VPD for Cannabis 2026: The Chart That Actually Makes Sense
Most indoor growers ignore VPD — and wonder why growth stalls. Get the exact VPD targets for seedling, veg, and flower, plus a simple setup to automate climate control in under 10 minutes.
The VPD Chart
Want your exact number instead of reading the grid?Use the VPD calculator and chart: enter your temperature and humidity and it gives your VPD in kPa, whether it suits seedlings, veg or flower, and what to change.
Every cell below is the VPD your plants are actually experiencing at that combination of air temperature and relative humidity. Read down to your temperature, across to your humidity, and check the colour: green bands are the working zones, blue means the air is too wet to pull moisture through the plant, and red means it is pulling harder than the roots can refill.
The numbers are calculated at leaf temperature, not air temperature. A transpiring canopy under LED sits a couple of degrees cooler than the air around it, and a chart that ignores that difference will tell you that you are in the green when you are not.
VPD in kPa at leaf temperature, assuming the canopy runs 2°C below air temperature. Find your air temperature down the left, your relative humidity across the top, and read the cell where they meet.
Air temp °F
30%
35%
40%
45%
50%
55%
60%
65%
70%
75%
80%
64°F
1.18
1.08
0.98
0.88
0.77
0.67
0.57
0.47
0.37
0.27
0.16
66°F
1.27
1.16
1.05
0.94
0.83
0.72
0.61
0.51
0.40
0.29
0.18
68°F
1.36
1.25
1.13
1.01
0.89
0.78
0.66
0.54
0.43
0.31
0.19
70°F
1.46
1.34
1.21
1.09
0.96
0.84
0.71
0.59
0.46
0.33
0.21
72°F
1.57
1.43
1.30
1.16
1.03
0.90
0.76
0.63
0.49
0.36
0.23
74°F
1.68
1.54
1.39
1.25
1.11
0.96
0.82
0.68
0.53
0.39
0.25
76°F
1.80
1.64
1.49
1.34
1.18
1.03
0.88
0.72
0.57
0.42
0.26
78°F
1.92
1.76
1.60
1.43
1.27
1.10
0.94
0.78
0.61
0.45
0.29
80°F
2.06
1.88
1.71
1.53
1.36
1.18
1.01
0.83
0.66
0.48
0.31
82°F
2.20
2.01
1.82
1.64
1.45
1.27
1.08
0.89
0.71
0.52
0.33
84°F
2.35
2.15
1.95
1.75
1.55
1.35
1.15
0.96
0.76
0.56
0.36
86°F
2.51
2.29
2.08
1.87
1.66
1.45
1.23
1.02
0.81
0.60
0.39
88°F
2.68
2.45
2.22
2.00
1.77
1.54
1.32
1.09
0.87
0.64
0.41
90°F
2.85
2.61
2.37
2.13
1.89
1.65
1.41
1.17
0.93
0.69
0.45
Under 0.4 — too humid
0.4–0.8 — seedling & clone
0.8–1.2 — vegetative
1.2–1.5 — flower
Over 1.5 — too dry
The Physics of Transpiration
Plants don't 'drink' water like animals; they 'pull' it through themselves using evaporation. This process, called transpiration, happens via stomata—tiny pores on the undersides of leaves. For water and nutrients to move from the roots to the top of the canopy, the air must act as a 'sponge' to pull that water out. VPD (Vapor Pressure Deficit) is the measure of that sponge's suction power.
The Three Magic VPD Zones
If your VPD is off, your plant is either drowning (too low) or parched (too high). The plant shows you which before a meter does — canopy leaves folding upward at the edges ('taco-ing') is the classic high-VPD tell, while soft, heavy drooping with no wilt points the other way. If you are unsure which you are looking at, diagnose it from a photo before you start moving humidity targets.
The Clone/Seedling Zone (0.4–0.8 kPa): High humidity is essential because young roots are weak. High VPD here will shrivel a seedling in hours — the cannabis seedling guide covers the first three weeks in detail, when the margin for error on humidity is smallest.
The Vegetative Zone (0.8–1.2 kPa): This is the growth accelerator. Strong suction drives rapid nutrient delivery.
The Flower Zone (1.2–1.5 kPa): Lower humidity prevents mold and increases resin production.
