Indoor Grow Metrics, Indoor Grow Tips

The Downlow On Vapor Pressure Density — How Does VPD Affect Cannabis Plants?

How does VPD affect Cannabis grow?

Vapor pressure deficit (VPD) isn’t a term you’ll find in the first few chapters of a cannabis growing guide. Heck, there are plenty of experienced cultivators who don’t fully understand the intricacies of VPD.

Although monitoring VPD is an advanced growing technique, there are benefits to calculating this variable. Knowing the average VPD in your grow tent will help you adjust other critical metrics like temperature, humidity, and CO2. Keeping your plant in a healthy VPD range could also reduce the risk of deadly conditions like mold and root rot.

What’s The Significance Of VPD For Cannabis Plants?

To understand VPD, it’s essential to mention another technical term: “transpiration.” As you may remember from Plant Biology 101, transpiration is a process where water and nutrients move through a plant’s roots through the leaves. Technically, the “stomata” on cannabis fan leaves release water into the atmosphere.

VPD gives growers a glimpse into the transpiration process. But VPD is not just about how much water the plant gives off. A VPD reading will also alert growers to how the external environment affects transpiration rates.

It’s worth noting that “vapor pressure” refers to the conditions at which water changes its state into steam. The vapor pressure reading will differ depending on the external environment, which includes temperature, humidity, and spatial concerns.

The significance of knowing your VPD is that you can better understand your plant’s health in relation to other standard grow room metrics. As long as you know the healthy range for VPD, you could adjust aspects like your watering schedule, ventilation, or average humidity to align this score more with the ideal range.

In addition to giving you optimal buds, a solid VPD score could reduce your operating costs. Knowing how much ventilation you need could help you effectively budget HVAC expenses.

How Could “Bad” VPD Affect The Growth Of Cannabis?

The most obvious risk of having higher-than-average VPD scores is an increased risk for mold and mildew. Since these fungi thrive in excessively damp conditions, the extra moisture in the atmosphere creates a better breeding ground for these deadly conditions.

Higher VPD scores also tend to correlate with a greater likelihood of plant stress. This factor could stunt plant growth and increase the risk of hermaphroditism. Too much VPD also signals that the plant’s stomata won’t be as receptive to extra CO2 in the atmosphere.

However, scores that fall below the intended VPD levels also have issues. Most notably, cannabis that isn’t transpiring enough usually exhibits signs of nutrient deficiencies. These plants may also show slower growth, discolored & curling leaves, or burnt tips.

Microscopic stomata of cannabis.
Microscopic Stomata

What’s The Easiest Way To Calculate VPD For Cannabis?

Calculating VPD isn’t as simple as taking the relative humidity of your room with a hydrometer. While you could place a hydrometer near your cannabis, it won’t give you the true VPD. To get your VPD number, you’re going to need to do a bit more work.

If you don’t mind crunching numbers, here are the formulas you could use to calculate VPD:

SVP = 610.78 x e^(T/ (T + 238.3) x 17.2694))

SVP (1 – RH/100) = VPD

In these equations, “T” stands for temperature in Celsius, and “RH” is relative humidity.

The “e” in the first formula is a constant variable known as Euler’s number. Please plug in 2.71828 for accurate results.

After your conversions are done, you will have a VPD score in kpascals (kPa).

Thankfully, many tools on the Internet allow you to calculate VPD without as much of a chance for human error. Be sure to look up tools like Omni Calculator to figure out VPD scores with your tent’s relative humidity and temperature.

So, What’s The Best VPD For Cannabis?

Since VPD is so interrelated with temperature and humidity, the ideal VPD rate could vary depending on your indoor environment. VPD scores could also vary depending on your strain’s unique physiology and water requirements.

However, the standard advice for most cannabis strains is as follows:

  • VPD of 0.4 – 0.8 kPa for the preparation/early veg stage
  • VPD of 0.8 – 1.1 kPa for the late veg/early flower stage
  • VPD of 1.0 – 1.5 kPa for the late mid/late flower stage.

To get more specific, you could look up VPD charts that map the ideal kPa range compared with the average temp and growth stage. However, if you stay within these general limits, there’s a good chance your cannabis strains will turn out healthy.

Controlling both the temperature and humidity levels can meet the VPD needed at different stages, but the temperature and humidity are different at each stage of plant development, for example, the seedling stage needs higher humidity and the flowering stage needs a higher temperature. If you are worried that you may not manage it, you can visit a dynamic VPD Chart with the optimal level you’ll need in weed growing.

Cannabis leaf with water droplets.

How Could Marijuana Growers Control VPD?

To manage VPD, cultivators must understand how temperature and humidity affect this rate. In brief, temperature mimics VPD, but relative humidity has the opposite effect. So, as you increase your room’s temperature, the VPD will increase. However, if you increase relative humidity, VPD scores will decrease.

Remember that cannabis fan leaves absorb water through their pores, especially in the early stages of growth. The more water available in the air, the less likely plants will transpire as much on their own.

Mars Hydro ventilation system to control the temperature and humidiy of the cannabis grow room.
Ventilation System Helps Control VPD

Mars Hydro offers cannabis growers a wide array of inline duct fans to help control the ventilation of their grow room. These customizable fixtures come in 4 and 6 inches versions, bundled with other handy gadgets, such as thermostats and speed controllers. You could adjust your watering schedule or use a humidifier/dehumidifier to toggle your room’s conditions.

It’s also essential to consider light intensity when adjusting for VPD. Generally, the higher the power of your light, the more VPD will rise. Since light gives off heat, it will increase the temperature of your leaves and influence transpiration. The converse happens when you decrease light intensity.

Mars Hydro models like the FC 3000 or the FC-E 3000 come equipped with a dimming feature, which makes it easy for growers to adjust their light intensity on the fly. Our LED grow lights also have a low ambient heat, which makes them more effective than standard HPS models. With these features and more, Mars Hydro’s LED grow lights give cultivators the most control over indoor temperature and VPD readings.

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