HorticultureKeymasterNovember 15, 2016 at 12:20 pmPost count: 98
Recent advances in LED technology for creating luminaires that have increased light output and idealized spectra have enabled cannabis growers to reduce operating costs and improve their yields qualitatively. But when growing indoors we have to remember that with all these lights, we are trying to imitate the sun and the amount of heat that the sun delivers to the atmosphere is as important as any of the other wavelengths we want to our crops to bask in.
The heat of the sun not only ‘moves’ the atmosphere by heating air, but it also creates vapour pressure deficit (VPD). Removing heat from the indoor growing environment reduces this essential feature of the indoor atmosphere; we need VPD to be high or plants will not grow as fast due to reduced transpiration. Indoor growers can use fans to simulate the wind, but blowing air around does not increase VPD. VPD is the temperature-dependant water holding capacity of the air. VPD is like the ‘accelerator pedal’ that drives plant growth; if plants do not lose water to the atmosphere, they won’t grow much, they will just idle along.
Vapour pressure deficit can be seen as the capacity of the air in the grow room to “suck water out of the plants” and induce transpiration. A plant that transpires 10 liters of water a day to the air in a grow room will grow 5 times faster than a plant that loses only 2 liters of water. This movement of water occurs because of the high VPD of the air; we want the air to be drier for plants to transpire. The interested reader can review VPD concepts here:
VPD is increased when the air is less humid and warmer. LED lights simply do not warm the air enough to create an indoor atmosphere with high VPD. The grow room air will be cooler and more humid when growing with LEDs compared to using HID lamps. The Humidity can be reduced by using dehumidifiers, and these machines will slightly increase the air’s VPD as they run and collect condensate. However air conditioners are much more powerful at drying air, so when these machines run they increase the VPD of the grow room air much more than dehumidifiers alone.
Simply put, the result of using LED lamps instead of HIDs for indoor growing is that the air conditioners will operate less and therefore won’t dry the air as much. Even though this saves money, by operating the air conditioners less, growers will reduce VPD and slow the movement of water through the plants, and thereby slow crop growth rates.
A report by a manufacturer of LED luminaries has described how growing with LEDs requires an input of heat to maximize growth rates:
Crops that transpire less may have constantly wetter roots, will not uptake mineral nutrients from the soil as much and will grow less biomass than crops which lose a lot of water to the atmosphere. The best yields come from crops grown under high transpirational demand (ie; high VPD), which happens under warmer and drier air.
Some examples of this can be found in these links, where using HID lights induced more transpiration compared to LEDs and thereby crops had greater yields. In both cases the greatest growth and biomass at harvest was correlated to the crops having used more water in transpiration:
Cannabis growers who switch to LEDs will need to consider how much the crop is transpiring; if LEDs reduce the run-time of air conditioners, and thereby allow the relative humidity to increase, VPD is diminished and crops grow slower. To correct this simply warm and dehumidify the air to drop the relative humidity as low as 35-45%, while providing roots with unlimited water. LEDs are able to deliver a full and intense PAR spectrum of light for plant growth, but with less heat output growing with LEDs alone will lower VPD and slow growth.
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