Excerpt from Integral Hydroponics Evolution by G.Low (coming soon)


“Differences in radiation quality from the six most common electric lamps have little effect on photosynthetic rate. Radiation quality primarily alters growth because of changes in branching or internode elongation, which change radiation absorption.” Bugbee, B (1994) Effects of radiation quality, intensity, and duration on photosynthesis and growth.


Given that there is no sun in the indoor grow room, artificial lighting plays an extremely important role in producing indoor “under light” crops. That is, light is a key environmental factor. Without light, green plants cannot grow. Additionally, if we get the lighting wrong (too little light or the wrong colour light) plant growth and yields suffer. This said, artificial lighting is a subject that is too often misunderstood by indoor growers. This is perhaps not too surprising given that the science of light and plant growth is a complex subject to come to terms with.



Therefore, let’s go through some theory about light and then focus on how artificial light sources that are used for the production of indoor crops can be taken advantage of to reduce stretch.



Light Colour Spectrum, Intensity and Photosynthesis


Photosynthesis is the process in which light energy is converted into chemical energy.


The process of photosynthesis occurs when green plants use the energy of light to convert carbon dioxide (CO2) and water (H2O) into carbohydrates. Light energy is absorbed by chlorophyll, a photosynthetic pigment of the plant, while air containing carbon dioxide and oxygen enters the plant through the leaf stomata.


Glucose, a carbohydrate processed during photosynthesis, is mostly used by plants as an energy source to build leaves, flowers, fruits, and seeds. Molecules of glucose later combine with each other to form more complex carbohydrates such as starch and cellulose. This cellulose is the structural material used in plant cell walls.


We can express the overall reaction of photosynthesis as:


Carbon dioxide + water and nutrients (+ light energy) → Glucose + Oxygen


6CO2 + 6H2O (+ light energy) → C6H12O6 + 6O2


The rate of photosynthesis in plants is affected by a number of factors including light intensity, light colour, air temperature, CO2, nutrients and water. If any of these factors become limited this slows the rate of photosynthesis. That is, according to the law of limiting factors, put forward by F.F. Blackman in 1905, the rate of a physiological process will be limited by the factor which is in shortest supply. Any change in the level of a limiting factor will affect the rate of reaction.


For example, the amount of light will affect the rate of photosynthesis. If there is no light, there will be no photosynthesis. As light intensity increases, the rate of photosynthesis will increase as long as other factors are in adequate supply. As the rate increases, eventually another factor will come into short supply.


In understanding F.F. Blackman’s law of limiting factors we can then understand why plant growth must be seen as a ‘holistic process (i.e. characterized by comprehension of the parts of something as intimately interconnected) because all factors relative to growth must be maintained at ideals to achieve optimum growth. If any single factor is less than ideal, this single factor will limit growth.


For now though we are talking about light so let’s focus our energies here.


Light Colour, Light Intensity and Photosynthesis: Overview


Both light colour and intensity influence the rate of photosynthesis.


Light is measured differently depending on what part of the light spectrum is being measured. The total light spectrum coming from the sun (400 to 1100 nanometer wavelengths) determines the light intensity and it is measured in units of watts/m2. On a clear sunny summer day, there may be 1000 Watts/m2.


Plants have an optimal intensity of light. This is the point at which the process of photosynthesis is maximised and plant growth is greatest. If the level of light is less, growth is reduced. However, there is a point where an increase in light intensity will not increase photosynthesis any more. This point is called light saturation. Basically, a plant can only use so much light. If we provide more light beyond this point no growth benefits will be gained. See following image:




Excess light is unlikely to harm the plant; although, at very high light intensity, chlorophyll may be damaged and some research suggests that excess light may be detrimental to C3 plant growth rates.[1] However, plants which have adapted to high light environments have a range of adaptations to avoid or dissipate the excess light energy, as well as mechanisms that reduce the amount of injury caused.


