Understanding VPD: Why Temperature and Humidity Only Make Sense Together
Your readings look fine and the plant still won't play along? Then you're staring at two numbers that mean nothing on their own. Here's what VPD actually descr…
You've got temperature and humidity under control, both numbers look good – and the plant still isn't taking off. That's not bad luck, and it's not bad genetics. It's because those two numbers, on their own, tell you nothing about what's happening at the leaf. They only make sense together, and that relationship is exactly what VPD describes. 🌱
What actually happens at the leaf
The underside of every leaf is covered in tiny pores called stomata – in cannabis they sit almost exclusively down there, with only a handful on the upper side. Water evaporates through them. That evaporating water is the engine of the entire plant: it pulls fresh water up from the substrate, and the nutrients ride along with it. No evaporation means no supply – no matter how perfectly your tank is dialled in.
A little water is also lost through the waxy layer of the leaf, top and bottom alike. But that's a small share, and more importantly the plant can't control it – all the valves are on the underside. That's why air movement beneath the canopy matters more than most people think: if the air sits still down there, a damp layer hangs right in front of the stomata and throttles evaporation, even while your meter in the middle of the tent shows perfect numbers.
How hard the air pulls on that water depends on how much more water it can still hold. And here's the part most people skip: warm air can hold far more water than cold air. 60 % humidity at 20 °C is a completely different pull at the leaf than 60 % at 28 °C – same percentage, different world. If you only watch humidity, you're flying blind.
That pull is exactly what VPD measures. It stands for Vapour Pressure Deficit and it's given in kPa. You can forget the name – just remember this instead: VPD is the one number that tells you how hard the air is currently sucking on your plant.
Too little pull, too much pull
VPD too low (humid, cool air): The plant barely evaporates. Nutrient transport grinds to a halt, growth turns soft and lanky, leaves droop. And because the air is damp at the same time, this is the corner where mould and botrytis get started. In flower it's the most expensive mistake there is.
VPD too high (warm, dry air): Now the plant loses more water than the roots can replace. It hits the brake and closes its stomata – and the moment it does, photosynthesis stops too. You'll see leaves curling up at the edges, often with dry, crispy margins. The lamp keeps burning, the money keeps running, the plant takes a break.
Here's the thing to understand: both ends look like "the plant is thirsty", but they need the exact opposite response. Without VPD you're guessing.
Your target values
The calculator is set to these ranges:
- Vegetative: 0.4 – 1.0 kPa. Young plants have little root mass and can't keep up with a strong pull.
- Flowering: 1.0 – 1.6 kPa. Stronger pull, more transport, more metabolism.
- Late flowering: 1.4 – 2.0 kPa. Run drier, mainly to protect the buds from mould.
As a climate window that roughly means: vegetative 20–28 °C at 60–80 %, flowering 22–28 °C at 40–60 %, late flowering 20–26 °C at 30–50 %.
How to use the calculator
1. Open the VPD Calculator.
2. Pick your phase at the top – the target range switches automatically.
3. Enter temperature and humidity . Important: the values from inside the tent , measured at leaf height – not the ones from the weather app and not the ones from your living room.
4. The bar shows you instantly where you stand. Green is the target range, blue means too little pull, red means too much.
5. If the pointer sits outside, you have two screws to turn: VPD too low comes down with less humidity or more warmth. VPD too high comes down with more humidity or less warmth.
Play with the sliders once without changing anything in the tent. Within thirty seconds you'll see what two degrees actually do – and that temperature is often the stronger lever of the two.
Leaf VPD: the more accurate number
A leaf is almost never as warm as the air around it. Evaporation cools, so an actively transpiring leaf typically sits about 2 °C below room temperature . That's why the calculator uses a −2 °C offset as soon as you switch on Show Leaf VPD .
Why it matters: the plant doesn't experience air VPD, it experiences the VPD at its own leaf. With a cooler leaf the real value is lower than the air figure – meaning you're running wetter than your meter suggests, without noticing.
That offset isn't a law of nature. Under strong direct LED light with little air movement a leaf can also end up warmer than the room air, and then a positive value belongs in that field. If you want to know for sure, a cheap infrared thermometer does the job: hold it close to a large, healthy leaf, read the temperature, subtract the room temperature, enter the difference as your offset. It's the cheapest measurement in the whole tent with the biggest payoff. I use this infrared thermometer (affiliate link, German Amazon store) – around ten euros, t …
Understanding VPD: Why Temperature and Humidity Only Make Sense Together