Important Considerations for Providing Supplemental Light to Indoor Plants

Supplemental lights on plants — Supplemental light is beneficial for growing plants indoors. ⁽¹⁾

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 houseplants, starting seeds, or nurturing plants indoors. Because of this, many indoor plants benefit from supplemental lighting either during the winter or year-round.

Three important factors need to be considered when providing light for plants: Quality, Intensity, and Duration.

These three factors are used to determine the light requirement needed for best growth, called the daily light integral (DLI). This is one of the most important measurements for growing plants under lights.

Learn more about the important factors that need to be considered and how to calculate the DLI below.

Quality

Light quality refers to the wavelength of light produced by the supplemental light source.

Wavelengths of light used by plants — Wavelengths in the red and blue end of the light spectrum are most absorbed and used by plants for processes like photosynthesis.

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.

The wavelengths from these two ends of the spectrum influence different functions in the plant.
Blue wavelengths help regulate chlorophyll production and movement in the plant, control excessive stem elongation, and regulate stomata (where important gases are exchanged).
Red wavelengths are used extensively for photosynthesis, cause stretching and elongation, and promote more flowering and fruiting.

Good supplemental light sources provide light in both of these important wavelengths.

Measuring Light Quality

For the home gardener, determining the wavelengths of light in a supplemental light source can be difficult. This makes it challenging to ensure the red and blue wavelengths needed by plants (PAR) are present. While spectrometers can measure specific wavelengths, this light meter is not available or practical for most home gardeners.

Kelvin Color Temps by Zern Liew — The Kelvin rating can tell you your light source's approximate color (or dominant wavelengths) but not the precise wavelengths of light your plants will receive. ⁽²⁾

Light fixtures manufactured as grow lights will provide the necessary wavelengths for plant growth.

If a standard fixture is used, an approximation of the wavelengths emitted can be determined using units called "Kelvin." This measurement indicates an approximate color of the light source but does not specify the exact wavelength of light the plants will receive. Many light fixtures available at home improvement stores are sold with an indication of their color in Kelvin (K). Lower numbers are redder in color, and higher numbers are bluer.

The ideal fixture would be "full spectrum" (sometimes called "daylight") and typically have a Kelvin value in the middle, indicating it has some of both the red and blue wavelengths. Alternatively, "warm white" (redder, lower K value) and "cool white" (bluer, higher K value) fixtures or bulbs can be mixed to create a wider spectrum of light usable by plants.

Intensity

Succulents under lights — Because light intensity indoors is low, increasing it using supplemental light can benefit growth and development. ⁽³⁾

Light intensity refers to the amount of light produced by the supplemental light source. Indoor light intensity is frequently very low, especially in the winter. Because of this, increasing the light intensity is important for good growth and development, especially for plants that are grown for flowers or fruit. In many cases, the indoor gardener will be looking to provide as much light as possible.

Measuring Light Intensity

Light intensity is measured in several ways; some measurements are more accurate than others.

Watts
While one of the most familiar units of measure for light, watts are not a reliable indication of light intensity because they measure the amount of energy the fixture uses, which does not universally correlate to the amount of light being emitted.

Lumens & PPF
Many light fixtures are sold indicating the number of lumens they provide. Lumens are a measure of the amount of light you see. While a much more direct measure of light intensity than watts, it still does not indicate the amount of light available for the plant to use (called PAR). A fixture may provide a large amount of light as measured by lumens, but certain wavelengths, like yellow and green, are brighter to the eye and, therefore would increase the number of lumens provided by a fixture but not increase the PAR.

Photosynthetic Photon Flux (PPF) is a more accurate measurement for indoor plant supplemental light. PPF is the amount of light provided that is useful to the plant and is measured in micromoles per second (µmol/s). For grow lights, this value is typically provided by the manufacturer. For standard fixtures, this value is rarely indicated.

Lux vs PPFD Lumens vs PPF Lux & Footcandles
Light intensity is also influenced by the distance and angle the fixture is placed from the leaf surface. A fixture closer to a plant would provide a greater light intensity (over a smaller area) than one placed further away. Lux measures the number of lumens over an area. (lux = lumens/square meter) Footcandles (fc) may be a more familiar term and measures the same thing (1 fc = 10.76 lux).

The Best Measure of Light Intensity

Photosynthetic Photon Flux Density (PPFD)
Lux and footcandles are helpful, but because they are based on lumens, they provide information about the amount of light perceived by the human eye, not the amount of light in the wavelengths useful for plants (PAR). Photosynthetic Photon Flux Density (PPFD) is a more accurate measurement for indoor plant supplemental light. PPFD is the amount of PAR provided over a surface area and is measured in micromoles per square meter per second (µmol/m²/s). Essentially, it is the amount of light useful to plants that actually reaches the leaves. This makes PPFD one of the most useful units of measure used to determine the light intensity needs of an indoor plant.