Of more concern is that the heat excess light generates can be detrimental to growth. That is, excess light is associated with increase in the temperature of leaves which induces rapid transpiration and water loss. The guard cells lose turgor, the stomates partially or completely close, and the rate of diffusion of carbon dioxide into the leaves slows down. The rate of photosynthesis decreases while respiration continues resulting in low availability of carbohydrates for growth and development. Further, high leaf temperature inactivates the enzyme system that changes sugars to starch. Sugars accumulate and the rate of photosynthesis slows down.


For this reason, lamps should be placed at appropriate heights above the crop, and adequate air movement through the crop is imperative.


Having covered light intensity we then need to consider light colour. This is where things become very important when discussing plant architecture/morphology and stretch. That is, light colour plays an incredibly important role in plant stem elongation (stretch).


Light is composed of electromagnetic waves of different colour wavelengths. These wavelengths are what can be described as the light colours that make up light. When considering light to be a wave, wavelength is typically expressed in billionths of a meter, or nanometers which is symbolized as ‘nm’.


Between 400 and 450 nm violet is found; blue light occurs between 450 and 495 nm; green between 495 and 570 nm; yellow between 570 and 590 nm; orange between 590 and 620 nm and red between 620 and 700 nm. There are also other wavelengths that have some importance to plant growth. Light that has very short wavelengths is ultraviolet light and light with very long wavelengths is called “Infrared” light.


See following image that shows the different colour wavelengths (the light spectrum) of the sun.




Note that at the bottom of this image/graph, the colours are listed within their respective nm colour bandwidths. Between 400 and 450 nm violet is found; blue light occurs between 450 and 495 nm; green between 495 and 570 nm; yellow between 570 and 590 nm; orange between 590 and 620 nm and red between 620 and 700 nm.


The part of the spectrum that humans can see, called visible light (380 to 770 nanometer wavelengths), is measured in lumens. One lumen is approximately equal to the amount of light put out by one birthday candle that’s one foot away from you. To help you get an idea of the lumen scale, a standard 60-watt light bulb puts out around 750-850 lumens of light. The lumen is the metric unit of light intensity and the term lux refers to the number of lumens per square metre of surface area.


Where plants are concerned, the number of photons reaching the leaf surface is more important. Photons are packets of energy which make up a stream of light. The number of photons trapped by a leaf determines the level of photosynthesis and therefore the amount of plant growth.


The spectrum to which plants are most sensitive varies with species, but for most plants the spectrum is similar to the visual spectrum to which humans are sensitive, approximately 400-700 nm. Plant growth is optimized under the entire range of the spectrum.


Any photons within 400-700 nm that are absorbed by the plant will contribute to photosynthesis.


Because it stimulates photosynthesis, light within this range is called photosynthetically active radiation (PAR). This light is measured in units of μmol m−2s−1 and describes the ‘photosynthetic photon flux density’ (PPFD). The term photon flux density (PFD) is also frequently used to mean the same thing. Names aside, the PFD or PPFD is the number of packets of energy that reach the surface of the leaf.


Leaf photosynthesis of most light loving crops saturates around instantaneous light levels of 500 – 750 µmol·m-2·s-1 within the 400 to 700 nm waveband. For closed canopies of plants, the saturation point is higher. In species with photosynthetic systems adapted to high-light environments, the rate of photosynthesis saturates at a much higher irradiance, even as high as 2000 µmol·m-2·s-1.[1]


However, not all wavelengths are absorbed equally. Therefore, the colour of light plays an important role in photosynthesis. Plants use only certain colours from light for the process of photosynthesis. The chlorophyll absorbs blue, red and violet light rays. Photosynthesis occurs more in blue and red light rays and less in yellow and green light rays.


There are two peak light colours absorbed by plants for photosynthesis. These are blue light at around 450 nm and red light at around 670 nm.


Red light is critical for optimal flower/fruit growth. Flowering plants that are grown under high pressure sodium (HPS) lamps, which provide a high fraction of yellow, orange and red light, produce higher yields than flowering plants that are produced under MH lamps, which provide a higher fraction of blue light. Additionally, studies have shown that flowering plants that are grown under HPS lamps finish earlier than flowering plants that are grown under MH.[2]


Blue light is primarily responsible for vegetative leaf growth. MH, due to having a higher violet and blue fraction than HPS, stimulates healthier vegetative growth than HPS.


