You may see two other details provided in lamp specifications aside from the terms introduced in my last installment. They are not critical factors but can influence your satisfaction with how the lights look by themselves, and how plants, decorative pots, and other objects look underneath them.
The appearance of plants under the lights is not only important for aesthetics, like seeing the true colors of blooms, but also for detecting leaf discoloration, which can be a key symptom of malnutrition, light stress, or pest or disease damage.
Color Rendering Index (CRI)
While this doesn’t affect actual light intensity, it does impact our perception of how colors will look under a light and is a matter of personal preference.
On a scale of 0 to 100, the higher the CRI value a lamp has, the more accurate the colors will look compared to viewing in natural light. At the low end of the scale, colors are lackluster (desaturated) and less distinguishable from each other. Since incandescent, fluorescent, and LED lights all produce light by different means, they have different CRI ranges, though improvements in technology are closing this gap. Ideal ratings are in the 80s and above, with the 90s considered “high CRI.”
Referred to as color “temperature,” this doesn’t actually relate to heat when used as a lighting term. Instead, it compares the range of overall “white” light to colors we associate with temperature – warm, neutral, and cool. Compared with a purely neutral white, “warm white” is more pink/orange/yellow and “cool white” is bluer.
The lower the kelvin temperature rating a lamp has, the warmer the light will appear. For example:
- the redder light of low-angled sunshine is low on the Kelvin scale, in the 2000s
- warm-white fluorescents tend to be around 2700K
- direct sunlight is roughly 5000K
- cool-white fluorescents are about 6500K
- the upper end of the Kelvin scale is around 10,000K and is akin to looking up into the light of a clear blue sky.
As with the CRI value, the closer you can get to daylight, the more natural the plants will look.
You’ll most commonly encounter warm white/cool white differences when working with fluorescents and sometimes with LEDs. If you’re using a single fluorescent tube, opt for a grow light within the range of 5000-6500K. For multi-bulb/tube fixtures, you can mix cool and warm lights or keep them all the same. The slight differences in how much red and blue light these lamps emit can have different impacts on plant growth, so mixing them evens out the effects and gives you both benefits. LED fixtures in particular rely less on Kelvin ratings than fluorescents since their light output is more easily customized.
What about those odd single-color lights?
You know the ones…those LEDs in the glaring fuchsia-like color the internet has dubbed “blurple.” (The red and blue diodes together create a pinkish-purplish hue.) If plants use mostly red and blue light, aren’t they the most efficient and cost-effective?
Depending on your goals, maybe not, though studies of plant light use are ongoing. The key point is that plants use mostly red and blue light, but they do use other wavelengths as well. Unless you’re using these lights only in areas where aesthetics don’t matter, or for short-term crops (producing hydroponic sprouts or heads of lettuce, for example), it’s my opinion that a more balanced white light would be better for all involved. Plus, it’s way easier on the eyes. This leads us to the topic of…
Some grow lights are labeled “full-spectrum.” This means that the amount of each color of light it produces mimics natural sunlight, and the light will look fairly white. Ideally, manufacturers of plant lights will provide a graph of the spectrum produced so you can see the relative amounts of each color. While full-spectrum light is not required for acceptable plant growth and flowering, as I discussed above it does make viewing the plants easier and reduces eye strain compared with harsher red-blue lights.
You do have some control over how brightly your plants are illuminated by artificial light, beyond just the choice in fixture type. While most fixtures are not dimmable (except for some specialty LEDs), you can move the plants closer to or further from the bulbs, or raise and lower the fixture itself if hanging it from hooks and chains. Light levels decrease fairly rapidly the further you move from the bulb, so even a few inches in either direction can make a difference.
For long light tubes, especially fluorescents, the ends are usually dimmer than the center. Lower-light plants can grow at the edges and higher-light plants in the middle. If you’re growing one type of plant under light tubes (like a tray of vegetable seedlings) and want uniformity, just periodically switch the pots growing under the center of the light with those growing on the outer edges to even out the intensity each of them is receiving.
As with outdoor plants, daylength (here’s that final term – photoperiod) plays a role in regulating growth, dormancy, and blooming. Some plants are more sensitive to daylength than others. This is easy to manipulate with artificial lighting because you control how long the lights are on. Using a programmable outlet timer helps simplify this, and you can change its settings with the season if desired. If artificial lighting is a plant’s only light source, a commonly-used photoperiod keeps the lights on for 12-16 hours per day.
For plants that bloom in winter (short days, long nights), called “short-day” plants, ambient lighting in the home might interfere with their required period of darkness, even if those lights are too dim to support regular growth. In these cases, you may need to move the plant to a dark spot overnight, or find a way to block out the extra light. Poinsettias, Christmas Cactus, and Chrysanthemums are common examples, and to come into bloom they need a seasonal lighting reduction to 10 or fewer hours of light per day.
For plants receiving mainly natural light that are “long-day” types, you can use artificial lighting in winter to extend the photoperiod long enough to encourage continued growth or bloom. Plants that are “day-neutral” aren’t sensitive to photoperiod and might sporadically bloom all year.
In terms of energy collected, light levels are cumulative, meaning that both intensity and duration combine to result in total photosynthesis for that day. A shorter period of brighter light (within the plant’s tolerances) can provide similar levels of energy to a plant as a longer period of dimmer light. This benefits users of artificial light in several ways:
- brighter lights, which may run warmer or use proportionately more electricity, don’t need to be run for as long
- dimmer lights can be run for longer to make up for their lower light output, and generally have less heat output
- lower-powered lights may be cheaper
Gardening under lights safely
Straightforward and common-sense precautions should keep your artificial light setups safely functional for years.
- Due to their lower power usage, some fluorescent and LED fixtures can plug into each other so only one needs to be plugged into an outlet. Do not connect more units to each other (called “daisy-chaining”) than the manufacturer recommends.
- Make sure the outlet and any timer, extension cord, and power strip needed are rated for the total electrical draw your setup will be using to avoid components overheating. Aside from being a fire hazard, it could shorten the life of the fixtures or bulbs.
- Maintain good airflow around the sides and tops of light fixtures. Most designs will have the option to hang the fixture, which is preferable to mounting directly on a surface for this reason. This reduces warmth buildup which can shorten the life of the bulbs, especially for LEDs.
- If you mist plants underneath the lights, avoid getting water on the bulbs as the difference in temperature could cause breakage. This is critically important if you use HID lights, because the bulbs get hot. (Fortunately, HID fixtures tend to come with shields over the bulb.)
- If there is a risk of water running down the electrical cord, make sure the lowest-hanging point of the cord is not the plug itself in the outlet. Plan for enough slack to have a dip in the cord before the plug end. Aquarium keepers call this well-known precaution a “drip loop.”
- Use sturdy shelving or other supports, as some light fixtures can be heavy due to using metal housing. Plus, weight adds up quickly for a collection of well-watered plants, especially if you use ceramic pots.
- Make sure components have been tested by OSHA-approved laboratories, such as Underwriters Laboratories (“UL-listed”).
- When you need to dispose of used fluorescent bulbs/tubes, check with your local landfill or recycling center to see if they go into household hazardous waste collection.
Congratulations! You’ve slogged through the ins and outs of plant grow lights and are ready to make more informed decisions on purchases and can better interpret how your plants are fairing. You certainly can be successful without this knowledge, but it may help save you time, money, and guesswork when plant performance changes. It can also expand your horizons considerably when it comes to new adventures in growing plants indoors since any windowless room is now fair game for a great collection of tropicals.
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 Indoor lighting options: terms, types, and measurements.