Why should indoor cannabis growers understand vapor pressure deficit?
Because vapor pressure deficit (VPD) affects whether your plants can breathe, grow, and thrive!
You see, vapor pressure deficit readings tell you how much moisture is currently in your grow room’s air versus the maximum amount of moisture that air can hold. And this difference directly affects the transpiration rate of your plants: i.e., how much water they’re releasing into the air.
If your grow room VPD is off, your cannabis plants experience massive stress that severely lowers the size and quality of your harvest.
Why VPD Matters
Your cannabis plants breathe through mouth-like openings in the bottom of their leaves called stomata.
These stomata inhale carbon dioxide (C02). They exhale oxygen and moisture.
With an optimal VPD, your plants will open their stomata and transpire faster, fueling photosynthesis for vigorous growth. By transpiring at the optimal rate, your plants will also be able to increase nutrient uptake. And of course, open stomata means a greater amount of CO2 can be absorbed, leading to better overall development.
As you can see, achieving the right VPD is critical to maximizing plant performance.
What Happens When Your VPD Is Too Low or Too High?
A high vapor pressure deficit means your grow room air has the capacity to hold much more water than what’s already present…
And your cannabis plants will transpire more water out of their tissues into the air.
A low vapor pressure deficit means that your grow room air is at or near moisture saturation…
And your plants will be much less able to transpire water out of their tissues into the air.
Too high or too low vapor pressure deficits cause major problems, such as…
- Your plants shut down C02 intake, slowing down or stopping photosynthesis altogether.
- Your plants take in too much water, causing potential nutrient burn.
- Your plants transpire less, negatively affecting their metabolism, water, and nutrient intake.
- Your leaves become susceptible to molds, fungi, pests, and other problems.
- Clones and seedlings struggle or die.
- Buds get moldy.
- Powdery mildew spreads like wildfire.
As you can see, improper vapor pressure deficit can lead to plant harm and even grow room disasters.
And even worse, growers who don’t know about vapor pressure deficit might misdiagnose the problem.
For example, they might think their plants are wilting or overwatered…
Overfed or underfed…
Or sick from a disease that’s sapping them of their growth rate and energy.
Well, here’s the good news. You CAN control VPD — and when you do, you’ll see much better performance from your cannabis plants…
Because you’ll spare them plant stress that can cost you harvest weight and potency.
Pinpointing Your Optimal Vapor Pressure Deficit Zone With a VPD Chart
Below is a VPD chart for cannabis showing you the ideal vapor pressure deficit zone for various temperatures in your grow room. The temperature indicates the maximum amount of water your grow room’s air can hold, while the relative humidity indicates how much water vapor is currently present in the atmosphere.
Take a look below.
The “green zone” is where you want your cannabis grow room relative humidity to be.
Imagine it’s 77°F at canopy level and you’re in the late grow phase.
According to this VPD chart, you’ll want a range of 65-80% relative humidity to maintain the optimal vapor pressure deficit.
And if you’re growing fat Indica buds, you’d choose 65% relative humidity for your grow room, to minimize the risk of botrytis and other pathogens.
Or say you’re running C02 and your leaves at canopy level are 84°F.
You want your relative humidity to be 70-85%, approaching the lower level if you have fat, dense buds on your plants.
How To Adjust Relative Humidity In Your Grow Room
Lowering relative humidity is easy—you use a dehumidifier.
However, in many cannabis grow rooms (especially in the western United States), relative humidity is too low, creating an unfavorably high VPD.
Simply put, a high VPD means the pressure inside your cannabis plants is much higher than in the outside air.
Because of this, your plants will transpire an abundant amount of moisture into the air, which can…
- Dry out their stomata.
- Cause them to uptake so much water that they overdose on nutrients.
- Trigger stress responses that use up your plants’ energy and metabolism trying to deal with high VPD rather than focusing on growth and bud production.
To add moisture into the air, cannabis growers use grow room humidifiers.
And keep in mind, the VPD chart also enables you to…
Measure Your Cannabis Leaf Temperatures
Remember, the temperature will tell you how much water your grow room air can hold.
Instead of relying on a general thermometer reading for the entire room, use a handheld digital thermometer to measure the temperature at the bottom of your leaves.
You can also consider using a vapor pressure deficit meter rather than relying on a grow room temp and humidity chart.
As with most cannabis grow room strategies and conditions, it’s crucial to monitor and adjust your use of vapor pressure deficit in order to achieve optimal results.
Take into account whether your grow room or strains are susceptible to pests, gray mold, powdery mildew or other problems affected by relative humidity… And do your best to control relative humidity to optimize vapor pressure deficit, without creating conditions that assist gray mold, powdery mildew, etc.
It may take time to get it right, but once you master your vapor pressure deficit, your plants will reward you with optimal performance and a much more satisfying harvest.
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