I try to stay close to it except after about week 4 in flower because my buds in a 4x4 can’t handle that high of humidity. Would need 2-3 more fans and I prefer 2-3 more plants.
I try to stay around 60% RH.
That chart is more for veg VPD #s.
Here's the formula. Start at 0 (left corner)
How you can influence and change VPD:
- Temperature
- Increase temperature (run a heater or reduce AC): increase VPD
- Decrease temperature (increase AC): decrease VPD
- Humidity
- Increase humidity (run a humidifier): decrease VPD
- Decrease humidity (run a dehumidifier): increase VPD
- Light Intensity
- Increase light intensity (move lights closer, etc): increase leaf temps : increase VPD
- Decrease light intensity (move lights farther, etc): decrease leaf temps : decrease VPD
Ideal VPD for Different Stages of Growth
So now you know what VPD is, how it impacts plants, and how you can change VPD. All that’s left is to figure out how to use this information in your own grow. Ideal VPD, as a general rule for plant growth, is around 0.8 – 1.2 kPa (kilopascals). However, your plants have different needs during different stages of growth. You need to tailor your grow environment to the stage of growth that your plants are in. Below are some general recommendations, but keep in mind that these recommendations can vary strain to strain, and setup to setup. As always, observe your plants and tweak things accordingly.
- Ideal VPD for Clones
- Clones are baby plants, they can’t handle a lot of stress because they are still just trying to form roots. Target a higher humidity and VPD closer to the lower end of the general range.
- The ideal VPD for clones is closer to 0.8 kPa.
- Ideal VPD for Vegetative Stage
- In veg (vegetative) stage the plants are bigger and more robust. You can reduce the humidity in your environment to increase VPD. This will increase water and nutrient uptake, but you don’t want to increase VPD too much. This will cause the stomata of the plants to close, causing them to absorb less CO2. CO2 is particularly important in the vegetative stage, because that’s the main ingredient that plants use to grow large.
- The ideal VPD for the vegetative stage is close to the middle of the general range, around 1.0 kPa.
- Ideal VPD for Flower Stage
- In flower stage the plants are robust, but the flowers are sensitive to various issues. You need to avoid excess humidity.
- The ideal VPD for the flower stage is closer to the top end of the range, 1.2kPa – 1.5kPa.