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Journal American Rhododendron Society

Current Editor:
Dr. Glen Jamieson ars.editor@gmail.com


Volume 60, Number 3
Summer 2006

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It's Your Money: Using Better Lighting to Grow Rhododendrons
E. White Smith
Portland, Oregon

        Lighting energy use accounts for about a quarter or more of the estimated one trillion kWh (kilowatt-hour) of energy yearly expended by the US commercial and industrial sectors. The cost to light US offices, factories, retail stores, warehouses and other commercial operations combined with outdoor lighting is close to $20 billion (kWh 1 kilowatt hour = 1000 watts per hour; all we need to understand is that a watt is a measure of electricity). Tacoma Power charges about $0.06 per kWh. Portland General Electric charges $0.0872 or almost 9 cents per kWh minus some adjustments. In most of the US the cost of electricity is much higher. Take a calculator and add up your electric light use. Bulb Watts x hours used = kilowatts used per bulb x days used = a lot of money The US Department of Energy says that a 100 watt incandescent bulb that only cost 75 cents will cost you almost $40 in electricity use and bulb replacement over just three years. A 23-watt Compact Fluorescent will cost about $20 for power over three years and the bulb will last another three years.
        The cost of electricity is always going to go up. Many rhododendron growers use lights to help root cuttings or grow seedlings and the lights work just fine. But the problem I have is the cost of running these lights. Our electric bill at Bovees Nursery in Portland Oregon runs about $140 per month. Some of that cost is for rooting bed heating cables and fans.
        Many of the lighting systems in use today are old technology. Also some of the lighting solutions are not very efficient dollar wise. But many new lighting systems are coming onto the market.
        Okay, our problem is cost of electricity to do the lighting we need. We have six, 8-foot by 3-foot rooting beds. Our beds are "sweat box" type with heat cables under the pots/plants. The beds are each 3 feet tall and the whole thing is covered with clear poly film. The beds are tall so that there is working room inside and they have fold-up or pull-up front sides. Last year each of these rooting beds had two, 8-foot, fluorescent units. These units each had two 80 watt tubes, so 80 x 4 = 320 watts per 8-foot bed. We have now changed these fluorescent fixtures to the new "High Output" ones. The new fixtures are also 8 feet long and contain two 110 watt High Output bulbs, and we only use one unit per rooting bed instead of two, so 2 bulbs X 110 watts = 220 watts for a power saving of 110 watts per hour of usage. The one new High Output fluorescent unit is much brighter than the two fixtures it replaced, so we gained all around, more light and less power used. Yes, the new high output fixtures cost more, $45 compared to $25 for the old ones. And the 110-watt fluorescent bulbs cost more, about $5 each compared to about $3 for the old 80 watt ones. So more light and less power used and we might come out even some day. We were replacing the old fixtures anyway when they went bad. The high output fixtures are "instant on," "no flicker" and "no noise."
        We run the lights at night in the winter for the added heat they give off, with a fan up high to direct the heat back down from the ceiling. We rarely provide any other supplemental heat in the propagation house. Because our propagating house is really a greenhouse, we were getting a lot of "light leaking" to the outside at night, and the leaking light was not doing the cuttings any good. I bought a roll of aluminum roof flashing material, 10 inches by 50 feet and cut it into 8-foot pieces. The fluorescent fixtures are hung just above the top poly covers, and I just laid the aluminum strips up against the light units like a lean-to tent. Boy, did that stop a lot of light leaking to the outside and a lot more light was directed down unto the plants. Good deal. Next time I put new poly on these beds I am going to try some aluminum stuff on part of the sides also to reflect more light.
        We also have a couple of seed growing chambers that had 48-inch fluorescent units in them. The fixtures had two 40 watt T12 tubes and I have replaced them with new two tube 32 watt T8 units which seen to give a lot more light. Here again the old fixtures were going bad so they needed to be replaced anyway. The T8 fixtures are cheap, cheap, cheap, only about $10 at Home Depot. So now we have again better light and less power used, instant on, no noise, and no flicker. Even the best T8 lamps will produce less light than the best T12 ones, but the T8s use less power.
        Light is measured in lumens. One lumen is equivalent to the light given off by one candle whereas a watt is the amount of power a bulb uses to produce light. kWh = 1 kilowatt hour = 1000 watts of electricity used per hour.
        Lumens divided by watts = Lumens per watt
3,300/40 = 82.5 lumens per watt (for the best T12 40 watt fluorescent lamp)
2,650/34 = 77.9 lumens per watt (for the best T12 34 watt fluorescent lamp)
2,950/32 = 92.19 lumens per watt (for the best T8 32 watt fluorescent lamp)
        (The following fluorescent tubes are 4-foot long T12 types. Remember all of my numbers are what I could find at Home Depot or Lowe's so they are plus or minus.)

