Lets discuss The importance of VPD – Vapor Pressure Deficit. But first let’s start with the basics and come back to VPD later. So, humidity.

During the course of learning to grow, almost every gardener will discover that the temperatures and humidity of the environment have quite an effect on your plants. A good understanding of how temperatures and humidity affect the plant’s growth will give the gardener a real advantage in optimizing their growing environment, and their plants will be happier and more productive for it. 

Humidity and temperature have a direct effect on two very important aspects of gardening. The first of these is something called plant transpiration; the second of these is dew point, the resulting elevated risk of fungal attacks, rot and mould. So, to get a better idea about these effects, let’s take a look at how a plant works. 

First of all, let’s take a look at plant’s transpiration. Most growers already know that plants obtain and draw their water by drawing it through the roots. This water is distributed through the plant via stems, eventually ending up in their leaves and flowers. On the underside of the plant’s leaves are microscopic breathing pores called stomata. The plant uses stomata to absorb CO2 as required for photosynthesis. they're also used to release oxygen, which is created as a waste product. However, because the lining of the stomata needs to be wet in order to be able to absorb CO2 from the air, it means that, through these stomata, the larger proportion— of up to 90%—of plant water loss occurs. Because there is a cost of photosynthesizing to the plant in terms of water, the stomata of the plants have the ability to close when the cost would be too high. To keep the plants as productive as possible, ideally, the stomata would only close during the dark periods, or at night for outdoor plants, which is when the plant has no need for light to photosynthesize  and has no need to absorb CO2. 

However, the stomata will also begin to close if the plant begins to sense that payoff for having the stomata open, which enables photosynthesis, is not worth it in relation to expenditure of water loss. This may be due to one or more environmental factors, such as air temperature, humidity, light levels, CO2 concentration, or the roots can’t find any water, ultimately affecting yield. So, there’s a great need to get the temperature and humidity combination right. 

Relative humidity is not the only factor that affects water loss through the stomata. It’s the combination of relative humidity and temperature that actually determines this. A combination of dry, environmental, air, low humidity, and warm temperatures would cause waters to be lost at a faster rate than would be in human conditions. The plant recognizes this as a problem and again will close the stomata to conserve water. The close in the stomata is a gradual process, and they also can be partially closed, too. The amount the stomata will close is somewhat relative to the level of risk of running out of water.

As I mentioned a moment ago, plants need CO2 to photosynthesize. If a plant has to shut its stomata in order to conserve water, then it’s unable to absorb CO2 and photosynthesis stops. If photosynthesis stops, the plant cannot create the sugars that it needs in order to grow. Obviously, we need our plants to keep their stomata open during the lights on period. This is where knowing about vapor pressure deficit comes into its own as a far more accurate way of predicting water loss than considering just relative humidity. So, that leads us nicely into the start of the next part of this article.  let’s now talk about ways in how to monitor your humidity and how to increase it.

So, there are number of simple ways to monitor humidity, like digital thermometers and hygrometers that have maximum and minimum settings on them, they give you a current reading of relative humidity , as well as low and high temperatures  that can be cataloged and tracked. There is also more sophisticated ways, to track these variables, whole room meters and software can be used to automatically  track temperature ,humidity , VPD, lux and co2 ppm (parts per million) 

Okay. So, where does your humidity need to be? On a basic level, your relative humidity in veg and early flowering needs to be in the area of about 70 to 80 percent, give or take, then lowering it in the late flower in the region of 50 to 60 percent. But these are guidelines for those who have limited ability to control and monitor their humidity. Later on, we’ll go into more depth about the best ways to calculate where this needs to be and how to adjust it. If you need to increase your humidity, look at buying some humidifiers. Ideally, use them with a hydrostat where you can set your humidity to the level you need it to be. This might need a little bit of manual calibrating to get it just right. And if you need to lower your humidity, try to avoid dehumidifiers, as these take water from the easy source, which is usually your plants, and will cause them to dry out. 

I hope this has been interesting and informative, and we will carry on where this left off next time, getting deeper into vpd and its effect on your growing environment.

