A Complete Guideline of VPD for Indoor Growing
As an indoor gardener, you may have come across the term VPD (Vapor Pressure Deficit). If your plants are struggling despite maintaining ideal grow tent temperature and humidity, VPD could be the missing factor. In this guide, we’ll break down what VPD is, why it’s important, and tips to maintain optimal VPD. Let’s get started!
What Is VPD
Vapor Pressure Deficit (VPD) is the difference between the current amount of moisture in the air and the maximum amount of moisture the air can hold when it is fully saturated at a given temperature. This measurement is expressed in kilopascals (kPa) and reflects the dryness of the air relative to its temperature.
How Does VPD Affect Plants
VPD is a crucial factor in plant growth, as it directly influences transpiration, nutrient uptake, and overall plant health. When VPD is within the ideal range, your plants can breathe easily and absorb nutrients efficiently.
However, incorrect VPD can negatively impact plant health. A VPD that is too high causes excessive water loss, leading to dehydration. Conversely, a VPD that is too low creates overly humid conditions, which can hinder the plants’ breathing, cause root rot, and encourage fungal growth.
VPD is especially important in grow tents because the closed, small volume of air allows heat and moisture to build up quickly. Plant transpiration, lighting systems, and ventilation can all cause significant environmental fluctuations in a short period.
What Is the Ideal VPD Range of Every Growth Stage
For indoor growers, Leaf Vapor Pressure Deficit (LVPD) is more important than the room’s VPD. Due to transpiration, plant leaves are typically 3°F to 5°F cooler than the surrounding air, resulting in a slight difference between LVPD and the room’s VPD. This cooling effect, caused by foliar evaporation, makes LVPD a more accurate measure of conditions directly affecting plant growth.
To determine the optimal VPD, you should consider the plant’s growth stages. Here are the recommended Leaf Vapor Pressure Deficit (LVPD) levels for each stage of the plant's life cycle. Note that variations may occur between different strains.
Propagation - Early Veg Stage
- Temperature: 70°F(21.1°C)
- Relative Humidity: 60%
- Min-Max Leaf VPD: 0.8-1.0kPa
- Goal: Maintain a relatively low VPD with evenly distributed humidity and avoid localized moisture buildup.
Late Veg - Early Flowering Stage
- Temperature: 75°F(23.8°C)
- Relative Humidity: 50%
- Min-Max Leaf VPD: 1.0-1.2kPa
- Goal: Maintain a moderate VPD with stable temperature and humidity.
Mid - Late Flowering Stage
- Temperature: 75°F(23.8°C)
- Relative Humidity: 40%
- Min-Max Leaf VPD: 1.2-1.6kPa
- Goal: Maintain a higher VPD and avoid excessive humidity at night, which can increase the risk of mold and mildew.
How to Calculate VPD
The formula for calculating VPD involves knowing the temperature and relative humidity in your grow tent. While there’s a more complex formula to calculate it manually, using a VPD calculator is the easiest method. We will provide you with an online VPD calculator for the following - you just need to enter the temperature and relative humidity to calculate the LVPD:
In addition, we also provide a comprehensive set of VPD charts, specifically designed for growth and flowering stages. You can simply take your measurements and refer to the appropriate VPD chart adjusted for leaf temperature. These charts are a powerful tool to help you maintain optimal growing conditions for your plants at each stage of growth.
How to Control VPD in a Grow Tent
How to Lower VPD in a Grow Tent
As we have already explored the concept of VPD, it follows that when VPD rises above the ideal range, raising humidity and regulating temperature will effectively bring it back down.Setting Up a Plant Humidifier
A plant humidifier adds moisture to the air to maintain stable humidity, preventing dry conditions and helping keep VPD in the optimal range. The Mars Hydro Plant Humidifier helps create a consistent humidity environment. Ultrasonic Technology: The cutting-edge ultrasonic system lets you craft the ideal microclimate with pinpoint precision. Mist, timing, humidity—everything fine-tuned to perfection.
Precise Humidity Control: Connect external temperature & humidity sensors to keep a constant eye on your environment, automatically adjusting mist output to maintain flawless conditions.
Smart Control: Connect to the Mars Hydro iControl system and MarsPro app to unlock advanced features for remote monitoring, scheduling, real-time data, and centralized control.
Unmatched Consistency: Stable humidity reduces plant stress, improves water and nutrient uptake, and supports healthier growth, bigger yields, and better-quality flowers.
