Further Evaluation of LED Light on seedling development

You may recall the short trial I ran using LED, 600mm long grow tubes which were procured by the Society about a year ago trying to compare the effectiveness of the Blue (80% Blue / 20% Red) and the Red (70%Red / 30% Blue) lights growing seedling and to recap this trial we said:

LHS Photos

  1. 1.        Spindlier growth on the open type Lettuce
  2. 2.        Smaller root development
  3. 3.        Foliage tended to spread out further, longer leaf stems
  4. 4.        Paler colour of the cultivar




RED 70% and BLUE 30%           BLUE 80% and Red 20%

Unfortunately I was not able to do more extensive trials with these lights as it was late in last year’s seedling growing season and my tomatoes and capsicums were already planted out in the greenhouse so this year I started with the LED lights and succeeded to at least grow and observe the tomatoes under the lights. I had 2 red and 2 blue tubes separated at 80mm apart and staggered for more even distribution -red/blue/red/ blue.

It was interesting to note that the 4 varieties of plants which I had all behaved similarly. They all tended to grow chasing the location of the blue tubes. What was more interesting was that the LHS of the growing tray is exposed to natural lighting as it is located against a window facing the north and very considerable natural light is present and even that failed to divert the growth pattern toward the blue light. Even moving the plants to different locations again showed this very strong affinity for the predominant blue emissions as the following photos will illustrate.


The growth rate was similar to the rate experienced with the 4x40watt fluoro tubes used in the past except that the power consumption was halved. The highest growth rate will be achieved by having the shortest distance between the light and the foliage as the light energy drops off as the square of the distance. Basically what that means is when light travels twice a certain distance it has to cover 4 times as much area so that the original energy is spread out over more area.

I tried to maintain a 2 cm height of the lights above the foliage and unless your system has continuously adjustable height of the tubes then there will be occasions when the foliage will touch the tubes as happened in my case often (see the next photo). There was damage to the foliage when this occurred, not sufficient to affect the final result, nonetheless surface damage was present.


It was not due to the tube temperature as that was recorded at 32⁰C max.  and that was not high enough to cause damage. I would guess that either the contact was sufficient to stop proper transpiration of the leaf or the actual light energy caused the top cells to be damaged and this then allowed the bottom cells to grow resulting in a leaf curl as shown on the next photo.

LED effects 001b

About 4 weeks from seed to plant out was typically what I was getting with the fluoro tubes so 600 mm long LED’s have proven to be successful and certainly suitable for seedling growth. By the middle of September the plants have grown well (See above) and the 2nd trusses are opening currently.

A second benefit was derived in that the algae growth which usually forms on the bottom of the seedling tray and on top of the grow block was considerably suppressed as the light intensity was too low for rapid growth as the tubes were always positioned just above the canopy and little light of sufficient energy  penetrated to the base.

There is no doubt that further work needs to be done for the best colour combination to use on seedlings and even though the suppliers spell out light frequencies in their sales pitch rarely do they result in ideal frequencies as these LED’s are mass produced for other markets where the wavelength  is not critical. A lot of research work has been done in Europe with variable frequencies of the LED’s output to control the light spectrum and maybe we will be looking at this type of LED system in the future.

10th Sep 13 006



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