The chart below shows VPD levels that are widely accepted and recommended for growing cannabis. The top of this chart is the RH or Relative Humidity in your grow room. The left displays temperature in both F and C of the air in and around the canopy of the plants. VPD (Vapor Pressure Deficit) is a control value for evaporation and plant transpiration.
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The above values are measured in KpA or kilopascals. The higher the value in the chart, the higher the VPD is. The lower the value, the lower the VPD. Ideally, you want your numbers to be in the green zone for optimal vegetative and flowering growth.
The higher the VPD, the faster the plants will transpire, and the quicker your root zones will dry out. In high VPD situations, nutrients will accumulate faster and plants will favor water over food. If you’re stuck in a high VPD situation and can’t fix it, it’s always better to feed on the lighter side of things to avoid over-feeding your plants.
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If VPD is too low, plants will start showing Calcium deficiencies and more Calcium will not fix the problem. In low VPD environments plants struggle to move Calcium through their tissue and spots will begin to occur on the leaves, starting at the top and moving down. Low VPD environments are also notorious for breeding fungal diseases like powdery mildew.
The ideal VPD for cannabis clones is around 0.8kPa
The ideal VPD for cannabis in the vegetative stage is around 1kPa
The ideal VPD for cannabis in the flowering stage is 1.2 – 1.5kPa
*Important Note: Prevent relative humidity from going over 80% or below 50%. If you are unable to hit your desired targets, use the chart to find a combination that gets you as close to the green area as possible. Most growers understand that during the veg stage of growth a higher humidity is acceptable and if you are able to control your environment with both a humidifier and dehumidifier, we suggest you shoot for a higher VPD during the vegetative growth stage.