vapor pressure deficitPrint out this chart and put it on your marijuana grow room wall.

Vapor Pressure Deficit Vs. Relative Humidity for Your Marijuana Grow Room

Why should indoor marijuana growers understand vapor pressure deficit?

Because vapor pressure deficit affects whether your plants can breathe or not!

Vapor pressure deficit readings (VPD) tell you precisely how atmospheric factors are affecting your cannabis plants’ ability to take in and transpire water.

The pressure differential between the inside of your plants and their outside atmospheric environment impacts the rate at which your plants transpire moisture into the air.

If your grow room VPD is wrong, your cannabis plants experience massive stress that severely lowers the size and quality of your harvest.

Vapor pressure deficit affects the ability of your plants to breathe.

Your marijuana plants breathe through mouth-like openings in the bottom of their leaves called stomata.

Stomata inhale carbon dioxide (C02).

They exhale oxygen and moisture.

A high vapor pressure deficit means grow room air has the capacity to hold a lot more water than is already in it.

Your marijuana plants will transpire more water out of their tissues into the air.

A low vapor pressure deficit means that grow room air is at or near moisture saturation.

Your marijuana 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 troubles such as:

  • Your plants shutting down C02 intake photosynthesis stops or slows down.
  • Your plants taking in too much water, so they take in too many nutrients and get burned.
  • Your plants unable to transpire enough water, so their metabolism, water, and nutrients intake are negatively affected.
  • Your plants’ leaves becoming susceptible to molds, fungi, pests, and other problems.
  • Clones and seedlings struggle or die.
  • Buds get moldy.
  • Powdery mildew spreads like wildfire.

Plant harm and even grow room disasters may arise due to improper vapor pressure deficit.

Growers who don’t know about vapor pressure deficit might think their plants are wilting or overwatered.

They might think their plants are overfed or underfed.

They might think their plants are sick, because growth has slowed.

They might think there’s a disease sapping their plants’ growth rate and energy.

The good news is you can control VPD and you’ll see much better performance from your marijuana plants.

You’ll spare them plant stress that can cost you harvest weight and potency.

Here’s a precise chart showing you the ideal vapor pressure deficit zone for various temperatures in your marijuana grow room:

vapor pressure deficit

See the green zone?

That’s where you want your cannabis grow room relative humidity to be.

Let’s use an example:

It’s 77°F at canopy level and you’re in late grow phase.

The chart shows a range of 65-80% relative humidity to maintain the optimum vapor pressure deficit.

If you have fat Indica buds, you’d choose 65% relative humidity for your grow room, to minimize risk of gray mold.

Another example…

You’re running C02 and your leaves at canopy level are 84°F.

You want your relative humidity to be 70-85%, going towards the lower level if you have fat, dense buds on your plants.

Lowering relative humidity is easy—you use a dehumidifier.

But in many cannabis grow rooms (especially in the western United States), relative humidity is too low, creating an unfavorably high VPD.

The high VPD means the pressure inside your cannabis plants is much higher than in the outside air.

Your plants will transpire lots of moisture into the air.

It can dry out their stomatas.

It can make them suck in so much water that they overdose on nutrients.

It creates stress responses that use up your plants’ energy and metabolism trying to deal with high VPD rather than growth and bud production.

To add moisture into the air, cannabis growers use grow room humidifiers.

A key feature of using the vapor pressure deficit chart is to measure your marijuana leaf temperatures.

Instead of relying on a general thermometer reading for the entire room, use a handheld digital thermometer such as this one to measure the temperature at the bottom of your leaves.

You could also consider using this Growtronix vapor pressure deficit meter, rather than relying on the chart.

As with most marijuana grow room tactics, conditions, and materials, you monitor, modify, and customize your use of vapor pressure deficit.

Take into account whether your grow room or strains are susceptible to pests, gray mold, powdery mildew or other problems affected by relative humidity.

Do your best to control relative humidity to achieve optimized vapor pressure deficit, without creating conditions that assist gray mold, powdery mildew, etc.

Be sure to watch the YouTube videos embedded in this article.

When you master vapor pressure deficit, you’ll see way better plant performance!

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