the leaves.) PPFD is measured in micromoles per square meter per second (µmol/m2/s).
- Watts - The amount of energy used by the fixture.
- Wavelengths Used By Plants - Plants use a specific range of wavelengths in the light spectrum to photosynthesize (referred to as PAR: Photosynthetically Active Radiation). Plants primarily use red and blue wavelengths in the visible light spectrum.