Growth Stage
Target kPa
Why?
Seedling
0.4 - 0.8
Prevent Wilting
Veg
0.8 - 1.2
Maximize Growth
Flower
1.2 - 1.5
Flavor & Safety
VPD Through Flowering: Stretch, Bulk, Ripen
Flowering is not one VPD target. The plant's needs change across the eight to ten weeks of bloom, and the humidity that is right in the second week will rot a dense bud in the last.
The stretch (weeks 1–2), 1.0–1.3 kPa: The plant is still putting on height fast, so it sits across the veg–flower boundary — from the top of the green veg band into the bottom of the flower band.
Bulking (week 3 until two weeks out), 1.2–1.5 kPa: Buds are gaining mass. This is the chart's flower band, and drier air is what keeps the inside of a swelling bud from staying damp.
Ripening (the final two weeks), 1.3–1.5 kPa: Dense, finished buds are at their most vulnerable to rot. Some growers push briefly to 1.6 kPa (about 35% RH at 75°F) to encourage resin; the chart marks that as too dry, so keep it short and watch the leaves.
Flower Phase
Target kPa
RH at 75°F (24°C)
Stretch (weeks 1–2)
1.0 - 1.3
About 45–55%
Bulking (week 3 until two weeks out)
1.2 - 1.5
About 38–48%
Ripening (final 2 weeks)
1.3 - 1.5
About 38–45%
The humidity column assumes the leaf runs 2°C cooler than the air, like the chart above. If you drop the temperature late in flower — many growers do — drop the humidity with it, and read the new target off the chart.
Device Setup: Controlling the Climate
You cannot manually chase VPD with a spray bottle. You need automation.
Place your humidity sensor at canopy level, NOT on the floor. Use a smart controller like a smart fan controller to sync your fans and humidifiers. This ensures that as the temperature rises under your LEDs, your humidity adjusts dynamically to stay in the green zone.
Advanced: VPD & CO2 Interaction
If you use CO2 supplementation, you can run higher VPD levels safely. Higher CO2 levels cause stomata to close slightly, which naturally slows transpiration. This allows you to run warmer, more intense environments (up to 85-90°F) without stressing the plant, provided your VPD stays under 1.6 kPa.
VPD (Vapor Pressure Deficit) measures the difference between the amount of moisture in the air and the maximum moisture the air can hold at a given temperature. For cannabis, VPD determines how hard the plant must work to transpire water. Too low (below 0.4 kPa) causes stomata to close and nutrient uptake to stall. Too high (above 1.5 kPa, where the chart turns red) causes wilting and drought stress.
What is the ideal VPD for cannabis seedlings and clones?
Seedlings and clones do best at 0.4–0.8 kPa. Their roots are too small to replace water quickly, so the air has to stay soft. At 75°F (24°C) air, with the leaf 2°C cooler as in the chart above, that is roughly 62–75% relative humidity. Let VPD climb higher and a young plant can wilt within hours.
What is the ideal VPD for cannabis in vegetative stage?
The ideal VPD for cannabis vegetative growth is 0.8–1.2 kPa. This range keeps stomata fully open, maximizing CO2 absorption and nutrient transport. At 75°F (24°C) air, with the leaf 2°C cooler as in the chart above, that is roughly 48–62% relative humidity. Lower humidity or higher temperatures will push VPD too high.
What is the ideal VPD for cannabis flowering?
During cannabis flowering, target VPD of 1.2–1.5 kPa. At 75°F (24°C) air, reading at leaf temperature like the chart, that is roughly 38–48% relative humidity — low enough to reduce bud rot risk, increase trichome density, and concentrate terpene production. In late flower (last 2 weeks), some growers deliberately push VPD to 1.5–1.6 kPa as a controlled stress to trigger resin — past the point the chart marks as too dry, so treat it as a short, watched push rather than a steady state.
How do I lower VPD in my grow tent?
VPD is determined by temperature and humidity. To lower VPD: increase relative humidity (use a humidifier), or decrease temperature. The most effective approach is to raise humidity. Lowering canopy temperature by adjusting light height also helps. A smart controller like a smart fan controller can automatically manage your fan and humidifier to maintain target VPD.