Close Menu

VPD: A focus on flowering

Key Takeaways (TL;DR)

Good ol’ Vapor Pressure Deficit, or as the cool kids call it, VPD. The concept, once largely misunderstood in the cannabis industry, is now commonly applied by top-tier cultivators around the world.

The reason VPD is popular for indoor cannabis cultivators is simple in theory: When you understand the relationship between plant transpiration, humidity, and temperature, you can dial in your plant growth.

While the effects of VPD are relevant at all stages, most growers prioritize the concept when their plants are flowering. When VPD is dialed in, growers can adjust levels to bring out brilliant colors, terpenes, aromas, and resin – not to mention they can create bigger, healthier buds. In essence, it’s one way growers can add another custom touch that separates their product from others. Because of that, we get a lot of questions related to VPD and flowering. Before jumping in, though, let’s level set on some high-level VPD knowledge:


At its core, VPD is a numerical value attached to how a plant is responding to temperature and humidity in the environment.

VPD describes how the plant “feels”, and whether it’s holding onto water or pulling water from its roots and out through stomata, where it transpires freely.

If VPD is too low, it means the air is full of moisture and the plant holds onto too much water. Think about a bathtub being full and you have a bucket of water. Since the tub is full, you have nowhere to dump it so you hold onto it. Eventually, like the plant, you become weak.

When the air is dry because of high VPD, the plant transpires too quickly, leading to wilting plants and other issues. For us humans, consider drinking water in the desert where the heat is ungodly and humidity is low. As quickly as you drink the water, it’s out of your system.

Cultivators pay attention to the nuances of VPD because it’s a variable that affects how plants transpire, which in turn affects how cannabis plants develop and grow.

VPD is most often measured in millibars, and the Quest crew uses the chart below as our baseline for each stage:


Propagation/Early Veg: 0.4 to 0.8 kPa; 4.5 to 7.5 mbar
Mid/Late Veg: 0.8 to 1.1 kPa; 7.5 to 10.5 mbar


Two-Step Flower:
Late Veg/Early Flower: 0.8 to 1.1 kPa; 7.5 to 10.5 mbar
Mid Flower/Late Flower: 1.0 to 1.3 kPa; 9.5 to 12.5 mbar

Three-Step Flower:
Late Veg/Early Flower: 0.8 to 1.1 kPa; 7.5 to 10.5 mbar
Mid Flower/Late Flower: 1.0 to 1.3 kPa; 9.5 to 12.5 mbar
Late Flower: 1.3 to 1.6 kPa; 12.5 to 15.5 mbar*
*Beyond 1.3 kPa might be considered VPD high to some growers. For example, a room sitting at 75F (23.9C)/50%RH has a VPD value of about 1.4 kPa (depending on kPa chart); it’s not noncommon to find flower rooms at this condition at some point. 

VPD kPa values when leaf temperature is 1F cooler than air temperature
VPD kPa values when leaf temperature is 2F cooler than air temperature
VPD kPa values when leaf temperature is 3F cooler than air temperature

If you want to go deeper into the basics, check out “Vapor Pressure Deficit and Why It Matters.”


A focus on VPD

As the industry has become more sophisticated and cultivators need to separate their product from the competition, we’ve seen an increased focus on VPD, according to Santo Bonelli of Quest, who works directly with cultivators of all sizes throughout the Pacific Northwest.

Running high VPD in your cultivation (low humidity) during flowering can create beautiful buds, full of various colors. From purples and reds to even blue, pushing a high VPD during flowering is what allows cultivators to bring out those hues.

The reason high VPD has this effect: Cold temperatures and low humidity lead to slow nutrient intake. These stressors emulate fall-like conditions and depending on the plant’s genetics they can inhibit chlorophyll production, letting those colors shine through. When controlled, this is beneficial during flowering.

Additionally, a higher VPD can help create a more resinous (aka, stickiness) and aromatic crop. Depending on your specific goal and the plants’ genetics, these VPD levels can vary.

For that reason, Bonelli likes to work on gradients when recommending VPD levels from early to late flowering. For reference, check out the chart at the beginning of this article.

“Speaking just to relative humidity, we generally recommend starting with 65-70% RH during early flower, 55-65% RH for mid-flower, and ultimately as low as 50% RH or below during late flower,” Bonelli said.

Of course, you need to remember that VPD is only one variable to consider. When using VPD in your cultivation, growers need to pay attention to their specific leaf or room temperatures and humidity.

Without that type of specific data, VPD values offer little guidance. For example, 86F (30C)/75%RH and 68F (20C)/55%RH share similar VPD values, both at about 1.0 kPa (depending on kPa chart), but the room conditions are drastically different. 

The key, as is always the case with VPD, is finding that sweet spot where your plant transpires just enough so it avoids drying out, but also isn’t so full of water that other issues arise.

The downfalls of extremes

of VPD in your cultivation

Like most things, you can go too far with VPD.

In a state of extremely high VPD, where the disparity between RH and temperature is too great, you risk nutrient burn. This occurs when plants transpire too fast, the nutrient solution isn’t adjusted to reflect the high transpiration rate, and excess nutrient collects in the plant tissue, usually first noticed on leaves.

Nutrient burn can destroy plant cells and tissues, leading to necrotic patches. As a preventive measure, plants will close their stomata to reduce transpiration and roll their leaves to mitigate light exposure. If the root system senses there’s little to no water, it’ll close its stomata.

On the flip side, very low VPD in your cultivation (high humidity) makes it difficult for a plant to transpire. If the plant can’t move water up and out of its leaves, a couple of things happen. One, growth can slow because the plant isn’t transpiring correctly and nutrients aren’t continually flowing through the plant. And two, all that moisture becomes a hotbed for disease. Molds and mildews like botrytis love to take hold in a low-VPD situation.

If VPD gets too low, then the moisture gradient becomes incredibly small between the humid air and the saturated leaf tissues. At that point, plants can’t pull water from their roots, up through their shoots, and out through their leaves due to excessively high humidity levels.

A lack of nutrients can lead to deficiencies. - Santo Bonelli, Quest regional sales manager

“Because there’s very little to no transpiration occurring in that type of situation, plants can’t take in CO2 and pull nutrients up from their roots. A lack of nutrients can lead to deficiencies, even in flowering, that can make plant tissues weak and easy for pests, molds and disease to take hold,” Bonelli said.


OK, so high VPD in a cultivation during flowering can bring out fantastic colors and aromas. That sounds pretty great, right?

Well, yes. But again this is why extremes are dangerous for your plants. If you’re pushing VPD high to prevent mold and mildew, do so with caution. Spores can handle extreme conditions on both ends of the spectrum. And when the air gets dry (high VPD), they release spores.

While those spores may not grow in your cultivation during flowering if VPD remains high, they will potentially strike during your next grow cycle unless your room-cleaning protocol is near medical grade.

“VPD high is worthwhile when trying to stop the spread of things like botrytis, but not as much so for prevention,” Bonelli said. “For prevention, we like to look at both proper dehumidification and airflow in, above, and below the plant canopy to ensure RH levels are within acceptable ranges.”