Installing a Smart Fan
While increasing humidity is the primary way to lower VPD, improving airflow also helps reduce localized heat and maintain a more stable VPD. The Mars Hydro iFresh Smart Fan delivers consistent air circulation for healthier plants.
Smart Climate Monitoring: Connect to the MarsPro app with the iFresh Add-On Pack to monitor real-time VPD and adjust temperature, humidity, and fan speed remotely.
Whisper-Quiet Yet Mighty: The Mars Hydro fan delivers robust airflow that keeps your plants thriving while operating so quietly that you’ll hardly notice it’s running.
Consistent Airflow: The Mars Hydro Ventilation Fan distributes air evenly across your entire grow space, ensuring each plant receives fresh, balanced airflow.
Advanced Mixed-Flow Design: This fan combines the power of axial and centrifugal fans, delivering superior airflow with steady pressure.
Using LED Grow Lights
Unlike traditional lights that generate excessive heat, LEDs run cooler, allowing you to maintain the ideal temperature and humidity balance. This means your plants grow stress-free in the perfect environment, leading to bigger yields and better results.
Additionally, consider running your grow lights during the night and turning them off during the day to reduce heat production. This takes advantage of cooler nighttime temperatures to help prevent overheating in your grow tent.
How to Increase VPD in a Grow Tent
When VPD is below the ideal level, lowering humidity and controlling temperature can effectively increase the VPD.
Using a Dehumidifier
A dehumidifier removes excess moisture from the air to maintain optimal humidity in your grow space. The Mars Hydro Compressor Dehumidifier provides powerful, efficient moisture removal for stable growing conditions.
High-Efficiency Dehumidification: High-performance compressor removes up to 38 pints/day of moisture. Dual ventilation ports create a closed-loop airflow system for improved dehumidification and air circulation.
Smart App Control: Connect to the Mars Hydro iControl System for automatic humidity adjustment and coordinated environmental control to maintain optimal VPD.
Stable Operation: Features automatic defrost, anti-freeze protection, power-loss memory, and sensor fault alerts for stable, worry-free performance.
4 Working Modes: 4 working modes to suit different needs. HOLD maintains your target humidity, CONT provides continuous dehumidification, DRY enables fast drying, and FAN circulates air without dehumidification.
Installing an Air Conditioner
An air conditioner cools the air while removing excess moisture, helping lower both temperature and humidity. However, it costs more to buy and operate than a dehumidifier. For most small to medium grow tents, a dehumidifier is the more cost-effective choice.
How to Stabilize VPD Through Multi-Device Collaboration
While methods for individually increasing or lowering VPD have been mentioned, it's important to note that environmental swings are far more extreme in sealed grow tents than in open spaces. Moisture builds up quickly, temperatures shift often, and plant activity constantly alters the microclimate. No single device can handle the full range of daily fluctuations, so a multi-device strategy is essential.
Stabilizing VPD requires a system-level approach with 3 layers: a sensing layer that monitors temperature and humidity in real time, an execution layer that responds through humidifiers and dehumidifiers, and a regulation layer that uses fans to prevent localized extremes. Together, they form a closed loop of monitoring, adjustment, and re-monitoring that keeps the environment continuously in check.
In practice, users often struggle with equipment matching, placement, and threshold settings, which stem from a single-device mindset. Mars Hydro views the grow tent as an integrated ecosystem where lighting, airflow, and climate control work together. The result is a stable VPD environment, consistent plant growth, and ultimately higher yields with lower risk.
Conclusion
Managing VPD is crucial for optimizing plant growth, particularly in controlled environments like greenhouses and grow tents. By understanding the factors that influence VPD and using the right tools, you can create and maintain ideal growing conditions. Follow Mars-Hydro and embark on your indoor growing journey!
FAQs
Q1: Is 1.2 VPD too high?
It depends on plant type and growth stage. 1.2 kPa is generally suitable for late veg and flowering, but it is too high for seedlings and clones, which prefer lower VPD levels.
Q2: Can I fix VPD with misting?
Not recommended. Misting only causes a temporary drop in leaf temperature. Once it evaporates, VPD returns to baseline. Effective control comes from managing temperature, humidity, and airflow together.
Q3: Will 60 RH cause bud rot?
Not directly. 60% RH is not an immediate guarantee of bud rot, but it increases the risk in late flower. The ideal RH during the flowering period is 40%-50%. When the humidity exceeds 60%, measures must be taken.
