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Also known as: vapor pressure deficit chart · VPD lookup table · VPD heatmap

VPD Chart for Cannabis

A lookup table linking temperature and humidity to optimize leaf evaporation and plant health.

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VPD charts bundle temperature and humidity into one number that reflects what your plant's leaves actually experience. The physics is solid. But the stage-specific target ranges you see in most grower forums and equipment manuals? Those come from horticultural convention, not cannabis-specific trials. Use the chart as a starting point and watch your plants. Leaf curl and transpiration rate are your real guides.

What VPD is and why it matters

Vapor pressure deficit (VPD) is the difference between how much water vapor the air is currently holding and how much it could hold at saturation, expressed in kilopascals (kPa). Strong evidence[1] It depends on both temperature and relative humidity: the same RH at 20°C and 30°C creates different evaporative demand on the plant.

A VPD chart is simply a lookup table or heatmap. You find your air temperature on one axis and relative humidity on the other, and read off the resulting VPD in kPa. Cannabis-specific charts add colored bands suggesting target ranges for seedling, vegetative, early flower, and late flower stages.

The underlying physics is standard plant science: low VPD means air near saturation that pulls little moisture from leaves; high VPD means dry air that pulls water aggressively through the stomata. Strong evidence[1][2] In horticultural research across many crops, managing VPD influences stomatal conductance, transpiration, nutrient uptake, and disease pressure. Strong evidence[2][3] High VPD (dry air) can cause stomata to close and slow photosynthesis; low VPD (humid air) slows transpiration, encourages leaf condensation, and favors molds like powdery mildew and botrytis. Strong evidence[3][4]

For cannabis specifically, peer-reviewed studies directly comparing yield or cannabinoid content to specific VPD targets are rare. Weak / limited However, the general principle—that extremes in evaporative demand harm plant physiology—is well supported. Strong evidence[1]

Commonly cited target ranges

Most cannabis growers and equipment makers reference these VPD ranges by growth stage:

These ranges originate in horticultural rules of thumb and grower observation, not in controlled cannabis trials. Weak / limited They assume leaf temperature equals air temperature—which is rarely true. Under strong LED or HPS light, leaves can run 2–3°C cooler than air due to transpiration, shifting the actual leaf-to-air VPD lower than the chart suggests. Strong evidence[1][5] Serious growers measure leaf temperature with an infrared thermometer and use a leaf-temperature-aware calculator to account for this gap.

Treat these ranges as starting points, not absolutes. Watch your plants for signs of stress: leaf curling, prayer position (leaves folding upward at night), or visible slow transpiration (dry leaves). These signals matter more than hitting an exact kPa number.

How to use a VPD chart, step by step

  1. Place a calibrated thermo-hygrometer at canopy height. Do not place it on the floor, wall, or outside the light spray. Shield it from direct light. Verify calibration against a known reference (a salt-test kit) at least once per grow.
  1. Identify your growth stage and choose a target VPD range from the chart.
  1. Record temperature and RH at lights-on, mid-cycle, and lights-off. Climate swings between these times matter as much as the average. Create a simple log with date, time, temperature, RH, and the corresponding VPD.
  1. Look up your VPD on the chart. If you have an IR thermometer, also measure a sun-exposed leaf and use a leaf-aware calculator (many free web calculators include leaf-temperature correction). This is more accurate than air temp alone.
  1. Adjust one variable at a time. To raise VPD: lower humidity using a dehumidifier or increase exhaust fan speed, or raise air temperature. To lower VPD: run a humidifier, lower temperature, or reduce exhaust. Avoid rapid swings—aim for stable values within your target range.
  1. Wait 15–30 minutes for the room to stabilize after each change, then re-check readings.
  1. Log daily. Patterns emerge in logs: for example, RH spiking at lights-off when transpiration stops but temperature drops. This pattern is a classic setup for mold.
  1. Adjust targets as the plant matures. A bigger canopy transpires more and naturally raises tent humidity. Late-flower plants are more prone to botrytis and mold; move toward the higher end of your VPD range (1.2–1.6 kPa) and improve air circulation.

Common mistakes

Leaf temperature and why it matters

A leaf is not the same temperature as the air around it. Under strong grow lights, transpiration cools the leaf; in darkness or low light, the leaf may warm above air temperature. This gap directly affects the VPD at the leaf surface, which is what actually drives water loss and stomatal response.

If you measure air temperature at 28°C and 60% RH on a standard VPD chart, you might read a VPD of ~1.3 kPa. But if the actual leaf is 25°C (due to transpiration under LED), the true leaf-to-air VPD is lower. Strong evidence[1][5] An infrared thermometer (non-contact, ~$20–50) pointed at a sunlit leaf gives you this correction. Most online VPD calculators now include a leaf-temperature field; if yours doesn't, switch to one that does.

For seedlings and clones under low light, this correction is small. For a mature canopy under high-intensity LEDs, it can shift your reading by 0.2–0.4 kPa—enough to change your management decision.

VPD management pairs naturally with several other cultivation practices:

For sealed rooms with AC and dehumidification, VPD becomes your primary climate dashboard. For tent growers with limited equipment, cheap VPD wins are usually a small dehumidifier in flower (to combat mold at lights-off) and an oscillating fan to break up stagnant boundary layers around leaves.

Sources

  1. Peer-reviewed Grossiord, C., Buckley, T. N., Cernusak, L. A., Novick, K. A., Poulter, B., Siegwolf, R. T. W., Sperry, J. S., & McDowell, N. G. (2020). Plant responses to rising vapor pressure deficit. New Phytologist, 226(6), 1550–1566.
  2. Peer-reviewed Zhang, D., Du, Q., Zhang, Z., Jiao, X., Song, X., & Li, J. (2017). Vapour pressure deficit control in relation to water transport and water productivity in greenhouse tomato production during summer. Scientific Reports, 7, 43461.
  3. Peer-reviewed Punja, Z. K., Collyer, D., Scott, C., Lung, S., Holmes, J., & Sutton, D. (2019). Pathogens and molds affecting production and quality of Cannabis sativa L. Frontiers in Plant Science, 10, 1120.
  4. Peer-reviewed Punja, Z. K. (2021). Epidemiology of Fusarium oxysporum causing root and crown rot of cannabis (Cannabis sativa L.) plants in commercial greenhouse production. Canadian Journal of Plant Pathology, 43(2), 216–235.
  5. Peer-reviewed Chavan, S. G., Maier, C., Alagoz, Y., Filipe, J. C., Warren, C. R., Lin, H., Jia, B., Loik, M. E., Cazzonelli, C. I., Chen, Z.-H., Ghannoum, O., & Tissue, D. T. (2022). Light-limited photosynthesis under energy-saving film decreases eggplant yield. Food and Energy Security, 11(2), e364.

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Oct 30, 2026
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