grow cycle growers know
You Down With
VPD ?
Knowing the temperature and relative humidity levels in your growroom can help you better grasp your plants ’ transpiration levels . Knowing the vapor pressure deficit , however , gives you an even better picture of what is happening directly around your plants ’ stomata .
by Austin Yeany
While many indoor gardeners grow successfully with only a simple hygrometer measuring temperature and relative humidity , they would be better off knowing the vapor pressure deficit ( VPD ). This measurement describes the difference between the actual moisture content in the air and the moisture content that the air could hold when saturated . In terms of plant growth , it relates the difference between the pressure inside the leaves and the surrounding atmosphere . It ’ s kind of like rolling temperature and humidity into one measurement to get a clear picture of what ’ s going on directly around the stomata , as opposed to just knowing what the conditions are like in the room itself . More specifically , VPD provides a clearer insight into how much water is required by the plants to maintain its stomata moisture levels . The higher the VPD , the greater the drying effect the air has on the plant ’ s ability to transpire . The key is making sure that everything is sitting in the right ranges . In most cases , a certain amount of pressure deficit between the leaves and the outside air has a positive effect because some level of transpiration from the stomata is needed to promote the water and nutrient flow from the roots to the rest of the plant . If the VPD is too high , however , plants will lose too much water through the transpiration and the stomata will close . Alternatively , a low VPD indicates that the air is holding a lot of water , which slows down the plant ’ s transpiration rates . Transpiration rates that are too low will also slow down the movement of water and nutrients from the roots , inevitably causing problems leading to nutrient deficiency issues . As VPD is a pressure reading , it ’ s usually written in millibars ( mb ), though you may also see VPD expressed in kilopascals ( kPa ). Converting between the two is easy enough ; just divide the millibar figures by 10 to get an appropriate kilopascal measurement . For low transpiration , VPD usually ranges between four to eight millibars , or 0.4 to 0.8 kPa , and is typically seen cutting into veg . Healthy transpiration for flowering is eight to 12 mb , or 0.8 to 1.2 kPa . High transpiration , which usually occurs during peak flowering times , ranges from 12 to 16 mb , or 1.2 to 1.6 kPa . To calculate VPD , you ’ ll need a temperature probe , a humidity probe , and an infrared pocket thermometer to take a reading from the leaf temperatures at the top of the canopy . Yes , you could take all the readings and do the subsequent ( somewhat complicated ) calculations yourself , but the easiest way of getting accurate VPD readings is to purchase a smart controller set-up that can do all the measuring and calculating for you . Once you ’ ve got your readings , you can then make the relative changes to your set-up to keep the VPD in the perfect spot . This usually means adjusting your humidity with the use of humidifiers or dehumidifiers , adjusting your atmospheric temperatures to ensure the room is cool or warm enough , and / or adjusting your plant canopy temperature , which can be done by simply lifting or moving the lights closer or further away from your plants .
Knowing the VPD allows you to keep your growroom in its sweet spot for your plants , ultimately maximizing your yields .
144 grow cycle