Fluorescent Lights - Part 1
By Jerry Meislik
By way of introduction I have been growing plants, mostly bonsai trees, indoors under lights and on windowsills for nearly 30 years. Over time I have graduated to growing most of my bonsai trees under metal halide lamps in a plant room. The growth of my trees is quite spectacular and is about the best that I have seen accomplished under artificial light. There are several other categories of artificial lamps that includes fluorescent, incandescent, halogen and LED's or Light Emitting Diodes. Each has advantages and disadvantages and more detailed information can be found on my site by clicking here, here and here.
With the more or less recent advent of LED plant lighting I was very interested in seeing how LED lights would work given my growing conditions. LED lights have the advantages of having low energy requirements and yet high efficiency in producing light that is tailored to the frequencies needed by plants. Excess energy is not wasted on producing heat or in producing light frequencies not used efficiently by plants. The disadvantages of LED lights are high initial cost for the lights and a lifespan variably predicted as 5-12 years and requiring replacement of the whole light apparatus since individual LED's in the arrays are not usually replaceable.
In order to see how LED's perform I set up an experiment comparing one 22 inch long, 9 watt LED plant array with five light clusters containing multiple colored LED's. This light bar was suggested to cover a 1 foot by 3 foot area with good light coverage. This was compared to a fluorescent light setup consisting of four, 20 watt , fluorescent bulbs, 24 inches in length. The fluorescent bulbs consisted of two daylight bulbs and two plant bulbs. The fluorescent and the LED bulbs were hung at 12 inches (*1) above the seed/plant tops. The lights were cycled off and on with a clock timer keeping the lights on for 16 hours each day.
The test plants were seeds of various vegetables and herbs put into commercially sold peat pots (*2) and sitting in plastic flats that were 12" wide by 20" long. The pots were watered with reverse osmosis water and not fertilized during the growth period. The growth period from seeding to the first comparison period was 4.5 weeks.
The room temperature where the experiment took place varied from a low of 60F at night to about 70F during the day. Temperatures underneath the fluorescent lights was about 2 degrees F warmer than under the LED's due to the warmth generated by the fluorescent bulbs.
At 4.5 weeks the plants in both groups were measured for height and also visually compared to see under which light system the plants had more colorful, compact and vigorous growth.
The plants were examined at 4.5 weeks and we concluded that the LED lights did not improve plant growth over the 4 tube fluorescent bulbs under the conditions of my experimental setup. See picture 4 below.
Other non-involved individuals were informally asked to compare the two groups of plants and they all selected the florescent grown plants as "looking better and stronger".
The cost to run the LED lights is 1/10th the cost of the fluorescent lights. Initial cost of purchasing the LED lights is higher and replacement costs for the LED are higher as well since the individual LED bulbs can not be replaced but the entire unit must be replaced. See cost comparison table.
Initial Cost of Fixture and Bulbs $260
Replacement cost LED fixture if calculated yearly over 8 year lifespan.
Electricity costs of LED bar per year
Initial cost of fluorescent fixture and bulbs $77
Replacement cost of fluorescent bulbs every 1.5 years
Electricity costs fluorescent lights per year
CUMULATIVE COST COMPARISON (Over 8 Year Life Of LEDs)
1*. LED fixture replacement arbitrarily set at 8yrs/96 months. Since LED's are not warranted or guaranteed we assume an 8 year life of the LED array. Should the LED's last longer or shorter than that period the cummulative cost for the LED's would be lower or higher than in our 8 year life assumption.
2*. Yearly cost electricity and fluorescent bulbs replacement each 1.5 years.
Depending on how you cost account replacement costs for the light fixtures the payback of the LED occurs somewhere about the 3rd or 4th year due to the much lower electricity cost of the LED. However, many manufacturers claim a life of 5-12 years for their LEDs but do not guarantee that lifespan. If we arbitrarily assume that the LED unit must be replaced every 8 years for $260 vs a bulb replacement for the fluorescent every 1.5 years, the cummulative yearly cost to own an LED array varies as in the above chart. During years 4-8 the LED cumulative cost is cheaper but in year 8(month 96), assuming the LED array must be replaced, the cumulative cost of LED vs. fluorescent is now nearly the same. Additional calculations for heating, cooling, ventilating your specific growing situation may vary widely and affect your financial calculations.
Of course in our age of conservation, global warming and the buildup of greenhouse gases spending less money and using less electricity definitely is a major consideration for all of us.
Conclusions and comments:
The four bulb fluorescent lights won the contest quite easily. Plants were taller, more colorful, had more leaves and sturdier in nearly all the categories of vegetables and herbs. One specific pea variety grew more compactly and had a thicker stem under the LED's but that variety did not sprout at all under the fluorescent lights. Reason unknown?
Inserted below are comments by my LED supplier as to possible complicating factors explaining why the LED's performed poorly under my test conditions.
1. The LED bar should not be mounted closer than 18" from the top of your plants. For seedlings 20 to 24" should be about right. By placing your bar closer than the recommended mounting distance plants received too much light (photo-inhibition), and the light does not have a chance to mix properly before reaching your canopy. This would cause growth to slow and the plant leaves to grow in an unnatural shape.
2. Peat is not considered a porous medium. It is actually designed to keep your seedlings moist for longer periods. We recommend the opposite - a porous medium that will dry out and allow air circulation to the root zone. Maybe mix dirt and vermiculite for instance. Overwatering plants under LEDs is not uncommon even with this more porous medium. Your plants need far less water without the radiant heat of traditional lighting.
3. Temperatures were 2F cooler under the LED's and a heating cable should be used to keep the seedlings warmer.
Further experiments will be performed with bonsai tree material comparing the two light systems. Perhaps trees will respond differently than vegetables and herbs.
The LED light will be moved to the recommended 24" height above the plants.
A porous soil mix will be used for the bonsai.
To see the second part of my LED experiment click here.