Additionally, light colour wavelengths influence the plant’s architecture/morphology in different ways. For example, numerous studies have shown that blue light in the 400- 500nm range inhibits stem elongation while red light in the 600 – 700 nm range promotes stem elongation.


Therefore, plants grown under light with a higher blue fraction will stretch less than plants grown under light with a higher red fraction. This is an important point for indoor growers to understand.


Light Colour, Photomorphogenesis and Plant Architecture/Morphology  


Through its impact on photosynthesis and ‘photomorphogenesis’, light is the environmental factor that most affects plant architecture/morphology.[1]


Light intensity for growth measured through PAR watts plays the most important role in photosynthesis. However, it is light colour that plays the most important role in photomorphogenesis.


Photomorphogenesis refers to light-mediated development where plant growth is altered in response to light signals. Photomorphogenesis only describes how plant growth patterns respond to light spectrum, which is a separate process from photosynthesis where light is used as a source of energy. [2]


Photomorphogenesis is directly affected by three factors: light quality, light intensity, and photoperiod. All three of these factors are perceived by a plant’s light perception network which is composed of primarily three types of photoreceptors: phytochrome, cryptochrome, and phototropin.[3] The red/far red photoreceptors are called phytochrome. There are at least 2 classes of blue light receptors; cryptochrome recognizes blue, green and UV-A light, while phototropin perceives blue light.


Current knowledge about photomorphogenesis indicates there is a complex cross-talk between the photoreceptor signaling pathways. Additionally, plant hormones (e.g. gibberellins, brassinolide and ethylene) are thought to be involved in photomorphogenesis. These hormones play an important role in plant stretch. For example, gibberellins and brassinolide increase stretch while ethylene reduces stretch (put simply).


Plants contain multiple blue light photoreceptors that have different functions. Based on studies with action spectra, mutants and molecular analyses, it has been determined that higher plants contain at least 4, and probably 5, different blue light photoreceptors.


Cryptochromes were the first blue light receptors to be isolated and characterized from any organism. Cryptochromes are responsible for the blue light reactions in photomorphogenesis. The proteins use a flavin as a chromophore. The cryptochromes have evolved from microbial DNA-photolyase, an enzyme that carries out light-dependent repair of UV damaged DNA.


One recent study that looked at blue light’s ability to reduce stem elongation indicates that phytochrome and cryptochrome work synergistically to reduce stem elongation and specific leaf area.[4]


Phototropins are also likely to be involved in the inhibition of stem elongation induced by blue light.[5]


The photoreceptor, phytochrome, is responsible for the physiological responses incited by changes in red (600–700 nm) and far-red (700–800 nm) light. This photoreceptor has been linked to stem elongation (plant stretch).[6] Basically, where phytochrome stimuli is high (i.e. high levels of red and/or far red light are present) stem elongation is promoted.


For the record, this is very much the dumbed down version of photomorphogenesis – the academic version being largely indecipherable to mere mortals who lack PhDs in plant physiology. In fact, the processes of photomorphogenesis are still not well understood by science. However, what is certain is that there is an extremely complex interplay occurring in the photoreceptor signaling pathways of the plant and the various colour wavelengths of light at different levels and ratios affect outcomes where plant architecture/morphology is concerned.


This said, complexities aside and where we are concerned, phytochrome is related to stem elongation (stretch), and the stimulus for phytochrome is red and far red light.


In contrast to this, the photoreceptor cryptochrome which is stimulated by blue light is associated to inhibiting stem elongation.


Additionally, phototropins are probably involved in the inhibition of stem elongation induced by blue light.


In layman’s (decipherable) terms:


Blue light = stretch inhibition

Red and far red light = stretch promotion


Photoreceptors, Shade-Avoidance Syndrome (SAS) and Plant Spacing


Plants rely on the availability of photo-synthetically-active radiation (PAR, 400–700 nm) to produce the carbohydrates used in their metabolism. The reduction of PAR below saturation levels lowers photosynthesis and can seriously compromise plant fitness. This has provided the evolutionary force to generate shade-avoidance responses. As a result of these shade avoidance responses, the presence of neighboring vegetation modifies the light environment experienced by plants, generating signals that are perceived by phytochromes and cryptochromes.[7] This results in plants competing for light and stretching.


