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PAR and Visible Light Spectrum- The spectrum of light that is visible to the human eye is approximately 380-780 nm (nanometers). Interestingly, plants also use almost the same spectrum of light visible to the human eye. This part of the electromagnetic radiation, which powers photosynthesis, is named PAR (Photosynthetically Active Radiation) and is usually found in wavelengths from 400 to 700nm. PAR is often confused as a type of measurement, but is more the designation of a region of light within the spectrum that is usable to plants.

PPDF- (Photosynthetic Photon Flux Density) is a measurement that is very important to your plants. Unlike lumens which will only measure the brightness of a light source, PPFD measures the number of photons within the PAR region that reach the target each second. Simply put, a PPFD measurement will tell you how much necessary light (within the PAR region 400-700nm) is actually making it to your plant.

Growth stages of Green Plants:

There are also generally 4 growth stages in green plants that have different spectrum requirements.

  • Vegetation – In Vegetation (VEG) stage, rapid, healthy overall plant and root growth is desired, and in general most growers desire maximum growth but with shorter compact plants with short inter-nodal spacing preferred.
  • Pre-flower – Pre-flower is the period from when the 12/12 flower cycle is first initiated, to roughly the end of the second week (in an 8-week flower), or until the small flowers are prevalent and the rapid growth stretch slows. Again, for most growers, the desire in this stage is to maximize SIZE, while limiting stretch.
  • Flower – The peak Flower period is generally from week 3-7 and is the time when the plant (stem / leaf) growth stops and all the plant energy focuses on flower production. Maximum flower matter size and good structure is generally the goal here.
  • Ripen or Finish – The Ripen period is generally from week 7 to finish (in an 8-week flower) where the Flower growth, (i.e. size) slows and plant energy refocuses on resin and terpene production. This is the period where the flower acquires a significant portion of it’s density, ie. resin content. This transition is not clearly defined, and some strains have big increases in resin production during this period, and others not as much.

Heat circulation - HPS vs LED

HPS explanation:

The beam heat from HPS will raise the temperature in the top of the greenhouse. This heat will circulate to the temperature sensor, which will close / reduce the bottom heat. This results in “cold roots”, which affects the crop’s growth. To avoid this situation, the windows open, temperature is dropping and allowing the temperature sensor the opportunity to open the bottom heat. As a result, the open windows will cause loss of heat and CO2.

LED explanation:

At a working temperature at only 45°C for the Senmatic LEDs, the climate condition for the crop is significant more comfortable. Bottom heat can be active under all conditions, to secure active roots and resulting in optimum growth conditions. The significant lower working temperature of the LED fixtures allows less ventilation and protect heat and Co2 loss, too.