Everywhere I look I see people getting into growing indoors. Gardening indoors extends the season for most of us and allows us to control the environment much better than we can outdoors. While growing houseplants and starting some flower and vegetable transplants indoors has been the obvious choice for indoor gardening, lately more people are getting into growing things like microgreens, hydroponic lettuce, and legal cannabis.
This has spawned an interest in grow lights, particularly LED’s. It has also spawned a lot of confusion over what the best light is for different growing conditions. Most of this confusion, I believe, is over the different terms used to describe light. So let’s see if we can simplify things so you, the grower can make the best choice possible for your indoor growing situation.
Understanding the terms
It’s important to understand the terms used to describe light fixtures so you know what you’re buying.Terms like watts and lumens have really stuck in the minds with the consumer but in reality they don’t address the quality and kind of light needed for the different growing phases of plants.
Watts only speaks to the energy a bulb is using and lumen refers to the brightness and efficiency of a lamp. Since plants see light differently than us humans, the measure of lumens is irrelevant when it comes to selecting a good lamp for your growing operation.
In the spirit of the phrase “keep it simple stupid” I can tell you that there are only a few terms and concepts we need to learn about our current lighting or the lights that we are considering purchasing.
Veg or flowering?
Light in the blue spectrum has a short wavelength and is used in the veg state of growing when you’re only concerned with establishing green growth. Light in the red spectrum has a long wavelength and is used in the flowering stage. Cannabis growers use lights in the red spectrum when their plants are in the flowering stage.
I’ve always used a fluorescent shop light (a T12) to grow my tomato and pepper transplants and a few flowers. However, a few years ago I picked up a relatively inexpensive T5 fluorescent fixture with 5 fluorescent bulbs or tubes. Compared to the T12, the T5 delivers more usable light than a T12. It is used when your plants are in the nonflowering or veg state. If I wanted them to flower and set fruit indoors I’d have to obtain a grow light that delivers light in the red spectrum.
There are LED fixtures on the market that, rather than having to change from a “veg” bulb to a “flowering” bulb, you can simply flip a switch. Prices range from $150-$200, which is much cheaper than light fixtures were selling for just a few years ago. Plus, LED light bulbs last longer than fluorescent and high pressure sodium bulbs used in traditional lighting.
Understanding Photosynthetically Active Radiation
One measure of light that is critical to growers and can be measured with a meter is PAR (Photosynthetically Active Radiation). PAR is a measure of the usable light plants have for productive growth. When you purchase lights, especially the more expensive type, the manufacturer should be able to provide you with the PAR range of the light fixture. PAR is a measure of the quality of light and is measured in micromols per second. PAR readings for a light fixture should be between 400-700 and may fluctuate, which is okay. Like measuring pH, if PAR is in the acceptable range you’re all set.
Quantum PAR meters can be VERY expensive, but the makers of this one have told me it will be sold at a reasonable cost. It is very easy to use and gives readings for both PAR and Daily Light Integral (DLI), which is the cumulative reading of PAR for any given period. DLI is a measurement that is very important to determine the total number of PAR units that reaches your plants during the time your lights are on; it can also be used to determine the PAR in your greenhouse on a typical day.
PAR and DLI are important readings to take in a greenhouse where coverings, especially older coverings, can reduce the amount of light that actually reaches and is usable to a plant. It can help you determine if it is time to replace your covering.
This is an intentionally over simplified description of light and how to measure it. If you want to learn more about lighting and the light needs of specific plants, I urge you to pick up a copy of Leslie Halleck’s excellent new book, Gardening Under Lights. Her book is a practical guide for growing virtually anything under lights.
You can purchase Gardening Under Lights by following the link to Amazon.com on the right column. If you’re interested in trying out the SpotOn Quatum PAR Meter and live in the area (Sault Ste. Marie, MI) give me a shout and perhaps I can stop by so you can take a reading of your light fixture or available light in your greenhouse.
Check out my blog for more information on growing and the green industry.