Given this information, when considering lighting for indoor crops and optimum yields, it is important to stress that one of the most common mistakes made by indoor growers is that they fail to appreciate that fewer plants can mean more yield. Too many plants crowded into a small space will compete for available light and as a result stretch as they compete for light. This is called shade avoidance syndrome (SAS) where plants respond to competition signals generated by neighbors by evoking SAS, which results in stem elongation, increased internode distance, altered flowering time and reduced shoot branching. [8] Basically, what happens with SAS is a plant grown under canopies of other plants perceives the reduction in the ratio of red to far-red light as a warning of competition, and enhances elongation growth in an attempt to overgrow its neighbors. As a result, plants stretch in order to compete with one another for light. [9] As Morelli et al (2000) put it: “The most dramatic shade avoidance response is the stimulation of elongation growth.” [10]


Therefore overcrowding causes stem elongation and less than optimal yields.


Because of this it is imperative that plants aren’t overcrowded and competing for light. This means that appropriate plant spacing needs to be considered as an optimal growing practice.






See following images of metal halide (MH) and high pressure sodium (HPS) light colour spectrums in ‘nanometers’ (nm).






Compare the colour spectrums of MH and HPS lamps and you will note far higher relative degrees of blue light in MH and higher levels of yellow, orange and red light in HPS. That is, the spectral output of HPS lamps is primarily in the region between 550 and 650 nm and is deficient in the violet and blue region while the spectral output of MH is more balanced across the spectrum and possesses higher levels of violet and blue between 350 and 495 nm.


When HPS an MH lamps are measured for the percentages of blue and red light that each lamp provides we can see that HPS emits about double the amount of red light as MH. Conversely, MH provides about five times more blue light than HPS. See following table.



Source: Zheng, Y. Zhang, P and Dixon, M. Evaluation of Four Lamp Types for the Production of Tomato Plants in Controlled Environments. HortTechnology 647, July–September 2005 15(3)

Let’s now also compare the colour wavelengths of HPS and MH lighting to the sun. See following image.




The graph demonstrates the limited colour ranges that artificial light sources provide when compared to the sun. Quite simply, direct full sunlight is very different from HPS and MH lighting when it is analyzed from an nm colour spectrum perspective.


It must be recognized that no artificial light source exactly duplicates the desired natural light spectral range. Studies show that plants grow best under natural sunlight due to the color distribution. Even HPS lamps, which are the dominant supplemental light system used in indoor growing, only convert 30% of electrical energy into photosynthetically active radiation. Thus, all artificial light sources have recognizable limitations as the sole light source to grow plants, as compared to sunlight.


It is important to note that HPS and MH lamps were mainly designed for outdoor lighting where high electrical efficiency (lumen per watt) and high light intensity (lumen output) were the sort after qualities. HPS and MH lamps were never designed for plant lighting. Early indoor growers who experimented with ‘High Intensity Discharge’ (HID) lighting found that these lamps could be used for effective indoor crop production. The fact is, however, that it was a case of adapting existing technologies to a process that the lamps were never designed for… a compromise at best!


HPS lamps, due to having higher efficiency and PAR output than MH lamps, are often used in greenhouse agricultural settings as supplemental lighting with daylight. The combination of daylight and HPS creates a reasonably ideal source of light for plant growth because daylight provides blue, violet and red light while HPS provides high levels of yellow, orange and red light. Thus, the two light sources combined provide a wide spectrum of reasonably ideal light for plant growth.


However, in indoor, under light settings where daylight/sunlight isn’t present this isn’t the case; the plants are supplied with light from only artificial light sources. These light sources typically only supply a limited quality of light for plant growth. This means that plants are being grown under lighting which is less than ideal for stimulating all of the physiological responses that sunlight would evoke, meaning that the majority of indoor growers are producing plants in less than ideal environmental conditions. Of course, based on F.F Blackman’s law of limiting factors, this means that the majority of indoor growers are getting less than optimal yields.


