This is the third in our four-part series of articles about indoor lighting for plants. You can also read the first, second, and fourth articles.
Artificial light sources come in several forms, all relatively easy to acquire. Costs can vary wildly, and some are more electrically efficient than others. The variety of available options allows you to customize setups to your needs and the preferences of your plants. Before you dive into an overwhelming list of web search results, here are traits of the basic categories:
Light-Emitting Diode (LED)
- best energy-efficiency in terms of light produced per watt consumed (especially if the light has the ideal spectrum)
- coolest to the touch except for high-powered units, which usually have small built-in cooling fans
- can be expensive for high-quality fixtures, though costs are decreasing
- light output does not dim significantly over time, though diodes do have a finite lifespan
- reach full brightness immediately or very quickly when turned on
- diodes can either be exposed or under a frosted or textured cover to help diffuse the light
- diodes are directional, meaning they don’t emit light in every direction the way a fluorescent tube does, so reflectors aren’t usually needed
- more even light output from one edge of the fixture to the other
- can be round like a spotlight (with a cluster of diodes) or straight strips (or strips inside a tube) with one or more rows of diodes
- some replacement “tubes” can be used in place of fluorescent tubes in a fluorescent fixture, but you must check with the fixture’s manufacturer for compatibility as mixing components is a matter of electrical safety
- good energy-efficiency
- relatively cool to the touch, but high-intensity fixtures can run warm
- fairly affordable, though replacement tubes for high-intensity fixtures can be moderately expensive
- light output dims as the tube ages, so learn about the expected lifespan from the manufacturer and be prepared to replace them when needed
- can take a few minutes to reach full brightness after being turned on, especially in cooler temperatures
- bulbs can be Compact Fluorescent Lamps (CFLs, in coils or U-shaped) or straight tubes, with the latter being the most commonly-used shape for plant lights
- since light is emitted from the entire bulb’s surface, reflectors are helpful to bounce the light from the top of the bulb back down onto the plants
- light output is lower at the ends of the tubes, and the ends will become darker gray as the bulb ages
- bulbs usually operate in pairs, so if one burns-out, the other may not light until the burnt-out tube is replaced; smaller fixtures may be designed with a single tube
- linear tube size designations start with “T” (for tubular) and are numbered referring to the tube thickness (in eighths of an inch); sizes are rarely interchangeable because of incompatibility in how they insert or lock into place
- T12 bulbs are the oldest style and the thickest and are being phased out since the technology is decades old
- T8 bulbs are slimmer
- T5 bulbs are the slimmest and are available in a higher-wattage, brighter “high-output” (HO) option
- bulb lengths fit the standard sizes of 2’ or 4’ long fixtures
- poor energy-efficiency
- warm or hot to the touch and will damage plants touching the bulb
- often not manufactured with light spectrums ideal for plant growth
- being phased-out of general production due to energy inefficiency
High-Intensity Discharge (HID)
- made in several forms, including High-Pressure Sodium (HPS), which look orange-white, and Metal Halide (MH), which look blue-white
- intense output means that lights need to be mounted much higher above plants than the other fixture types; while a useful feature for tall, high-light plants, it’s harder to find a secure way to mount them indoors
- often impractical for home use due to high energy usage (even though their energy-efficiency isn’t bad), higher heat output, higher initial fixture cost, and weight of the components
Determining Light Intensity
Human eyesight adjusts so readily and quickly to changes in our environment that our eyes unfortunately aren’t a good judge for actual light intensity. Additionally, our eyes and plant pigments have different sensitivities to different wavelengths (colors) of light.
Light-metering devices can help reveal the true brightness of a space so you can determine the best locations for indoor plants. With artificial light, you cannot always judge relative brightness by energy usage (like wattage) alone. While special meters are not necessary for successful gardening under lights, they can help compare the outputs of different lights when the manufacturers don’t provide data. Whether you measure lights yourself or just read the specs included with a particular product, understanding the terms used in reporting light output will help you make the best selections for your situation and budget.
Understanding Terms: How is Light Measured?
When shopping for light fixtures, specific terms are used to describe their efficiency and performance, which helps you rate how they compare with other lamps. Not all lights will specify measurements useful in determining their suitability for growing plants, but they can still provide a baseline for comparison.
