Shining A Light On The Photoperiod & Light Spectrum

Shining A Light On The Photoperiod & Light Spectrum

The main benefit of using a hydroponic system is the ability to control high energy sources to maximise plant growth precisely. Plants use energy in light, known as photosynthetic assimilation, to create specific organs such as foliage (Basil, Lettuce,), flowers (Rose, Gerbera), or fruit (Tomato, Cucumber, Eggplant). All these organs require a significant amount of photon energy to lead to their creation; therefore, they are grown under a controlled regime of 18 hours (hrs) to 22hrs of illumination per day.

It is up to the gardener to adapt the duration of illumination (and intensity) according to the stage of growth of the cultivated species. Some plants see their life cycle entirely "guided" by the duration of illumination whereas others require specific low light periods (e.g. cuttings in rooting phase require a lower illumination than a young plant).


Seasonal Control

Plant germination and growth periods occur naturally in early spring due to the favourable high temperatures before the summer solstice (June 21st in the northern hemisphere) as the duration of the day decrease steadily until the winter solstice (December 21). The plant will "measure" the period of illumination via its chromoproteins, specifically phytochromes, then signal individual organs to ensure the sustainability of the species: the need to develop flowers for pollination and the growth of seeds. It takes some time for the plant to achieve all these steps and anticipate the rigours of winter. The photoperiod is a way for the plant to find its way in the seasons; whereas other plants use the thermoperiod.

Four-arrow circle with superimposed images representing four seasons of the year

This natural phenomenon involves the transformation of vegetative buds into floral growth. The variation of the duration of the day is a signal to some plants to acquire their flower buds. First highlighted scientifically in France by Dr Julien Tournois in 1912 on plants of the Cannabaceae family: Hemp (Cannabis sativa) and Hops (Humulus japonicus). The botanist noticed that by artificially shortening the duration of lighting in the greenhouse, the plants began to bloom.

Plants, such as Hemp, which react to the decrease in day length are identified as being "nyctiperiodic". To return to the example of Hemp, this plant will receive 18 hours of light per day and will begin to bloom quickly under a regime of 12hrs. Conversely, Poppy, Spinach, and Fennel only flower when the length of the day increases (this is why poppies are in bloom in summer in the fields of cereals). Plants of this type are called  "hemeroperiodic"; with "hemeros" being Greek for the day. Others are not sensitive to the variation of the duration of the day: which is the case for the Tomato, requiring only a minimal duration of illumination for their sustenance (photosynthesis).

The main advantage of using propagation tents (free of natural light) is the ability to choose at any moment of the year the blooming stage of the culture. Equally, the flowering will be decided by the horticulturist via modifying the duration of the lighting according to the cultivated species. The horticulturist can, therefore, cause the plants to bloom:

  • by reducing the day/night period (occultation)
  • by increasing the day length (photoperiodic lighting)

These occultation and lighting techniques are frequently used in commercial horticulture, especially for Chrysanthemum production.

All of the grow tents available from Hytec Hydroponics have been tried, tested and are trusted by hydroponic experts. Our grow tents are covered in durable and robust canvas, to be entirely hermetic to outside climatic conditions. The interior of the tent is lined with highly reflective material, usually Mylar, but it can also be aluminium or the highly reflective white Orca covering. For additional information on Budbox Tents check out our Product Summary page.

Planned and Respected

Triggering the photoperiod requires 30 times the amount of light energy than that required for photosynthesis. It is in this case, where plants are sensitive to photoperiod, that the system must be meticulously planned and respected.

1.      The darkness must be total and continuous during night phases. It must not have a "parasitic light" that will thwart flowering during the dark period.

2.      A strict nyctyperodic plant (such as some Chrysanthemums) can be maintained in vegetative growth state by applying short cycles of illumination during the dark period: lighting 10 minutes every half hour.

By utilising these methods, the plant will maintain a vegetative growth state and will never flower as a result of the short cycles of illumination. However, "Flashing", which is the action of a lighting a plant during the rest periods of its reproductive stage, will stop flowering or considerably slow down this process to several weeks, and cause the plant to create deformed young leaves.

