Here is a chart showing the VPD levels recommended by Perfect Grower. The top of this chart is the RH or Relative Humidity in your growing environment. The left side of the chart is the temperature of the air in and around the canopy of the plants.
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. Anything in the green zone is optimal for the vegetative and flowering stages of growth.
VPD is a control dial for evaporation and plant transpiration. 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 of 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 garden.
If your 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 spotting will begin to occur on the leaves, starting at the top and moving its way down. Low VPD environments are also notorious for breeding fungal diseases like powdery mildew.
Great targets to shoot for in any growth stage are:
Lights on:
- Temperature: 75-78F
- Relative Humidity: 60-65%
Lights off:
- Temperature: 70F
- Relative humidity: 60-65%
It is important to make sure relative humidity doesn’t go over 65% or below 50%. If you can’t get your growing environment close to the targets above, use the chart to find a combination that gets you as close to the green area as possible.