Measuring the PPFD is beneficial but not always easy for the home gardener. Light meters that measure PPF (i.e. quantum flux meters) or PPFD can be purchased, but they can be expensive, and only the most serious indoor gardeners can justify their costs. Several smartphone apps are available to measure, lumens, PPF, and PPFD. These apps provide a level of accuracy adequate for the typical home gardener. Always measure light intensity at the foliage level for the most accurate reading. Regardless of the tool being used, be sure settings are adjusted or the meter is calibrated for the light source type being used (LED, fluorescent, etc.).

Methods to Measure PPFD

Consult the light source manufacturer for the PPF. Then use that value divided by the area lit in m²to determine the PPFD.
- PPFD = PPF / m²
Measure PPF with a quantum flux meter in several spots across the area. Average that number and divide it by the area lit in m²to determine the PPFD.
- PPFD = average PPF / m²
Measure with a light meter or smartphone app to get a value in lux or footcandles. Convert that value to PPFD using online tables or calculators.
Measure with a light meter or smartphone app that provides the PPFD. Be sure to adjust the light source settings on the device to get accurate values.

Duration

The duration of light refers to the amount of time indoor plants are exposed to light and dark periods. Generally, when growing plants indoors, it is best to supply light 12 to 14 hours a day. For home gardeners, the lights can be on for as little as 10 hours and no more than 16 hours a day.

Photoperiod

Some plants have certain growth and development processes (such as flowering ) that are cued by the duration of light and dark periods known as photoperiod. This is an important consideration when providing supplemental light.

Short-day plants require a certain period of light or less to respond (flower). Long-day plants require a certain period of light or more to respond.

In actuality, plants respond to the length of darkness. So short-day plants are more accurately referred to as "long-night" plants and require a certain amount of uninterrupted darkness or more to respond. Long-day plants are more accurately called "short-night" plants and require a certain amount of uninterrupted darkness or less.

Daily Light Integral

Seedlings Under Lights — Daily Light Integral (DLI) is one of the most important measurements for growing plants under lights. It basically tells you how much photosynthesis can occur. ⁽⁴⁾

The Daily Light Integral (DLI) is one of the most important measurements for growing plants under lights. It is the amount of light useful for plants (PAR) delivered over an area (PPFD) in a 24-hour period of time. Measured in moles per square meter per day (mol/m²/day), DLI basically tells you how much photosynthesis can occur. This is an important consideration, especially when growing something that produces flowers or fruit indoors (like citrus or tomatoes).

Calculating DLI

Calculate the DLI by multiplying the PPFD by the number of hours the supplemental light is on by a factor of 0.0036.

DLI = PPFD × # hrs × 0.0036

Online calculators can also be used to easily calculate the DLI provided by the supplemental light source.

What DLI is Needed for My Indoor Plants?

Tables and charts are available online to tell you the DLI required to grow different plant species successfully. In general, indoor plants can be divided into several groups.

DLI Needs of Indoor Plants

Category	DLI Value	Types of Plants
Low Light	3 to 6 mol/m²/day	Foliage houseplants
Medium Light	6 to 10 mol/m²/day	Foliage houseplants Flowering houseplants Vegetative cuttings Starting seedlings (on the high side of the range)
High Light	12 to 16 mol/m²/day	Flowering houseplants Succulents Starting seedlings (on the low side of the range) Herbs and vegetables grown for leaves (lettuce, basil, etc.)
Very High Light	18 to 30 mol/m²/day	Herbs Fruits Vegetables (both those grown for leaves, like lettuce, and those grown for fruit, like tomato)

Providing the Right DLI

Methods to change DLI If the DLI is not where it needs to be, you can adjust it in several ways.

Use a different light fixture that provides additional or fewer lumens/PPF
- Raising the light intensity (PPFD) by adding or swapping fixtures will increase the DLI.
- Lowering the light intensity (PPFD) by removing or swapping fixtures/bulbs will decrease the DLI.
Move the light fixture closer or further away from the plants
- Moving the fixture closer to the plant will increase the DLI.
- Moving light closer to the plant will reduce the square footage available for the plants to grow.
- If the fixture produces a lot of heat, moving the fixture closer can increase the temperatures around the plants, potentially causing damage.
Change the length of time the lights are on
- Increasing the number of hours will increase the DLI.
- This tactic cannot be used if plants are photoperiodic, as changing the number of hours could change the response a short-day or long-day plant would have.

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.

More Information

Photo credits: 1: Aaron Steil; 2: Zern Liew/AdobeStock; 3: Aaron Steil; 4: Aaron Steil

Authors:

Aaron Steil

Last reviewed:

December 2023