The Dynamic Duo—VPD and EC
Let me tell you a story that genuinely scrambled my brain.
It was spring—British spring, mind you—so the kind that can’t decide if it wants to be chilly, damp, or both at once. I was chatting with a fellow grower named Jason, a very experienced chap with an annoyingly large greenhouse. We’re talking chilies—and he’s grown way more than I have. I’m admiring the health of his overwintered plants, and I ask him about his feeding regime.
“Yeah, I’ve been feeding them at 4.5 mS.”
I nearly spat my tea. Four-point five?! I thought chilies—especially young ones—were light feeders! Seriously, I thought he’d lost the plot.
However, his plants were a testament to his wisdom. Glossy leaves, thick stems, zero stress.
Still, that number flabbergasted me. We chatted some more, and it turned out my thinking when it came to EC was a bit … basic and isolated. In other words, EC is only half the recipe. VPD is the other half.
So let’s talk about this dynamic duo because understanding their little push-pull act changed everything about how I grow.
Water In, Water Out: The Root of It All
You probably already know this, but it bears repeating: plants are basically bags of water doing chemistry in sunlight. Their entire life depends on a fluid logistics system—water coming in through the roots, evaporating out through the leaves, keeping them nice and cool, and—oh yeah—dragging nutrients along for the ride.
Now, two big players are in charge here:
- VPD (Vapour Pressure Deficit): the atmospheric demand for water. It pulls moisture out of the leaves.
- EC (Electrical Conductivity): the nutrient density of your root-zone solution. It pulls water and nutrients into the plant.
One pushes, one pulls. Ignore either one, and things get wonky faster than you can say “tip burn.”
VPD: The Atmospheric Thirst Trap
VPD is basically how much the air wants to suck water out of your plants. High? Hot and dry air slurping up moisture like it’s happy hour. Low? Cool and muggy—air’s already full and can’t be bothered.
Now, your plants aren’t as daft or passive as you might think. They’ll open or close their stomata depending on this “atmospheric thirst.” But here’s the sneaky part: you can have the same VPD at two totally different temps and humidities. A measure of 1.2 kPa might mean 26°C and 60% RH—or 30°C and 70% RH—and those are two completely different vibes for your plant.
VPD doesn’t just tell you how quickly plants dry out—it changes how they grow, affecting leaf shape, nutrient uptake, and overall metabolism.
EC: The Root-Zone Ringleader
While VPD is doing its sweaty, yoga-pants-clad sky dance, EC is handling business underground. EC tells you how “salty” your nutrient solution is. Higher EC = more ions = more nutrients. But it’s a balancing act. Too low, and your plants are hydrated but starving. Too high, and your plants find it harder to uptake that all-important water, and your root zone can quickly turn into a salty wasteland.
The key takeaway? The right EC depends on your VPD.
In other words, measure your VPD and adjust your EC accordingly. Trust me, it’s easier that way ‘round! If your VPD is low (humid and cool), your plants aren’t transpiring much. So you need to raise EC to get enough nutrients in.
If VPD is high (hot and dry), plants are pulling water quickly—you lower the EC so they can continue to drink without straining their roots.
The Jason Revelation: When High EC Works
Now, remember Jason’s chilies? Feeding 4.5 mS in cool, damp spring weather?
Most growers would call that insane.
But his plants were in perfect health. Why? Because even though the VPD was low, the plants still needed nutrients to grow fast. And because his root zone was warm enough, well-oxygenated, and irrigated just right, the plants could handle the higher osmotic pressure. No burn, no stunting—just lush growth.
That experience forced me to rethink the usual “low VPD = low EC” mantra. It’s not just about matching the numbers. It’s about understanding what your plant can handle physiologically at that moment.
Adjust as You Go
Plant needs evolve as they grow. Seedlings aren’t munching the same as blooming monsters. So your VPD and EC shouldn’t be static, either. Here are some general guidelines for commonly grown flowering annual plants:
In veg:
- VPD: 0.8–1.0 kPa (more humid, less stress)
- EC: 1.2–1.8 mS/cm (nutrient-rich, gentle)
In flower:
- VPD: 1.2–1.6 kPa (drier air for stronger transpiration)
- EC: 2.2–3.0 mS/cm (more food, more oomph)
Yes, this seems backward. “High VPD and high EC? Madness!”
But once the roots are mature and healthy, and the environment is dialed in, it works. It’s all about matching its appetite.
Don’t Get Caught Chasing Charts
Here’s where things go sideways for a lot of growers:
They download a VPD chart, stick it on the wall, and chase the numbers as if it were gospel. Meanwhile, their substrate is salty as crisps, and the plants are sulking.
Or they obsess over EC, get the ratios bang on, but forget their plants are sweating bullets in 30°C heat and can’t absorb anything anyway.
EC Explained by Everest
Everest Fernandez has created a series of videos that can help any level of grower. Understand the relationship between electrical conductance/conductivity (EC) and TDS meters (PPMs) when measuring the strength of your hydroponics nutrient solution.
VPD Explained by Everest:
Everest will break down Vapor Pressure Deficit in 5-ish minutes:
- What is it?
- How do you measure?
- What range is good for propagation, vegetative, and flowering?
- How are relative humidity, leaf temperature, and air temperature combined into VPD, and which units of pressure are used?
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