This is particularly true when considering that most indoor growers have settled on using HPS lamps alone from the start to finish of the crop cycle. However, we can see when comparing the wavelengths of the three light types (HPS, MH and sunlight) that HPS lighting provides a very limited range of light that is completely different from the sun. We can also see that much of this light falls within the yellow, orange and red wavelengths of light. We can further see that HPS light provides only very low levels of violet and blue light.


Given this and based on the theory that we have covered to date surrounding photosynthesis, light wavelengths, photomorphogenesis and stretch this helps us to understand that while HPS lamps provide high amounts of stimulus for flower production they are less than ideal as a standalone lamp for the production of indoor crops. That is, HPS lamps also provide a high degree of light that promotes stem elongation.


Lighting to Reduce Stretch


Growers who have experimented with different lighting combinations can/will tell you that different lighting configurations can produce very different results. For example, plants that are flowered under a combination of yellow, orange and red high pressure sodium (HPS) and blue spectrum metal halide (MH) lighting form very differently than plants that are flowered under HPS light alone. In simple terms, to repeat, blue light (400 to 500 nm) has been shown to have an inhibitory effect on stretch,[1] whereas red light, particularly red/far red light at 660:730 nm,[2] has been shown to increase stretch. Therefore, if we combine MH (higher blue fraction) with HPS (higher red fraction) we provide a higher quality of light for plant growth.


Way back in the 1970s Ed Rosenthal ran a trial where one set of cannabis plants were grown under high pressure sodium (HPS) lighting, another group of cannabis plants were grown under metal halide (MH) lighting and a third group of cannabis plants were grown under a combination of MH and HPS lighting.


Rosenthal found that the combination of HPS and MH produced the highest yields, HPS light alone, the next highest yield while MH light alone produced the lowest yield. What Rosenthal also found was that when HPS lamps and MH lamps were used in combination, where their light overlapped plant growth was optimized (i.e. the highest yields were obtained in the MH HPS overlap area). See following image.




Decades on, research by the scientific community surrounding photomorphogenesis explains Rosenthal’s findings and provides us with insights as to why the combination of MH and HPS lighting resulted in the highest yields. Further, comparative trials with HID lamps on crops other than cannabis substantiate Rosenthal’s findings.


For example, in one study with tomato by Zheng et al (2005) where four lamp types were evaluated (high-pressure sodium high output, high-pressure sodium standard, metal halide warm deluxe and metal halide cool deluxe) the authors concluded:




“The MH lamp types are also recommended for growing compact plants. Since the MH(CDX) had more than 34% blue, it may be used to replace some of the currently used chemicals for keeping some ornamental crops, such as transplants, short and compact. Under some circumstances, it may be beneficial to combine two or more lamp types for crop development, plant morphology, and crop yield and energy use efficiency. For example, to prevent abnormal stem elongation caused by HPS lamps, a small amount of blue light (supplied by MH) can be added…


Plants grown under the two MH lamp types were visually greener, shorter and more compact than plants grown under the two HPS lamps. Chlorophyll content index measurements showed that there was a general trend for leaves under MH lamps to have significantly… higher chlorophyll levels than those under HPS lamps…


Plants under the HPS systems grew faster and developed to the reproductive stage earlier than plants under the MH lamp types. For example, under the two HPS lamp types, the first fruit appeared on day 46, while the first fruit for plants under the two MH lamp types was observed on day 52, about 1 week later. The first ripe fruit was recorded earlier on plants under the HPS lamps than on those under the MH lamps; also, fruit ripened earlier on plants under the HPS(HO) than those under the HPS(STD)…


We recommend that the HPS lamp be used for flowering and fruiting crops and the MH lamp would be better used for foliar (vegetative) and compact crops.[1]


[End Quote]


What this tells us is that HPS light acts as the best stimuli for flowering. Flowering plants produced under HPS light produce higher yields and finish earlier than plants produced under MH light. However, HPS light also contains a high fraction of orange and red light which promotes plant stretch. This can lead to a reduction in yields in crops that are ideally produced as more compact plants with close internodes.


The bluer light of MH lamps promotes higher levels of chlorophyll production and stimulates vegetative production.


