VPD Chart for Growing Weed: Temperature and Humidity by Stage

A VPD chart tells you whether the air in your grow room is pulling water out of the plant at the right rate, by combining temperature and humidity into a single number in kilopascals. Seedlings and clones want 0.4 to 0.8 kPa, vegetative plants 0.8 to 1.2, and flowering plants 1.2 to 1.5.

This chapter explains VPD in plain terms, why it beats reading humidity alone, the leaf temperature offset that most online charts ignore, and the target temperature and humidity for each stage. The chart is computed, not copied, and the formula is given so you can check it.

What is VPD?

VPD stands for vapour pressure deficit. It is the gap between how much water vapour the air is holding and how much it could hold at that temperature. A high VPD means dry, thirsty air that pulls water through the plant fast. A low VPD means air so full of moisture that the plant can barely transpire at all.

Transpiration is how a cannabis plant moves water and nutrients from the roots to the leaves and cools itself, and the air drives that pump. At the right VPD the plant drinks steadily, stomata stay open, and growth is fast. Too high and the plant shuts its stomata to save water, stops taking in carbon dioxide, and stalls even with perfect light and feed. Too low and water sits in the leaf, calcium does not move, and mold gets its chance.

Relative humidity alone cannot tell you this, because 60 percent means very different things at 68F and at 84F. Warm air holds more water, so the deficit at 84F is far bigger. VPD folds the two readings into one number that maps onto what the plant feels.

Leaf temperature and the offset

The plant does not experience air temperature; it experiences leaf temperature, and a transpiring leaf runs 2 to 5F cooler than the air around it under most lights. A VPD chart built on air temperature alone overstates the deficit. The chart below assumes the leaf is 2F cooler than the air, which is typical under LED.

Under a hot HPS lamp the leaf can run warmer than the air and the offset flips. An infrared thermometer pointed at the canopy gives the real number. If your leaf reads more than 3F either side of the air, use the leaf reading and pick the nearest row.

VPD chart for growing weed

Find your air temperature in the left column and your relative humidity across the top; the cell is your VPD in kPa. Every value was computed with the Tetens saturation vapour pressure equation from FAO Paper 56: SVP in kPa = 0.6108 multiplied by e to the power of (17.27 T / (T + 237.3)) with T in Celsius. VPD = SVP at leaf temperature minus SVP at air temperature multiplied by RH.

VPD in kPa by air temperature and relative humidity, leaf 2F cooler than air

Values are rounded to one decimal. The seedling and clone range of 0.4 to 0.8 kPa sits in the 60 and 70 percent columns at 68 to 78F. The vegetative range of 0.8 to 1.2 kPa covers 50 to 60 percent humidity across most of the chart. The flowering range of 1.2 to 1.5 kPa is the 40 to 50 percent columns between 68 and 80F. Anything at 1.6 or above is the plant closing its stomata, and anything under 0.5 is a mold invitation.

Temperature and humidity for weed plants by stage

Seedlings and fresh clones have almost no roots and cannot replace water quickly, so they need 70 to 78F and 65 to 75 percent humidity, which lands them at 0.5 to 0.8 kPa. A humidity dome does this by itself. Once true leaves are working and the clone has rooted, start lowering the humidity.

Vegetative plants want 72 to 82F and 55 to 65 percent humidity, or about 0.8 to 1.2 kPa. A firm deficit here builds thick stems and a wide root system, provided watering keeps up. The stages of growth chapter says when each phase begins and ends.

Flowering plants want 68 to 78F and 40 to 50 percent humidity, or 1.2 to 1.5 kPa, and the humidity should fall through the stage rather than sit flat. In the final two weeks, 40 to 45 percent at the cool end of the range keeps dense buds dry inside and holds terpenes. Push humidity above 55 percent in week seven and bud rot is the usual result.

How to use VPD in a grow tent

Put a digital thermometer and hygrometer at canopy height in the middle of the tent, not hanging by the door, and read it with the lights on. Adjust temperature first, then humidity. A humidifier raises RH; a dehumidifier or a faster exhaust fan lowers it; dimming the light or raising it lowers leaf temperature.

Most home tents run hot and dry under a big LED and cool and damp at night. A night reading of 68F and 70 percent puts a flowering tent at 0.5 kPa for twelve hours, which is where mold starts. Run the dehumidifier on a humidistat, not a timer, and keep the exhaust moving in the dark. The grow tent chapter covers fan sizing and ducting.

Do not chase the chart to the second decimal. Sitting anywhere in the right band, day and night, beats hitting a perfect 1.2 for two hours and swinging to 0.6 overnight.

The mistake most growers make with VPD

The mistake is reading VPD off a meter clipped to the tent pole and never measuring the leaf. Growers see 1.2 kPa on a controller, assume the environment is dialed, and miss that the canopy under the light is 6F hotter than the sensor and the leaves are already curling. Those symptoms are on the heat stress page.

In eighteen years of answering Dear Danko letters, the grower who told me their humidity was "fine at 50 percent" without a temperature next to it was usually the grower with the problem. The two numbers only mean something together.

Next step: building the environment

With the targets set, the grow tent chapter shows how to hold them with fans, filters and a controller, and the grow guide follows the plant from seed to jar. If leaves are already tacoing or bleaching, go straight to heat stress.

Cultivation is for adults where it is legal.