This is a very common rating, especially for light bulbs not necessarily intended for use with plants. Keep in mind the rhyme “lumens are for humans.” This means that lumens are a way of measuring light as it helps humans to see – the brightness we perceive. Because our eyesight is most sensitive to the yellow-green part of the spectrum of visible light, this is the main consideration for lumen ratings. It does not give much weight to the redder and bluer wavelengths that plants need most for photosynthesis. As an example, a yellowish light (like an incandescent bulb) can be fairly bright to us and have a high lumen rating, but have so little red-blue output that it will be useless for growing plants.
This widely-used unit of measurement is applicable in horticulture for measuring levels of natural daylight. Even though footcandle meters only detect light wavelengths important to human-perceived brightness, we know that the sun’s full spectrum suits plants, so readings of natural light correlate to relative brightness for them as well.
In contrast, since white-looking artificial lights can have different spectrums than daylight, footcandles aren’t always an accurate measure of plant suitability. The levels of yellow-green light produced by two lamps may be similar, but the red and blue levels may not. When artificial light closely mimics the sun’s spectrum, though, footcandle measurements can be an acceptable way of approximating intensity. As you might expect, footcandles and lumens are related: one footcandle is equal to one lumen per square foot.
“Bright indirect light” is a common yet somewhat nebulous recommendation for many houseplants. Knowing the ideal footcandle levels instead can be much more useful, even if only for gauging relative brightness levels for different plants. Unfortunately, this isn’t a widespread practice yet, though orchids are one plant group where footcandle ranges are regularly given for lighting recommendations. As an example, the American Orchid Society recommends a maximum brightness of 1,500 fc for Phalaenopsis and 5,000 fc for Oncidium. In nature, Phalaenopsis grow in fairly low light, attached to tree trunks in the forest undergrowth. Oncidium grows higher, on upper tree limbs, with less light blocked by canopy foliage.
- unobstructed, midday summer sunlight is about 10,000 fc
- a heavily-clouded day is around 100 fc
- a typical ceiling-lit office space, well-lit classroom, or bright supermarket is less than 100 fc
- ambient light in a residential room is less than 50 fc
It’s amazing how easily our eyes adjust to a very wide range of light levels! Other than donning a pair of sunglasses, it doesn’t take much for us to quickly acclimate from one extreme to the other.
Footcandle meters are a relatively inexpensive type of light meter. Cameras can also be used to measure footcandles, and smartphone apps may have the ability to measure approximate levels as well.
PAR stands for Photosynthetically Active Radiation, and simply refers to the range of light wavelengths (visual radiation) that plants need for photosynthesis. While plants have multiple pigments and make use of several light colors, chlorophyll heavily depends on the red and blue ends of the spectrum.
PAR meters measure plant-usable light and give their readings in units called PPFD (Photosynthetic Photon Flux Density), measured in moles per square meter per second. Descriptions of light fixture performance sometimes use these two terms interchangeably. This is why you may see light fixtures labeled as “high PAR,” or see product literature showing distribution charts of PPFD readings over a given area below the light.
PAR readings give valuable and more fair comparisons of one light type to another, since it measures the most plant-important part of the spectrum that we cannot judge by sight alone. PAR meters are much more expensive than footcandle light meters, though some models can measure both. Certain PAR meter models may be better at measuring LED light than others, so check the manufacturer’s information.
These parameters are more familiar to aquarium hobbyists since they need to account for how water depth interferes with light availability to submerged plants or coral. Home horticulture is just starting to embrace this kind of measurement, though some niches (such as terrariums and hydroponics) are starting to more regularly work with PAR ratings.
Coming up: fine-tuning your lighting choices!
In our next installment, we’ll talk about how lights are rated (only three more terms, I promise) and how light quality, intensity, and duration impacts plants. We’ll wrap up with safety tips for your lighting steups.
By Miri Talabac, Horticulturist, University of Maryland Extension Home & Garden Information Center. Read the previous articles in this series, An introduction to gardening under lights, Why light levels are important for indoor plant growth, and more by Miri.
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