Growing Under Lights

By

Paula Szilard


In 1996, when I first started growing plants under lights, I used 40 watt fluorescent grow lights. At first I grew just seedlings for my veggie garden—tomatoes, eggplants, peppers, broccoli and kohlrabi. Then gradually the plant stand started filling up with African violets, Cape primroses (Streptocarpus) and other gesneriads.  I also started to propagate under lights. 


Then over time, as I read more about growing under lights I switched to 40 watt fluorescent lamps of the type used in shop lights. I used plain cool white and warm white fluorescents. The latter are sometimes labeled for “kitchen and bath.” I alternated cool and warm white bulbs in four lamp 48” fluorescent fixtures. Plants need the cool light or the blue/violet part of the spectrum for vegetative growth and the warm or red part of the spectrum for flowering. The point of alternating these lamps was to make the light more like what plants get in nature, in other words, more of a wide-spectrum light.  Doing this is similar to the way vegetarians complement proteins.  Similarly, we can enhance the quality of the light by combining two lamps that complement one another in order to provide sufficient amounts of both the blue/violet and red/orange parts of the visible spectrum. This low tech solution was far cheaper because replacing these plain fluorescent lamps was much less costly than getting expensive new grow lamps once a year or so.  It seemed to work reasonably well, even with the vegetable seedlings.


Packaging for all lamps now provides considerable information.  It tells you how many watts of electricity a lamp uses per hour, the number of hours you can expect it to last, the brightness or light output in lumens, the Color Rendering Index (CRI), which tells you how well the light emitted compares to sunlight, which has a value of 100. For instance, if you have a lamp that has a color rendering index of 93, it will be a pretty good grow light because it’s very close to sunlight.  It also has the Kelvin temperature of the lamp, measured in degrees Kelvin, which tells you how much violet/blue and how much red/orange light it emits. The higher the Kelvin temperature, the cooler, bluer the light.  The lower the Kelvin temperature the warmer the light. The Kelvin temperature is actually a measurement of the different colors emitted when a piece of platinum is heated to different temperatures on the Kelvin scale.  At the higher temperatures the metal turns blue.  At lower temperatures, it turns red/orange.


According to George Van Patten, author of Gardening Indoors, each lamp has an aggregate Kelvin temperature that denotes the bulb’s color output.” According to Patten, lamps with a Kelvin temperature of 3000-5500 are best suited to growing plants indoors.  The color of a clear blue sky is 9500K, for our purposes, the top of the scale.  A Kelvin temperature of 1500 is the color of a burning candle, the bottom of the scale.  Many full or wide spectrum grow lights are around 5000K or a little higher. Cool white lamps are about 4100K.  Warm white lamps are about 3000K.  Once you know the color temperature of a lamp, you don’t need to be concerned about what terminology is used on the packaging.


Everything was going quite smoothly.  I was growing most of my plants under a combination of cool white and warm white fluorescents for years. I  had a 400 watt HID (High Intensity Discharge) lamp for overwintering the patio plants, but after calculating that it cost me $50 per month to run it, I retired it.  Traditionally, HID lamps have been used for tall plants because the light is so intense it penetrates to the bottom leaves of a plant, whereas traditional fat fluorescents do not.  Lighting experts tell us with every foot away from the light source the amount of light diminishes roughly by half, so to reach the bottom leaves of a very tall plant, a lamp has to be extremely bright. The new skinny fluorescents are somewhat better at penetrating than conventional fluorescents, but not nearly as good as HID lamps.


Then, recently I noticed that technology had passed me by--completely.   I discovered that the T-12 fluorescent grow lights (the fat ones I had been relying on) are being phased out because they are not as energy efficient as the newer generation of fluorescents.  They also present a disposal problem because they contain worrisome amounts of mercury.


The fat T-12 tubes are being replaced by the new thin high output fluorescent tubes, called T-5’s.  These lamps are very efficient in converting watts to lumens.  They produce 40% more light than the old fashioned fluorescents. Also, they contain much less mercury.  Forget that sales pitch, though.  Sales people inevitably tell you they use less power, but that is generally not the case. The T-5 grow lights definitely use more power, but there may be other T-5 lamps that use less.   Per lamp, T-5 fluorescents use 50% more power per linear foot than standard fluorescents!  Instead of using 40 watts per 48” lamp, they use 54 watts for a 46” lamp, however, they certainly are more efficient in the amount of lumens they produce per watt of electricity consumed.

 

Conventional T 12 48” Fat Fluorescent Lamps

A four-lamp conventional fat fluorescent fixture will use 160 watts per hour. Multiply this by 12 if that’s how long you run your lights per day.  Then multiply the daily watts used by 30 to give you the number of watts used per month.  You then need to divide this number by 1000 to get kilowatt hours. 

160 x 12  = 1920 watts per day.

1920 x 30=57600 watts per month

57600 divided by 1000 to get 58 kilowatt hours per month


High Output T-5, 46” Lamps

A four-lamp fixture of high output skinny T-5 fluorescents on the other hand uses 20 more kilowatt hours per month as shown below:

216 x 12=2592 watts per day

2592 x 30=77760 watts per month

77760 divided by 1000 to get 78 kilowatt hours per month


Now that we have gotten that out of the way, let’s talk performance.  Yes, they do take more power!  But do they ever perform!!!  The 46” lamps put out a whopping 5,000 lumens of light when they are new.  This amount of light will gradually diminish as the bulbs age.  So, a 46” fixture with four new bulbs will produce a whopping 20,000 lumens of light!


This is so much better than the conventional 48” fluorescents, which vary in their light output anywhere from around 2500 to 3000 lumens per lamp. So, four 48” lamps of conventional, fat fluorescents put out between 10,000 to 12,000 lumens of light, roughly half of the lumens produced by the new high output “skinny” fluorescents. So, twice as much light for only 1½ of the power is a good deal.  My plants seem to thrive under them.  Last spring I produced the best vegetable starts ever using these new lights. 


When you buy a T-5 fixture, it usually comes with grow lights.  These are of two types:  those high in the violet/blue part of the visible spectrum and those high in the red/orange part of the spectrum.  Unfortunately, most fixtures you buy come with lamps high in the blue/violet part of the spectrum. When the lamps burn out, you can certainly replace these bulbs with regular cheaper T-5 lamps available in warm and cool white from your home improvement store, or you could alternate red/orange and violet/blue grow lamps.


Unfortunately, those of us who are looking to reduce our carbon footprints and our electric bills are not going to achieve these goals with the new T-5 fluorescent lamps.  We are looking for something better.  So far, LED plant light fixtures have simply not put out enough light to grow plants effectively.  In other words, their output of lumens is not adequate.  Those of us who garden under lights are still waiting for better LED lamps and fixtures.  In the last five years or so new LED lamps have become much more powerful.  Recently I purchased some LED lamps for my recessed kitchen lights that were indeed bright.  The amount of light they produce is equivalent to the output of a 65 watt bulb, or 750 lumens. Here is the great news:  They only use 13 watts of electricity!  Now we are on the right path, I think.  In a few years, we should have plant lights that use this new technology more effectively and have a greater output of lumens.