That is….


We recommend that the HPS lamp be used for flowering and fruiting crops and the MH lamp would be better used for foliar (vegetative) and compact crops.


Additionally, MH lamps act to inhibit plant stretch. As the authors note: “Under some circumstances, it may be beneficial to combine two or more lamp types for crop development, plant morphology, and crop yield and energy use efficiency. For example, to prevent abnormal stem elongation caused by HPS lamps, a small amount of blue light (supplied by MH) can be added… “


Based on this, MH is best used in the vegetative stage of the crop cycle. Additionally, due to its ability to inhibit stretch, MH light is best used during the preflower stage of the crop cycle (when rapid stem elongation occurs over a 2 – 3 week period) until the first signs of fructification become apparent (i.e. vegetative axillary buds begin showing signs of switching to flowering buds). At this point HPS lighting should be introduced because HPS best encourages flower growth.


However, as bud set is also occurring through the bud set bloom phase we also want some blue spectrum light (provided through MH) to help these buds set closely to one another. Therefore, a combination of MH and HPS lighting facilitates stretch inhibition while also promoting optimal flowering.


Basically, a combination of MH and HPS lighting in bloom will result in optimum yields.


The way I mix HPS and MH lighting is by using two 600 watt HPS lamps per one 400 watt MH lamp, so 1200 watts of HPS and 400 watts of MH. This means I have about 34% of MH lighting in conjunction with HPS during flower.


Quality (Essential Oil) and Light Colour Spectrum


An important point to consider when discussing the light/lamp types that are most suitable for indoor growing is that essential oil production is stimulated by a wide range of light colours and, therefore, the combination of HPS and MH lighting better promotes essential oil production than HPS alone. That is, research shows that a combination of blue light and red light promotes more flavonoid synthesis than where predominantly blue light or predominantly red light are used alone; however, other wavelengths and possibly ratios between wavelengths also have an effect. For example, one study compared multiple artificial light types and found that maximum flavonoid synthesis occurred under a continuous lighting spectrum of 400 (blue) to 750 nm (red).[2]  In other words, flavonoid synthesis is best stimulated by a wide band and ratio of colours. Both metal halide and HPS only provide limited colours at different ratios to each other. Therefore, by combining both MH and HPS we can increase the range of the colour spectrum and better promote essential oil production.


What this really means (to simplify) is that the all too common cultural practice of using high pressure sodium (HPS) lighting alone in indoor growing has played a significant role in promoting stretch and reducing yields and quality.


What this also means is that plants are ideally grown under MH lights (or other blue spectrum lighting) during the settling, vegetative and preflower/stretch phases and thereafter a combination of MH and HPS lighting will provide the best results where yields and essential oil production are concerned.


Therefore, one of the wisest investments indoor growers can make is in MH lighting. Think of the initial investment cost in MH lighting this way. Traditionally many growers have used synthetic PGRs to reduce stretch. Beyond having implications to the end users health and the quality of the harvested end product, these additives are not cheap and had PGR using growers instead invested in MH lighting they would have saved a great deal of money over the long term. Put simply, MH lighting in combination with HPS lighting will both improve your crop’s quality and yields and save you money in the long term. It’s a win/win situation. For an initial reasonably low investment cost of about $150.00 (MH lamp and ballast) growers can achieve higher yields and higher quality produce while saving money on consumables.


Why then has HPS Become the Standard in the Hydroponic Industry?


I expect the answer to this comes down to several things or variables thereof. Firstly, HPS lighting is superior to MH lighting where lumen per watt and PAR per watt is concerned. Electrical efficiencies of high-pressure sodium lamps are typically within 30% and 40%, which make them the most energy-efficient light sources used nowadays in plant growth. Further, HPS lamps have a longer lifespan than MH lamps making the use of HPS over MH less costly (i.e. electrical consumption per par watt and bulb replacement) all round.


Secondly, HPS lighting will provide higher yields in flowering crops than MH lighting when used at comparative wattages and photosynthetic photon flux*. Therefore, HPS becomes an obvious choice over MH when discussing standalone lamp types.


