As a new grower, I was aware that my temperatures and humidity should be moderate, and (ideally) stable–but I had no idea the extent their relationship impacted my plants.

Vapor Pressure Deficit (VPD) measures the difference in vapor pressure between the inside and outside of a plant.

Often, VPD is used to refer to the relationship between humidity and temperature in your garden. While this is not technically what the deficit refers to, these are the levers you push and pull to dial in a garden's VPD.

VPD affects photosynthetic and transpiration rates–how your plant makes energy and releases moisture.

If your environment is too hot and dry, your VPD will be too high, and your plants are working overtime to transpire.

Conversely, if your VPD is too low, your grow is too cold and humid, and your plants are unable to transpire properly, which can lead to moisture issues such as rotting.

If your plant is working against your environment, it's fighting an uphill battle, costing you quality and yields.

Dialing in your VPD guarantees increased photosynthesis activity, but not necessarily the healthiest or most resinous plants. And the healthiest plants are not necessarily the biggest yielders.

For this reason, VPD is best used as a guideline, not a rule.

As you get to know your garden, goals, and genetics, you'll learn to know what's best, even if it deviates from optimal VPD.