Stomata 101

What they are

Stomata are microscopic pores, mostly on the underside of leaves. Two guard cells open and close each pore – there are hundreds of them on every square millimetre.

The trade-off

Open stomata let CO₂ in for photosynthesis and water vapor out. Every gram of growth costs water. Closed stomata protect the plant from drying out – but stop growth.

What closes them

• Darkness
• Drought in the root zone
• A VPD that’s too high – the air pulls water out faster than the roots can deliver

The takeaway

That’s the whole point of every VPD chart: a climate where the plant can keep its stomata open and breathe freely. Check your VPD.