Third, many growers are just looking for something that works with the least effort and initial investment cost. These growers want to keep things as KISS (keep it simple stupid) as possible and, therefore, see combining lamp types as all too hard. I have my own views on the KISS principle. That is, if you are losing yield/money through applying the KISS principle, then you are indeed stupid…. thus, KISSS. Keeping It Simple Stupid is Stupid!


Fourth, lighting is a subject that is widely misunderstood. Many growers are possibly unaware of the fact that a combination of HPS and MH will provide higher yields and quality than HPS alone. Additionally, misunderstanding surrounding lighting for indoor crops has led to less than ideal information being passed through the grow scene, whether that be via forums, blogs, grower-to-grower (word of mouth) and indeed through some hydroponic retailers when advice is given out about the most suitable form of lighting for indoor crops. This results in misinformation and disinformation being circulated as fact, leading to ill-informed purchasing decisions re lamp/light choice by growers.


Fifth, configuring the ideal lighting system takes a bit more time and effort than simply running HPS lighting. Some growers can’t see past the initial investment cost (i.e. two types of ballasts and bulbs v. one type) and the minimal extra effort.


Sixth, some strains (e.g. short height early flowering genetics) tend to do very well under HPS lamps. Many growers get extremely good results using HPS lighting alone and, therefore, probably see no need to experiment with combining lamp types. Further, to some extent after many years of crop specific breeding programs and growers selecting genetics that produce the highest yields under HPS lamps a phenotypic plasticity* has resulted in some genetics that perform extremely well under HPS lamps.


Seventh, in SOG (Sea of Green) growing situations stretch isn’t quite the same issue as it is where growing larger plants. For this reason, SOG growers for the most part use only HPS lighting. This said, having grown SOG many times over the years, there are benefits to combining HPS and MH lighting in SOG growing. Additionally, I have met many SOG growers who apply PGRs to their crop shortly after flowering is induced. This is because, even where growing SOG, reducing the stretch that is caused by HPS lighting produces yield benefits.


Lastly, a culture of ‘the answer comes in a bottle’ seems to have evolved amongst large sections of the grow community. Basically, this means growers, rather than focusing on creating the ideal environment, in which plant growth is optimized, instead look towards an additive (or as the case may be, 5-10 additives) to increase yields. This has resulted in the widespread use of synthetic PGRs (subclass growth retardants). On this note, as a tip, many additives only elicit positive growth responses when they are used in subpar environments. That is, plants are extremely adept at reaching near to genetic potential when environmental factors and nutrition are at optimum. Therefore, if growers were to place more emphasis on getting all of the environmental and nutritional factors right there would be little need for answers in a bottle (bar for the essential and beneficial nutrients provided in bottles). One of the most important environmental factors in indoor growing is light, which is provided via artificial light sources. Unfortunately, the vast majority of growers are getting this factor wrong. Quid pro quo, the quick-fix for a subpar environment, therefore, comes in a bottle.


*Photosynthetic Photon Flux: PAR (photosynthetically active radiation) is normally quantified as µmol photons m−2s−1, which is a measure of the photosynthetic photon flux (area) density, or PPFD.



*Phenotypic plasticity is commonly viewed as an evolutionary strategy enabling organisms to adaptively match their phenotype to local conditions. Plasticity is expected to strongly affect phenotypic evolution by influencing the opportunity for selection through the ability of one genotype to produce more than one phenotype when exposed to different environments. The most phenotypically plastic organisms are plants. Unlike animals, plants are forced to stay put and must weather any and all conditions thrown at them by the environment with no option of an immediate migration or a run for shelter. Hence, they’ve evolved to be plastic in their physiology and development. Through phenotypic plasticity and selective breeding with sort after growth traits in mind it is highly possible that the normally long process of natural selection that occurs in nature has been greatly accelerated and genetics more suited to production under artificial lighting has resulted. Various researchers, for example, have noted the plasticity of cannabis; e.g. Indian varieties of cannabis when planted in England and France were indistinguishable from European varieties within several generations.[3]