(Reminder: repetition of lighting and extinction of lamps can significantly decrease plant lifespan)

Shining A Light On The Photoperiod & Light Spectrum - Propagation Tents

Whether using mechanical (notched) or electronic light relays, it is necessary to respect the power and the maximum intensity to be installed on each programmer. The maximum power is of the order of 3600W in 230V. The maximum intensity should be 16A. (U = IR). However, never install the 4 HPS 600W or the 3 HPS 400W on the same programmer.

These lamps require a high intensity at startup, which will far exceed 16A programmers. It is better to "sectorise" the supply of the discharge lamps by using several programmers, which will trigger the ignition at a few minutes/seconds offsets.


Light Spectrum

The visible light of the sun consists of several colours (radiations), which go from violet (wavelength of 380nm) to red (wavelength of 700). It is the "dosage" between these different colour spectrums that gives us the final colour that we perceive. For example, a "mix of paint" of blue paint and yellow paint gives the colour green; whereas the meeting of all the colours produces white light. What we are talking about here is the light spectrum.

As for plants, they only use part of this light spectrum to perform photosynthesis. This is due to the well-defined absorption spectra of the various pigments contained in the plant. These pigments (Chlorophyll A and B, carotene, and phycoxanthine, phycoerythrin in algae) have different sensitivity scales depending on the light wave (radiation, chlorophyll A, for example, is sensitive to red and violet radiation).

The combination of the absorption spectra of the different pigments gives the overall frequency of photosynthesis, i.e. the parts of the light spectrum absorbed by the plant and from which it derives its energy. Plants absorb the red, orange and blue/violet range; whereas on the other hand, there is only a very limited absorption of the green spectrum. It is for this reason that we see plants as green plants (the light ray is reflected on the human eye which then perceives the plant of green colour!)

This range of waves of different colours used by plants is called PAR light, (for Photosynthetic Active Radiation).

Light Control in Hydroponics

Plants, therefore, have specific requirements for the spectral composition of light. This will affect the choice of lamps used for lighting plants as they will have to convert as much as possible the electrical energy into assimilable light.

Shining A Light On The Photoperiod & Light Spectrum - Propagation Tents

The light spectrum should be balanced, with blue/violet and red/orange wave proportions that promote healthy growth while avoiding stem elongation. Indeed, the light spectrum influences the morphology of the plant. The red rays are the most active for photosynthesis but induce excessive elongation: the stem plant! Whereas blue rays favour short internodes.

To characterise the dominant colour of the light emitted by the lamp, we use the Kelvin scale, named after the Irish thermo-dynamicist Lord Kelvin who introduced the notion of "absolute zero" - the theoretical value of -273.15 ° C at which the energy of a molecule will be none existent. For your information, the lowest temperature obtained in the laboratory is 450pK which equals -273,149 ° C ~ close but no frozen cigar!

From this Kelvin scale, we can allocate the lights emitted temperature at being "warm" when lower than 3000K, and "cold" when above 3000K.

Temperature colour is indicated on the packaging of fluorescent lamps and tubes, either directly (2200K for HPS lamps, 4000K for HD) or indirectly on fluorescent tubes and energy saving lamps. You will find this information indicated on these lamps a three-digit number: for example, 827 or 940: the first digit 8 or 9 states the colour rendering index. The next two that indicate the colour temperature: 27 = 2700K Warm Light, and 40 = 4000K Cool Light.

The choice of a lighting system will, therefore, be between several criteria:

  • Light output: electrical energy converted into light energy;
  • The yield in the PAR;
  • The cost of use (the electricity bill will depend on the power used);
  • Lifetime

By utilising Hytec Hydroponics' lighting equipment, you will be able to recreate the conditions of natural light easily. Combined with a supply of water and high-quality nutrients, light activity will allow plants to assimilate carbon dioxide (CO2 = photosynthesis fuel) and release oxygen. Thus, the higher the luminous intensity, the more the plant will grow, remain perennial and produce abundant flowering.



Bulbs which recreate the conditions of natural light include:

Light is essential to the success of your hydroponics. To enhance the intensity of horticultural light, utilising reflectors, such as the maxibright horizon, is a perfect solution. Some bulbs, including HPS and MH bulbs, will need a digital or magnetic ballast - check out the maxibright daylight.  Perhaps you prefer to go to the basics, in this case Hytec Hydroponics offers pre-designed lighting kits for all surfaces.

For more information on Lighting check out our Product Summary page. Additionally, to speak to a Hytec team member please call 01142280024 or email us at

Posted by Hytec

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