Vapour-pressure deficit

Vapour-pressure deficit, or VPD, is the difference (deficit) between the amount of moisture in the air and how much moisture the air can hold when it is saturated. Once air becomes saturated, water will condense out to form clouds, dew or films of water over leaves. It is this last instance that makes VPD important for greenhouse regulation. If a film of water forms on a plant leaf, it becomes far more susceptible to rot. On the other hand, as the VPD increases, the plant needs to draw more water from its roots. In the case of cuttings, the plant may dry out and die. For this reason the ideal range for VPD in a greenhouse is from 0.45 kPa to 1.25 kPa, ideally sitting at around 0.85 kPa. As a general rule, most plants grow well at VPDs of between 0.8 and 0.95 kPa.[citation needed]

In ecology, it is the difference between the actual water vapour pressure and the saturation water vapour pressure at a particular temperature. Unlike relative humidity, vapour-pressure deficit has a simple nearly straight-line relationship to the rate of evapotranspiration and other measures of evaporation.

Climate

VPD can be a limiting factor in plant growth. Climate change is predicted to increase the importance of VPD in plant growth, and will further limit growth rates across ecosystems

Do I need a dehumidifier and vpd for a 4x4 tent Rdwc in a shed building?

Is there an easy way to measure vpd?

Not by my standards i’m afraid.

Computing VPD for plants in a greenhouse

To compute the VPD,[1] we need the ambient (greenhouse) air temperature, the relative humidity and, if possible, the canopy air temperature. We must then compute the saturation pressure. Saturation pressure can be looked up in a psychrometric chart or derived from the Arrhenius equation, a way to compute it directly from temperature is.

You also need Infrared Sensor to measure fan leave temps.. If the “Relative values” are in parameters you should be fine!

Welcome to the community @Great_white

@Ratch33 @Stoney1BudTokie @Guigrow

IS ANYONE INTERESTED IN SOME TOP SHELF SEEDS?

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If it’s NOT under the sun and under artificial lighting than you’ll always need to follow VPD chart. You’ll need to be able to control the environment so yes you’ll need an HVAC system in big room or tent you’ll need exhaust fan, humidifier, dehumidifier, AC, and intake fan to provide an ample amount of fresh air which will also prevent from mold and mildew.
Here’s a reliable VPD chart and information . Just make sure to invest in infrared thermometer to measure your leaf’s temperature because your leaves temperature will be roughly up to 5° cooler than inside tent and that’s called leaf temperature or offset temperature. You’ll understand why that’s important to know once you read the VPD guide.

You will be expecting an order of some of them for sure @DeAzNuTz, but the Nukedaddy s are insane the sizes of the colas lovely collection

Yeah bro, huge seeds! Makes sense why they grow out to be so thick and dense.

@DeAzNuTz just got my grow pack from Kelp4Less in the mail! Can’t wait to feed my girls next watering!

Oh cool, your plans will love you for using their nutrients and you’ll see how happy they will be and how easy it is to use those all in one nutrients that were carefully selected and dozed in perfect proportions. I was amazed how healthy my plants looked from seed to harvest with those nutrients. FYI… I don’t know if this is different for you but for me using Cal mag in dry form that comes with both grow and Bloom packs, was not enough and besides feeding the suggested amount during veg, when I flipped it to flower I had to add 5 to 10 mL of liquid Cal mag because I’m using RO water and apparently it needs more CalMag because the leaves started changing color to a little lighter green like a lime green and when I added more calmag the problem went away a few days later. I use everything organic so for me it takes a little longer for nutrients too convert and soil and then be available for uptake versus synthetic salts that are available right away. I just prefer organic because I grow for my own personal consumption and I want to keep it as clean and natural as possible.

I’ll look out for the calmag deficiency, thanks for letting me know. Yeah I started growing because I’m tired of dispensaries charging 100 a quarter. Or having to rely on the street where people will sell you anything for quick money. Growing my own medicine organically where I can be in control of what goes into the plant definitely has appeal

@DeAzNuTz question for you about Grove Bags, do I need a heat sealer to close the bags after I put the flower in? How does that work?

