04/27/2026
I’ve mentioned VPD in several posts over the past week, and before I get into the BRIX series next week, we need to take a deep dive into “V***r Pressure Deficit”
Think of V***r Pressure Deficit (VPD) as the "Thirstiness of the Air." For a home gardener, you don't need a physics degree to understand it—you just need to think about how a plant "breathes."
Plants are essentially living straws. They pull water up from the soil through their roots, and that water travels all the way up to the leaves. At the leaf surface, the water evaporates through tiny pores called STOMATA. This evaporation is what "sucks" the next drop of water up from the roots.
VPD is the difference (the deficit) between how much moisture is currently in the air and how much moisture that air could hold if it were totally saturated.
When the Air is "Full", there is LOW VPD. On a very humid, muggy day, the air is already packed with moisture. It doesn't want any more. Because the air won't take the water from the leaves, the "straw" stops pulling. The plant's internal transport system grinds to a halt. More on this later.
When the Air is "Thirsty", there is HIGH VPD. On a very dry, hot day, the air is like a sponge. It aggressively pulls moisture out of the leaves. If it pulls too fast, the roots can't keep up, and the plant WILTS to protect itself.
SO, why does the average gardener care about VPD? Most people focus on "Humidity," but VPD is more accurate because it accounts for TEMPERATURE. WARM AIR CAN HOLD WAY MORE WATER THAN COLD AIR!
Certain nutrients, SPECIFICALLY CALCIUM, can only move through the plant if water is moving! SO, If your garden or greenhouse is too humid (Low VPD), even if your soil is perfect, your plant might get "Blossom End Rot" or "Tip Burn" simply because the plant isn't moving the calcium to the new growth.
How does VPD affect disease and pests? If the air stays too humid (low VPD), the plant gets "sweaty." This moisture sits on the leaves and invites mold, mildew, and rot.
If it's too dry, like in arid climates, the plant gets stressed. Stressed plants are like a dinner bell for pests like Spider Mites, who love hot, dry conditions.
You don't need fancy sensors to find the "Sweet Spot" (where the air is just thirsty enough to keep the water moving, but not so dry that the plant panics). When the air is too full (low VPD), if it’s possible, put a fan on your plants! Moving air helps strip the "wet blanket" off the leaves so they can breathe.
When the air is too "Thirsty" (High VPD), you’ll feel it…cracking skin, static electricity, and the soil dries instantly. Mulch your soil and consider a light misting or grouping plants together to create a little "humidity bubble."
As we move into the BRIX series, this explanation will make it easier to understand how and when to apply your BRIX sprays.