For those wanting to really take their grow to the next level, optimizing your climate control with a VPD Chart is the next step.
This is something that we did not start practicing until recent years. But it makes a big difference in the outcome of our cultivation.
In this article, we will explain:
- What is a VPD Chart; and
- How to use the VPD Chart with cannabis
- Best VPD ranges for Propagating, Vegging, and Flowering stages of cannabis plants.
What is a VPD chart for cannabis?
VPD stands for vapor pressure deficit. It comes from the scientific discipline of physics, and is a unit of measurement that relates to the interaction between temperature and humidity. It is really deep science stuff, and is way over my pay grade to give you a proper scientific explanation.
The good news is that you don’t need to understand all of the science involved, and I don’t need to explain it. All of the benefits can be achieved by following a simple VPD chart, such as the one below:
Notes on using a VPD Chart for growing weed
So the idea behind using a VPD chart for growing cannabis is that there is an optimal RH% to match to your chosen temperature – or vice versa.
If you are stuck at a certain temperature, due to equipment limitations, for example, you can use the VPD chart to achieve the best outcome for that given temperature zone.
This means if you have an A/C unit go down and a spike in your temperature, you can adjust your humidity to achieve the best outcome at that hot temperature.
Once you really dive in and start applying the VPD Chart to your cultivation schedule, it will really change how you view temperature and humidity. You will start to realize that these two metrics will need to be in sync at all times. But by syncing these metrics, according to the VPD chart, you can achieve better outcomes than when you just allowed chance and circumstance to rule your environment.
A really important note about VPD: it refers to leaf temperature, not room temperature. And surface leaf temperature is generally higher than what you read on your thermostat.
VPD chart for cannabis propagation
During propagation of new clones, you will want higher temps and higher humidity. We like to stay in the range of 0.5 to 0.6, with a median or average around 0.55. You can see on the chart below, that puts us at around 81 to 84 degrees F, with a corresponding humidity of 85% for those temperature ranges.
VPD chart for the Vegetative Stage
For Veg, we want to continue the warmer temps. This requires a corresponding amount of humidity. We like to keep our VPD in the range of 0.7 to 0.9 during the vegetative stage, and keep temperatures in that 72 to 81 degrees Fahrenheit.
VPD chart for the Flowering Stage
Now for the flowering stage, we are going to move through this chart and use several different VPD points as we progress through flower.We like to start off during the initial weeks of flower mimicking our environment from Veg. And as we progress through the development of the flowers, we will adjust the environmental controls for the room.
For early flower, we stay in that same range you use for veg, which is 0.7 to 0.9, up to 0.1. Our aim is to keep temps and humidity a bit higher.
For the middle stage of flower (think weeks 3 through 6), we are getting the VPD above 1.0, with a target of around 1.3 as you get later into flower. You will want to end flower with a VPD of around 1.3 or so.
As we start to near the end of flower, we like to aim for lower temps, lower humidity, while still maintaining a VPD of 1.3. That means, if we want to really drop it down low in those last two weeks to, say, 67 or 68 degrees Fahrenheit, we have to drop humidity down to 45%. If you want to keep your temperature at 72 F, then you would stay at 50% RH, or even a bit higher.