                           VPD PART 2 

Hello to all you growers out there, and welcome back  to the second installment of Vapor Pressure Defecit

One thing an experienced grower knows only too well is that the environment in your room or tent plays the most important role in deciding your success. In part one, we touched on plant transpiration, essentially the way temperature and humidity conditions in your grow room affect your plant’s growth rates, the quality of any fruits produced, and ultimately, your ability to maximize your yields. But you want to know about vapor pressure deficit. Many indoor gardeners grow successfully with only a simple hygrometer, which measures temperature and relative humidity. However, measuring VPD gives much better information on the drying ability of the air around your plants, which provides a clearer insight into how much water is required by the plants to maintain its stomata moisture levels.

VPD, as a measurement, talks about the differences between the moisture content in the air and the moisture content the air can hold when saturated. In terms of plant growth, it relates to the differences between the pressure inside the leaves and the surrounding atmosphere. It’s kind of like rolling the temperature and humidity into one measurement to get a clear picture of what’s going on directly around the stomata, as opposed to just knowing what the conditions are like in the room itself.

Putting it more simply, the higher the VPD, the greater the drying effect the air has on the plant’s ability to transpire. The key is making sure that everything is sitting in the right ranges. In most cases, a certain amount of pressure deficit between the leaves and the outside air has a positive effect, because some level of transpiration from the stomata is needed to promote the water flow from the roots to the rest of the plant, and it is this flow that brings along with it the nutrients that are required in countless roles around the plants. If the VPD is too high, plants will lose too much water through the transpiration and the stomata will close. A low VPD indicates that the air is holding a lot of water, which slows down the plant’s transpiration rates. Transpiration rates that are too low will slow down the movement of water and nutrients from the roots, inevitably causing problems leading to nutrient deficiency issues. 

As VPD is a pressure reading, it’s usually written in mb, or millibars, but you may also see VPD expressed in kilopascals, or kPa. Converting between the two is easy enough. Just divide the figures in millibars by ten to get an appropriate number in kPa. That’s it.

Here are some typical ranges. For low transpiration, 4 to 8 mb, or 0.4 to 0.8 kPa, usually, if you cut into veg. Healthy transpiration, 8 to 12 mb, or 0.8 to 1.2 kPa, flower. The high transpiration, 12 to 16 mb, or 1.2 to 1.6 kPa, usually during peak flowering times. 

So, now we know what VPD readings relate to, we need a way of calculating them. By far, the easiest way of getting accurate readings is to use a piece of equipment that does the job for you. there are many brain type monitoring systems out there  that can take these readings and do the calculations for you  . There isn’t much in a typical grow room that can’t be connected to these systems . Lights, CO2 generators, CO2 sensors, humidifiers, heaters, can call be controlled and regulated digitally from a central point. 

To calculate VPD, you’ll need a temperature probe, the humidity probe, and the  plant camera, which takes a reading from the leaf temperatures at the top of the canopy. The system of your choice will take  all these readings to calculate your VPD. Once you’ve got the readings, you can then make the relative changes to your setup to keep the VPD in the sweet spot. This usually means adjusting your humidity with the use of humidifiers or dehumidifiers, adjusting your atmospheric temperatures, ensuring the room is being cooled correctly, or increasing the temperature if needed, or adjusting your plant canopy temperature. This can be done by simply lifting or moving the lights closer or further away from your plants. Assuming that you set this up, the controller will dim the lights if the plants go above your required settings. But use this as a last resort, as typically  we always want our  lights  on maximum power. 

If you don’t have a smart controller  set up, you can still monitor VPD, and here’s how. You will need an infrared pocket thermometer, which measures leaf temperature, and a hygrometer to take temperature and relative humidity readings from the room. Point the infrared thermometer at the canopy and  Measure to get the leaf temperature. Keep the distance relative to the size of the area you want to measure. If the leaf area to measure is 30 centimeters by 30 centimeters, keep the infrared pocket thermometer 30 centimeters away. You’ve now got your leaf vapor pressure. Then take the air temperature and relative humidity readings from your room with your hygrometer. Again, just check out where they intersect  and make a note of that number, there are conversion tables available online  which give you your air vapor pressure. So, subtract your leaf vapor pressure from your air vapor pressure and you’ve got your vapor pressure deficit. Then make changes as you see fit.

VPD allows you to keep your grow room in its sweet spot for your plants, ultimately maximizing your yields. I hope this helps you out, guys. Happy growing.