Light intensity
Red light is ideal for flowering and fruit set.
Light intensity is the brightness of light. The amount of light produced by a bulb is measured in a variety of ways and, unfortunately, two different bulbs may report their light output using different measurements, making it hard to compare. The distance between a light source and a plant impacts the light intensity.
While there are many ways to measure light, a few common measurements you are likely to see include:
- PPF (photosynthetic photon flux) is a measure of how much plant-usable light is released by a bulb per second and is measured in micromoles of light per meter per second (umol m-2s-1). You may also see PPFD (photosynthetic photon flux density), which is a measure of PPF as it reaches a surface like a plant leaf. PPFD goes down as your plants get further away from the light source.
- Foot-candle is the amount of light received by a 1-square-foot surface located one foot away from a light source equal to one candle. It’s not used as frequently though you may find this measure in older reference books.
- Lumens are less relevant when considering lighting for plants. Lumens measure how bright the light is to the human eye, and do not measure some of the important wavelengths that plants need to grow.
- Watts are a measure of the amount of energy needed to produce light, rather than a measure of the actual intensity of the light. Light bulbs should report both watts and another measure of light intensity such as PPF, lumens or foot candles. A more efficient lightbulb will produce more light with fewer watts of energy.
Distance from light source
Keeping sufficient distance between plants and a light source is especially important when using bulbs that produce a lot of heat, like incandescent and high-pressure sodium. But even with LED and fluorescent lights, maintaining a proper distance helps to ensure healthy plant growth.
- Seedlings: 4-6 inches (move your light up regularly as they grow)
- Hydroponic lettuce and herbs: 6-12 inches
- Foliage houseplants: 12-24 inches
- Flowering houseplants: 6-12 inches
Light quality
Light quality refers to the wavelength or color of light. The light spectrum is composed of red, orange, yellow, green, blue, indigo and violet light. Sunlight provides all colors of light.
The part of the light spectrum that plants use is called Photosynthetically Active Radiation, which is composed of primarily red and blue light.
As lighting technologies have become more efficient, grow lights that only emit light from the red and blue wavelengths of the light spectrum have become more common.
Check the packaging to see what type of light is emitted by a grow light before buying it; grow lights tend to be labeled as blue, red, or white/balanced light.
- Blue light or mixed light bulbs are suitable for starting seeds and leafy greens, as well as non-flowering house plants.
- Red light or mixed light bulbs are suitable for promoting bud formation in flowering plants as well as keeping the plants shorter.
- White lights or mixed/balanced light bulbs are suitable for most plants at any stage of growth.
Light duration
Light duration (photoperiod) is the number of hours of light a plant needs per 24-hour period. Plants are classified by photoperiod into three categories for flowering response: short day, long day, or day-neutral.
- Short day indoor plants, such as chrysanthemum, Thanksgiving and Christmas cacti and poinsettia, require short days to flower. You cannot reflower them indoors unless they are grown in short days.
- Long day plants, such as African violets, gloxinia and tuberous begonias, flower when the daylight exceeds the hours of the night period.
- Day-neutral plants are insensitive to day length differences for flowering and include indoor plants such as flowering maple (Abutilon), Crossandra, and gerbera daisies.
Use a timer to provide supplemental light if growing in a location with less natural light. Set your timer so that plants receive the following total light hours.
- Seedlings: 16-18 hours per day
- Hydroponic lettuce and herbs: 12-14 hours per day
- Foliage houseplants: 12-14 hours
- Flowering houseplants: 14-16 hours