I’m glad you’ve asked because it’s for every type of setup regardless whether you’re doing water culture or soil or coco or hydro because when you adjust the temperature and RH to provide the right environment, you obviously didn’t know this but that comes from the VPD chart and to simplify things people just go by temperature and RH but if you want to maximize on your yield and provide the best possible environment for your plants, if you want to control when and how much nutrients they update, I highly recommend look into the VPD and what it’s all about and how to follow the VPD chart. I’m going to provide all this information I just hope you will actually read it. Whether or not you follow it is your choice but you should definitely educate yourself on it and I’m pretty sure once you do you will never grow the way you did before without following VPD chart.

Chart: https://growsensor.co/indoor-growing/the-ultimate-vpd-calculator-environmental-timeline/

Chart: The Ultimate Vapor Pressure Deficit (VPD) Guide - Pulse Labs

Chart: https://www.dimluxlighting.com/knowledge/blog/vapor-pressure-deficit-the-ultimate-guide-to-vpd/

Make sure you buy yourself an infrared thermometer to measure your leaves temperature because to follow VPD chart properly you’ll need to measure the offset temperature of your leaves. Usually your leaves are going to be up to 5° F lower than your room temperature and that will change up your chart. Sometimes it’s 2° lower sometimes it’s the same temperature as your room so when you start doing things properly you will see how much better your plans are going to be growing and uptake will be optimal and respiration on point.

This is what professional grows at home and commercially are all following. When I first learned about it I quickly realized how stupid I am or uneducated because temperature and relative humidity adjustments come from the VPD . Who would have thought

I’m happy to share this knowledge so whoever is interested can increase their yields and performance!

You can hate seal it but I don’t. They recommend to heat seal the bags and they leave extra space to do it twice but like I said I don’t and it’s been working fine for me. I think it’s a marketing thing for them because once you heat seal it you’re going to have to buy new bags and that helps them sell more. See for yourself, heat seal one and don’t seal another and see if there’s a difference. I keep them in groove bags for up to 2 months and then I transfer them into glass jars for prolonged storage. They say that you don’t have to add humidity packs but I do after about a week being in the bag because I like my bud to be a little moist. It’s a personal preference so do whatever you like. Hope this helps.

Thanks a million @DeAzNuTz Iv downloaded all the vdp s and gone through Kelp4less and all the stuff I need is there I will have to wait till Wednesday to order! But belive me I’ll be watching all what you give me in information

That’s awesome! I just fed my outside batch with grow pack and they absolutely love it! You can start feeding them as early as four true leaves appear like mine

And I got my CO2 in there as well in a small 500 mL bottle where I put 300 mL of water with 120g of sugar, 5g baking soda and 5g active yeast. Yes you can supplement CO2 this young and you will see the accelerate and growth like Superman LOL. It does help them grow faster.

Until the four true leaves appear I just feed them water with CalMag and gray white mixed together and pH to 6.3-6.5 for soil grow.

The paper towel you see in there is one of my thermometerometers covered from the light which makes it higher temperature if uncovered which is not a real temperature. Guess how I learned that lol and lightly burned my seedlings. That was a long time ago but lesson learned! That usually happens with intense lights not cheap lights. When I was using Mars hydro, Roleadro Galaxyhydro, Atrium, and some other cheap Chinese crap, they don’t really affect the temperature on my thermometers because they are not that intense and they don’t reach them but once you step your game up and get better light like Platinum LED or BlackDogLED or Kind LED K3 or K5 series is what usually home growers get.

@DeAzNuTz I went and picked some up from my local grow shop today. They had qp bags for $5.

Just sent you a text from a 571 number.

@DeAzNuTz if you don’t heat seal the bags how do you close them? Do you leave them open? Lol