How to Read a Cannabis VPD Chart

The environment surrounding a cannabis plant dictates how it can breathe and, ultimately, how well it can grow. The transpiration and respiration of a plant allow water to flow from the media, through the plant, and into the atmosphere. This osmotic flow enables roots to efficiently uptake water and nutrients, and transport them throughout the plant to promote new growth. When temperature and humidity are kept at the proper levels, plants can easily absorb carbon dioxide (CO2) and release oxygen into the atmosphere. Understanding the relationship between these parameters gives cultivators insight into the vapor pressure deficit (VPD).

The VPD affects the rate at which water moves through a plant and into the environment. By using this parameter to understand the relationship between temperature and humidity, cannabis plants can be steered through their stages of growth to promote vigorous development and big yields across a variety of growing environments.

The role of temperature in the grow

Both air and leaf temperature directly affect transpiration rates. When the temperature becomes too hot, plants begin to transpire too quickly. To combat this, a stressed plant will begin to fold its leaves into a taco shape. This is known as leaf curling; it’s a plant mechanism that helps reduce water loss through transpiration. 

High temperatures may also cause plants to stop absorbing water and slow growth. This happens because the stomata close, and the plant stops transpiring to protect itself.

Cool temperatures will slow plant transpiration. The air can quickly become fully saturated or reach 100% relative humidity, especially in the colder temperatures at night. This creates an environment ideal for fungal and bacterial growth, something incredibly detrimental to plant health. 

When conditions are too cold, leaves will cup to help protect themselves. This helps reduce the leaf surface area that is exposed to cold and is also a mechanism to slow plant transpiration. The same can also occur if the temperature is too high: leaves will curl to help stop transpiration. 

By controlling the temperature throughout the growth cycle, cultivators can steer plant growth. During the seedling and vegetative stages, temperatures are maintained between 75°F and 85°F. During flowering, temperatures can be lowered to 65-75°F. Cooler temperatures are used during late flowering, motivating plants to finish at their full genetic potential.

The perfect humidity for cannabis

Humidity is the ratio of how much water the air can hold at different temperatures. It’s often measured in different ways, but cultivators typically use relative humidity (rH). This measure represents the ratio of the amount of water vapor present in the air to the maximum amount of water vapor the air can hold at a given temperature. 

High relative humidity may cause plants to stop transpiring, creating ideal conditions for fungal and bacterial growth. Maintaining optimal humidity levels is crucial for supporting vigorous growth throughout a plant’s life cycle.

During seedling and vegetative growth, humidity should be maintained between 60% and 70%. During flowering, humidity should be lowered to around 50%. To eliminate the guesswork, cultivators typically utilize environmental sensors to monitor temperature and relative humidity to maintain vigorous growth while preventing diseases.

Using a cannabis VPD chart

Vapor pressure deficit (VPD) is where temperature and humidity collide. VPD is calculated by subtracting the actual vapor pressure from the saturation vapor pressure. The actual vapor pressure is the amount of moisture in the air. The saturation vapor pressure is how much water can be in the air at the current temperature. Pressure is measured in kilopascals (kPa), a standard unit of pressure defined as one newton per square meter.

High-VPD cannabis indicates the air can hold more water, and that a plant is transpiring quickly. This means a plant quickly absorbs water with fast nutrient uptake, requiring more frequent watering and feeding. A high-VPD range occurs when temperatures are high and relative humidity is low.

When VPD is low, the air is near its water-holding capacity. A low-VPD indicates high humidity and cool temperatures. When this occurs, transpiration slows, and plants require less watering and reduced nutrients.

The ideal VPD for seedling growth is 0.6 to 1.0 kPa. The optimal VPD for vegetative growth is 0.8 to 1.2 kPa. In flowering, it is 1.2 to 1.5 kPa.

Adjusting the relative humidity in the grow room

To adjust the grow room humidity, you either need to remove water from the air or add water to the air. Humidifiers will add water to the air by spraying a fine mist or fog. Dehumidifiers pull air in that passes across its cooled coils; the cool air causes the moisture to condense, which is then collected in the machine’s drain for removal. In addition to removing moisture, a dehumidifier will also add heat to the space 

The importance of environmental monitoring

Cannabis thrives on consistency, so preventing large swings in any parameter can greatly improve plant yields. Maintaining a consistent environment, both day and night, allows the plant to transpire and respire as needed. When a plant can breathe easily, its stomata can fully open and close, allowing CO2 to be taken in during the day and waste O2 to be released at night. Easy breathing allows nutrients to be taken up and water to move freely from the media through the plant. 

Sensors should be used to monitor the ambient air temperature, leaf temperature, and actual humidity. Cultivators can use these parameters to calculate VPD, and online tools can be utilized to help with this calculation. Cropsalt is a popular and easy-to-use option for VPD calculations.

Ideal environment = ideal crops

Temperature is the underlying force that controls how quickly plants transpire and how much water the air can hold. Humidity indicates the amount of water currently present in the air. Low temperatures correspond to a reduced water-holding capacity and lower plant transpiration, whereas high temperatures increase water-holding capacity and plant transpiration.

The VPD is the relationship between air and leaf temperatures and actual humidity, providing a key indicator of how plants transpire. By managing the relationship between temperature and humidity, cultivators can increase growth and overall plant yield.

FAQ

What is the optimal VPD for propagation, vegetative growth, and flowering?

A low VPD, around 0.7 kPa optimal for propagation, for vegetative growth around 1 kPa, and for flower around 1.3 kPa.

Is VPD related to humidity?

Yes, VPD is a measure of the correlation between temperature and humidity. A higher humidity indicates a lower vapor pressure deficit (VPD). 

What happens if my VPD isn’t what it’s supposed to be?

Plants will struggle to transpire. This can cause nutrient deficiencies and watering issues because the plant won’t be able to breathe properly if the VPD is incorrect.