Phenotypic plasticity, however, has its limitations. For example, the earliest land plants were established in Mid-Palaeozoic era at high CO2 (1500–3000 ppm) before the beginning of a period of low CO2 (<1000 ppm).[4] Therefore, many plants evolved in conditions of higher CO2 than are currently seen as atmospheric CO2 norms today (350-500ppm). As a result, many plants evolved in conditions where they adapted to assimilating higher levels of atmospheric CO2 for biomass production then is the norm today. However, notably, many of these same plants today have higher photosynthetic rates (PR) under elevated CO2. Thus, it could be said that regardless of phenotypic plasticity, the genetic potential of many C3 plants can only be realized under elevated levels of CO2 similar to the conditions in which these plants evolved. In other words, phenotypic plasticity hasn’t resulted in a phenotype that can produce the same biomass at the same rate under lower levels of CO2. This probably relates to light quality also; i.e. light loving plants evolved under a light source (the sun) which provides a wide spectrum of colours which are pretty equally distributed among the different wavelengths. While phenotypic plasticity may have enabled plants to adapt to light sources that have more limited qualities, optimal plant growth in all genetics probably occurs under light with qualities similar to the sun.




[1] Zheng, Y. Zhang, P and Dixon, M. Evaluation of Four Lamp Types for the Production of Tomato Plants in Controlled Environments. HortTechnology 647, July–September 2005 15(3)

[2] Kotriana, S. (2013) The effect of light quality on tomato (Solanum lycopersicum L. cv ‘Efialto’) growth and drought tolerance

[3] Connell Clarke, R. Marijuana Botany: An Advanced Study: The Propagation and Breeding of Distinctive Cannabis. Ronin Publishing, Jun 15, 1981

[4] Leakey ADB and Lau JA (2012). Evolutionary context for understanding and manipulating plant responses to past, present and future atmospheric [CO2]. Phil. Trans. R. Soc. B., 367: 613– 629.

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[2] Parks, Brian M. (2003-12-01). “The Red Side of Photomorphogenesis”. Plant Physiology 133 (4): 1437–1444.

[3] Casal, JJ. 2000. Phytochromes, Chryptochromes, Phototropin: Photoreceptor Interactions. Photochemistry and Photobiology, 71(1):1-11.

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[5] Durner, E. F. (2013) Principles of Horticultural Physiology. The State University of New Jersey, USA, pp 144

[6] Lund, J. B. Blom, T. J. and Aaslyng, J. M. (2007) End-of-day Lighting with Different Red/Far-red Ratios Using Light emitting Diodes Affects Plant Growth of Chrysanthemum morifolium Ramat. ‘Coral Charm’

[7] Casal, J. J (2012) Shade Avoidance. Arabidopsis Book. 2012; 10: e0157. Published online 2012 Jan 19. doi:  10.1199/tab.0157 retrieved 23/1/15

[8] Krishna Reddy S, Finlayson SA (2014) Phytochrome B promotes branching in Arabidopsis by suppressing auxin signaling. Plant Physiol. 2014 Mar;164(3):1542-50. doi: 10.1104/pp.113.234021. Epub 2014 Feb 3. See also Ronald Pierik and Mieke de Wit (2013) Shade avoidance: phytochrome signalling and other aboveground neighbour detection cues

[9] Carabelli M, Possenti M, Sessa G, Ciolfi A, Sassi M, Morelli G, Ruberti I. 2007. Canopy shade causes a rapid and transient arrest in leaf development through auxin-induced cytokinin oxidase activity. Genes and Development 21, 1863–1868.

[10] Morelli G, Ruberti I. 2000. Shade avoidance responses. Driving auxin along lateral routes. Plant Physiology 122, 621–626.

[1] Michigan State University, Department of Horticulture, Developing Technologies and Practices for Sustainable Specialty-Crop Production. Retrieved 19/1/16

[2] Zheng, Y. Zhang, P and Dixon, M. Evaluation of Four Lamp Types for the Production of Tomato Plants in Controlled Environments. HortTechnology 647, July–September 2005 15(3)

[1] Ort, D. R. (2001) When There Is Too Much Light. Plant Physiology January 1, 2001 vol. 125 no. 1 29-32


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