Approximate Efficiency
Lamp Types Range lpw* Color Rendering Ability
Standard Incandescent Bulbs 7 to 15 Excellent (100)
Tungsten Halogen 15 to 25 Excellent (100)
Compact Fluorescent Bulbs 25 to 75 Good (70+) to Excellent (100)
Full Size Fluorescent Tubes 65 to 95+ Medium (60+) to Excellent (80+)
Metal Halide 45 to 95+ Fair (50+) to Good(70+)
  *Light Per Watt  

Demystifying Fluorescent Lamp Size
Fluorescent lamps are classified according to their diameter in increments of ⅛ inch. Such as, T12 Lamp diameter is 12/8 inches or 1.5 inches.
        T8 Lamp diameter is 8/8 inch or 1.0 inch (next generation lamps that require an electric ballast. They are Instant Start, don't flicker, and make no noise).
        T5 Lamp diameter is 5/8 inch or 0.625 inch (another new generation lamp/bulb).
        Some facts (note that different manufacturers have different numbers (little bit different). At this time I suggest you only use fluorescent lights to grow plants because of the cost of electricity. If you buy new fluorescent fixtures be sure that they have Electronic Ballast. The old type ballast were called magnetic ballast; the new ones are solid-state electronic ballast.
        Remember these new fluorescent fixtures require the new "solid state" ballast, not the magnetic type ones. Beware of so called "energy saving" bulbs. They might save 15% of the electricity but are not as bright defeating the energy savings. In most cases you get what you pay for. I have tried to get good numbers for all of my "facts" but that is very hard to do. If you go to a store and look carefully at bulbs, you may or may not learn something. Sometimes they don't tell you about light output so how can you compare?

Type of bulb Watts used Lumens output Life of bulb/hours Bulb cost
Incandescent 100 watts 1690 750 hours $0.50 to $1.00
Incandescent Rough Service 100 watts 1160 1000 hrs $2.00
Incandescent 75 watts 1170 750 Not much
Incandescent 60 watts 840 1000 At $0.12 per kW, 24 hour use cost 17 cents
Compact Fluorescent 13 watts 900 10,000 $2.50 At $0.12 per kW 24 hour use cost $0.037 that's almost 4 cents
Compact Fluorescent (GE) 20 watts = 75 watts 1200 8,000 $5.00
Compact Fluorescent (GE) 26 watts = 100 watts 1500-1700 10,000 $4.00
Compact Fluorescent 42 watts = 150 watts 2600 10,000 $9.00
Fluorescent tubes  4 foot long T12 25 watts 1860 15,000 $2 +
Fluorescent tubes  4 foot long T12 Philips Advantage 40 watts 3250 12 to 15,000 $3
Fluorescent tubes  4 foot long T8 32 watts 2900 20,000 ?
Fluorescent tubes  8 foot long T12 60 watts GE XL 5400 15,000 $4 to $8
Fluorescent tubes  8 foot long T12 75 watts GE XL 6700 15,000 ?
Fluorescent tubes  8 ft long T12 HO High Output 110 watts     $4 to $7.00
Fluorescent tubes  8 foot long T8 57 watts 5400 18,000 ?
Fluorescent tubes  57 inches long T5 35 watts 3650 20,000 ? If you can find them anywhere??
Fluorescent tubes  57 inches long T5 HO High Output 49 watts 4900 20,000 ? If you can find them anywhere??

        Light level goes down by a factor of 4. If you triple the distance it goes down by a factor of 9. I think I remember that light is lost by the square of the distance by the foot.
        You can also get T5 fluorescent grow lights and compact fluorescent grow lamps. There is one that uses 125 watts and puts out 9500 lumens for $70 and it might be a good deal if you could build a reflector for it to screw into. The commercial reflectors cost about $60 with hardware.
        Enough of that fluorescent stuff. Are there better ways to get cheap light. You bet and it's called L.E.D.s, Light Emitting Diodes. And you can buy plant growing fixtures that have LED bulbs but they are very expensive for now (like $250 to light four square feet). Some day! The big advantage of LED is that they use very little electricity and last almost forever. But LEDs are little bitty things and you need a lot of them to get much light. Probably all of the little lights on your computer, TVs, etc., are LEDs. Some of the fancy new cars have LED taillights so LEDs are coming but it will be a while. Sure wish I could figure out a way to hook some up and try them. If any one has some suggestions about using LEDs with out spending a lot of money please let us know.
        Some people might question the distance of our lights from the cuttings. The purpose of the lights is not necessarily to push top growth, but to promote root growth by providing night and day periods, and at least a 12-hour day of light. Also we are not concerned about the frequency or color of the light. For our use, light is light. We get excellent results with both vireyas, hardy rhododendrons and hardy rock garden plants.


Volume 60, Number 3
Summer 2006

DLA Ejournal Home | JARS Home | Table of Contents for this issue | Search JARS and other ejournals